enhancement of solubilization and sorption...

ENHANCEMENT OF SOLUBILIZATION AND SORPTION

BEHAVIORS OF POLYCYCLIC AROMATIC HYDROCARBONS

THROUGH INVOLVEMENT OF GEMINI SURFACTANTS IN

SOIL-WATER SYSTEMS

A Thesis

Submitted to the Faculty of Graduate Studies and Research

In Partial Fulfillment of the Requirements

For the Degree of

Doctor of Philosophy

in

Environmental Systems Engineering

University of Regina

By

Jia Wei

Regina, Saskatchewan

April, 2013

Copyright 2013: J. Wei

UNIVERSITY OF REGINA

FACULTY OF GRADUATE STUDIES AND RESEARCH

SUPERVISORY AND EXAMINING COMMITTEE

Jia Wei, candidate for the degree of Doctor of Philosophy in Environmental Systems Engineering, has presented a thesis titled, Enhancement of Solubilization and Sorption Behaviors of Polycyclic Aromatic Hydrocarbons Through Involvement of Gemini Surfactants in Soil-Water Systems, in an oral examination held on April 5, 2013. The following committee members have found the thesis acceptable in form and content, and that the candidate demonstrated satisfactory knowledge of the subject material. External Examiner: *Dr. Huining Xiao, University of New Brunswick

Supervisor: Dr. Guo H. Huang, Environmental Systems Engineering

Committee Member: Dr. Guoxiang Chi, Department of Geology

Committee Member: Dr. Liming Dai, Industrial Systems Engineering

Committee Member: Dr. Stephanie Young, Environmental Systems Engineering

Chair of Defense: Dr. Dongyan Blachford, Faculty of Graduate Studies and Research *participated Video Conference

i

ABSTRACT

Contamination of soil, sediment and water by polycyclic aromatic hydrocarbons

(PAHs) has been recognized as a major, widespread, environmental waste concern. As a

consequence, surfactant enhanced remediation (SER) has emerged as a promising

technology for the removal of toxic PAHs. Gemini surfactants as a new generation of

surfactants have structures and properties that are unique to the world of surfactants,

such as greater efficiency in reducing surface tension and unusual aggregation

morphologies in comparison with conventional surfactants. They have generated a

growing interest owing to their superior performance in soil and water remediation

applications.

In this research, systematic studies have been physicochemically investigated in

detail via tensiometric, conductometric and solubilization techniques to get insight into

the micellar, interfacial and enhanced solubilization aspects of selected single and

equimolar bi and ternary Gemini/Gemini and Gemini/conventional surfactant systems. A

variety of mutual interaction parameters associated with the micellarization and

solubilization process have been correlated through several theoretical treatments to

understand the synergism and antagonism in solubilization capabilities of mult i-

component surfactant systems. Nonideality has been found due to a change in the

microenvironment of the surfactant solution when various surfactants are mixed. The

results have shown the solubilization power depends on the micellar and interfacial

properties of the surfactant and their association with the hydrophilic/hydrophobic

properties of solutes.

ii

Based on the solubilization analysis, the binding of select symmetric and

dissymmetric Gemini surfactants with soil particles and their equilibrium distribution

between solid and aqueous phases were evaluated. The adsorption isotherm is plotted

and modeled to measure the surface coverage by surfactant molecules under a given

condition to study the adsorption mechanism and, hence, determine the interfacial

properties of modified solids. The sorption capacity of modified soils and natural soils

and the overall partitioning of representative polycyclic aromatic hydrocarbons (PAHs)

in a soil-water-surfactant system with a soil-sorbed Gemini surfactant and Gemini

micelles are compared and related coefficients are developed.

Major contributions coming from this research include: 1) A set of methodologies to

evaluate and assist in the design of multi-component Gemini surfactant systems to

enhance the solubility of polycyclic aromatic hydrocarbons (PAHs); 2) A clearer view of

the effect of the structure of surfactants including the hydrophilic head group,

hydrophobic chain length and spacer length, and the chemical nature of the solute on the

micellar, interfacial and solubilization properties of surfactant systems; 3) Optimum

Gemini surfactant mixtures for potential engineering application; 4) An explanation of

the contaminant distribution pattern produced by the behavior of Gemini surfactant

adsorption onto soils; 5) An investigation into the partitioning behavior of PAHs in a

soil-water-Gemini surfactant and a development of the relative coefficients to produce an

in-depth understanding of the mechanisms for surfactant enhanced remediation

technology. The outputs of this study will be useful to understand and predict the

solubilization and adsorption properties of Gemini surfactant systems and provide proof

for exploring new surfactant systems for practical engineering applications.

iii

ACKNOWLEDGEMENTS

First and foremost, I am deeply grateful to my honorific supervisor, Dr. Gordon

Huang, for his patient supervising and generous support during the course of my Ph.D.

study. I have been extremely lucky to have a supervisor who cared so much about my

work and responded to my questions and queries so promptly. His positive outlook and

confidence in my research inspired me and gave me confidence. I have learned a lot from

him, without his help I could not have finished my dissertation successfully.

My great gratitude must also extend to his family, particularly to Ms. Chunling Ke,

for her kind help during my study while living in Regina.

I am grateful to the Faculty of Graduate Studies and Research and the Faculty of

Engineering and Applied Science at the University of Regina, for providing various

scholarships and awards for my Ph.D. study.

I would like to extend great thanks to my friends in the Environmental Informatics

Laboratory, Hui Yu, Shan Zhao, Yao Yao, Renfei Liao, Ping Guo, Yongping Li,

Xianghui Nie, Wei Chen, Gongchen Li, Yumin Chen, Xiaodong Zhang, Yanpeng Cai,

Qian Tan, Wei Sun, Zhong Li, Jiapei Chen, Xiujuan Chen, Yurui Fan, Guanhui Chen,

Xiuquan Wang, Sheng Li, Mengyuan Wang, Shuo Wang, Yang Zhou, and Hua Zhu, for

their friendships and unselfish helps during my Ph.D. study, which led to an enjoyable

and precious time in my life.

iv

DEDICATION

This dissertation is dedicated to my entire family, who gave me love, strength and

support throughout this important period of my life.

I am very grateful for my parents in China, Mr. Hua Wei and Ms. Bin Kong, for their

endless love and unconditional support. Their firm and kind-hearted personality has

affected me to be steadfast and never bend to difficulty. They always let me know that

they are proud of me, which motivates me to work harder and do my best.

Many appreciations are also due to my parents- in-law, Mr. Zixin Zhang and Ms.

Yimei Zhu, for their kind understanding and encouragement.

My deepest gratitude must be reserved for my husband, Lei Zhu. He took every

responsibility and suffered all the bitterness to take care of my family and our parents.

Without his understanding, support, encouragement and sacrifices, all that I have

achieved would have been impossible. I owe my every achievement to him.

Last but not the least, I am thankful to my dear grandparents who gave me strength,

courage and endless love.

v

TABLES OF CONTENTS

ABSTRACT ...................................................................................................................... i

ACKNOWLEDGEMENTS ............................................................................................. iii

DEDICATION .................................................................................................................. iv

CHAPTER 1 INTRODUCTION ..................................................................................... 1

1.1 Background.............................................................................................................. 1

1.2 Challenges ............................................................................................................... 4

1.3 Objectives ................................................................................................................ 8

1.4 Organization .......................................................................................................... 10

CHAPTER 2 LITERATURE REVIEW ...................................................................... 11

2.1 Research Regarding Surfactants ............................................................................ 11

2.2 Surfactant Solubilization ....................................................................................... 20

2.3 Surfactant Sorption ................................................................................................ 24

2.4 Applications of Surfactants for Soil and Water Remediation ............................... 30

2.5 Literature Review Summary.................................................................................. 41

CHAPTER 3 EFFICIENCY OF GEMINI SURFACTANT ON THE

SOLUBILIZATION OF PAHS IN COMPARISON WITH

CONVENTIONAL SURFACTANTS ................................................... 43

3.1 Introduction ........................................................................................................... 43

3.2 Materials and Methods .......................................................................................... 45

3.3 Results and Discussion .......................................................................................... 53

3.3.1 Surface properties of selected surfactants ................................................ 53

3.3.2 Solubilization by surfactants..................................................................... 57

3.4 Summary................................................................................................................ 69

CHAPTER 4 INVESTIGATION ON THE SOLUBILIZATION OF PAHS IN THE

PRESENCE OF SINGLE AND BINARY GEMINI SURFACTANT

SYSTEMS WITH VARIOUS HYDROPOBIC CHAIN LENGTHS . 71

4.1 Introduction ........................................................................................................... 71

4.2 Materials and Methods .......................................................................................... 74

vi

4.3 Results and Discussion .......................................................................................... 81

4.3.1 Critical micelle concentration properties of the single and equimolar

binary Gemini surfactants ....................................................................... 81

4.3.2 Solubilization of PAHs by single Gemini surfactants ............................... 84

4.3.3 Solubilization of PAHs by equimolar binary Gemini surfactants ............ 90

4.4 Summary............................................................................................................. 102

CHAPTER 5 ENHANCED SOLUBILIZATION CAPABILITIES OF BI AND

TERNARY CATIONIC GEMINI AND CONVENTIONAL

NONIONIC SURFACTANTS TOWARDS PAHS ............................ 104

5.1 Introduction ......................................................................................................... 104

5.2 Materials and Methods ........................................................................................ 107

5.3 Results and Discussion ........................................................................................ 113

5.3.1 Critical micelle concentration properties of all surfactant systems ....... 113

5.3.2 Surfactant-Surfactant interactions in mixed micelles ............................. 114

5.3.3 Solubilization of naphthalene and pyrene .............................................. 120

5.4 Summary.............................................................................................................. 133

CHAPTER 6 IMPROVED SOLUBILITIES OF PAHS BY MULTI-

COMPONENT GEMINI SURFACTANT SYSTEMS WITH

VERIOUS SPACER LENGTHS .......................................................... 134

6.1 Introduction ......................................................................................................... 134


6.3 Results and Discussion ....................................................................................... 142

6.3.1 Critical micelle concentration properties and surfactant-surfactant

interactions ............................................................................................ 142

6.3.2 Counterion binding ................................................................................. 148

6.3.3 Properties at the air/water interface ...................................................... 153

6.3.4 Solubilization abilities of selected surfactant systems towards PAHs.... 154

6.4 Summary.............................................................................................................. 164

vii

CHAPTER 7 GEMINI SURFACTANT-ENHANCED ADSORPTION AND

DESORPTION OF PAHS IN A SOIL-WATER-SURFACTANT

SYSTEM................................................................................................. 166

7.1 Introduction ......................................................................................................... 166



7.3.1 Sorption of Gemini surfactants ............................................................... 175

7.3.2 Solubilization of naphthalene by Gemini surfactants ............................. 178

7.3.3 Mobility of naphthalene in soil-water-Gemini surfactant systems ......... 182

7.3.4 Immobilization of naphthalene with Gemini surfactants ........................ 183

7.4 Summary.............................................................................................................. 191

CHAPTER 8 DISTRIBUTION OF PAHS IN A SOIL-WATER SYSTEM

CONTAINING A GEMINI SURFACTANT ...................................... 192

8.1 Introduction ......................................................................................................... 192



8.3.1 Solubilization of PAHs in the presence of Gemini surfactant ................ 200

8.3.2 Gemini surfactant sorption onto soil ...................................................... 206

8.3.3 Distribution of PAHs in a soil-water-surfactant system ......................... 209

8.4 Summary.............................................................................................................. 223

CHAPTER 9 CONCLUSIONS .................................................................................... 224

9.1 Summary.............................................................................................................. 224

9.2 Achievements ...................................................................................................... 226

9.3 Recommendations for Future Work .................................................................... 227

REFERENCES .............................................................................................................. 229

viii

LIST OF TABLES

Table 2.1 Size limits of different soil constituents (Paria, 2008) ...................................... 35

Table 3.1 The formula and physicochemical properties of the selected surfactants ......... 46

Table 3.2 Experimental critical micelle concentration (CMCexp), ideal critical micelle

concentration (CMCmi), interaction parameter (β), micellar mole fraction (Xi)

and activity coefficients (fi) values of equimolar surfactant mixtures at 25 ◦C a 54

Table 3.3 MSR, Log Km, 0

sG , R and B of phenanthrene in various single and equimolar

mixed micellar systems at 25 ◦C a ...................................................................... 67

Table 4.1 The formula, molecular properties and experimental critical micelle

concentrations (CMCexp) of the selected Gemini surfactants ............................. 75

Table 4.2 The formula and physicochemical properties of the selected PAHsa ............... 76


concentration (CMCmi), interaction parameter (β), micellar mole fraction (Xi),

ideal micellar mole fraction (Xideal) and activity coefficients (fi) values of

equimolar Gemini mixtures at 25 ◦C a ................................................................ 83

Table 4.4 Log Km, Log Km12, 0

sG , R and B of PAHs in various equimolar mixed Gemini

surfactant systems at 25 ◦C a ............................................................................. 101

Table 5.1 The structure and physicochemical properties of the selected surfactants and

solutesa in this study ........................................................................................ 110


concentration (CMCmi), interaction parameter (β), micellar mole fraction (Xi)

and activity coefficients (fi) values of equimolar surfactant mixtures at 25 ◦C a117

ix

Table 5.3 Molar solubilization ratio (MSR), micelle-water partition coefficient (Km),

deviation ratio (R) and free energy of solubilization (0

sG ) of naphthalene and

pyrene in various single and equimolar bi and ternary micellar systems at 25 ◦Ca131


concentration (CMCmi), interaction parameter (βm) and micellar mole fraction

(Xi) of equimolar surfactant mixtures at 25 ◦Ca ............................................... 146

Table 6.2 Surface excess concentration (Гmax), minimum area per molecule (Amin),

counter ion binding (g1) and free energies of micellization (0

mG ) of single and

equimolar mixed surfactant systems in aqueous medium at 25 ◦Ca ................. 149

Table 6.3 MSR, Log Km, R and 0

sG of PAHs in various single and equimolar mixed

Gemini surfactant systems at 25 ◦Ca ................................................................ 161

Table 7.1 The formula and experimental critical micelle concentration (CMCexp) of the

selected Gemini surfactants ............................................................................. 170

Table 7.2 Selected properties of the soil sample ............................................................. 171

Table 7.3 Parameters of selected Gemini surfactants and the distribution of PAHs in the

presence or absence of Gemini surfactant systemsa ........................................ 185

Table 7.4 Equilibrium Gemini surfactant concentrations and calculated surfactant-

derived organic carbon contents (fsoc) in soils.................................................. 189

Table 8.1 Selected physicochemical properties of surfactant and PAHs in this study ... 199

Table 8.2 Parameters of the distribution of PAHs in a soil-water-Gemini surfactant

system............................................................................................................... 204

x

LIST OF FIGURES

Figure 2.1 Schematic diagram of a surfactant molecule .................................................. 13

Figure 2.2 Surfactant molecules exist as monomer and micelle in aqueous solution ...... 14

Figure 2.3 Schematic diagram of the variation of solute solubility, surface and

interfacial tension with surfactant concentration (Sabatini et al., 2000) ........ 15

Figure 2.4 Schematic diagram of a Gemini surfactant molecule ..................................... 18

Figure 2.5 Schematic presentation of typical four-region adsorption isotherm (Atkin et

al., 2003) ......................................................................................................... 28

Figure 2.6 The sorption isotherm of TX-100 onto natural soil (Zhou and Zhu, 2005a) .. 29

Figure 2.7 Technologies selected for source control at superfund remedial action sites

(Fiscal year 1982-2002) (U.S. EPA, 2003; Paria, 2008) ................................ 31

Figure 2.8 Typical in situ soil flushing installation (Mulligan et al., 2001) .................... 38

Figure 2.9 In situ modification of aquifer material to create a pollutant sorptive zone

coupled with the biodegradation of pollutants for groundwater remediation 40

Figure 3.1 (a) Plots of surface tension (γ) vs. the total surfactant concentration (Ct) of

single and equimolar binary C12-2-12 and CPC ................................................ 49

Figure 3.1 (b) Plots of surface tension (γ) vs. the total surfactant concentration (Ct) of

single and equimolar binary C12-2-12 and DTAB............................................. 50

Figure 3.1 (c) Plots of surface tension (γ) vs. the total surfactant concentration (Ct) of

single and equimolar binary C12-2-12 and Brij 35 ............................................ 51

Figure 3.1 (d) Plots of surface tension (γ) vs. the total surfactant concentration (Ct) of

single and equimolar binary C12-2-12 and TX-100 ........................................... 52

Figure 3.2 (a) The solubilization of phenanthrene in single C12-2-12, TX-100 and

equimolar mixed C12-2-12/TX-100 surfactants................................................. 60

xi

Figure 3.2 (b) The solubilization of phenanthrene in single C12-2-12, Brij 35 and

equimolar mixed C12-2-12/Brij 35 surfactants .................................................. 61

Figure 3.2 (c) The solubilization of phenanthrene in single C12-2-12, CPC and equimolar

mixed C12-2-12/CPC surfactants ....................................................................... 62

Figure 3.2 (d) The solubilization of phenanthrene in single C12-2-12, DTAB and

equimolar mixed C12-2-12/DTAB surfactants .................................................. 63

Figure 4.1 (a) Plots of the surface tension (γ) vs. the total surfactant concentration (Ct) of

the single Gemini surfactant combinations .................................................... 79

Figure 4.1 (b) Plots of the surface tension (γ) vs. the total surfactant concentration (Ct)

of the equimolar binary Gemini surfactant combinations .............................. 80

Figure 4.2 (a) Water solubility enhancements (Qs) of PAHs by C12-2-4 ........................... 86

Figure 4.2 (b) Water solubility enhancements (Qs) of PAHs by C12-2-8 ........................... 87

Figure 4.2 (c) Water solubility enhancements (Qs) of PAHs by C12-2-12 .......................... 88

Figure 4.2 (d) Water solubility enhancements (Qs) of PAHs by C12-2-16.......................... 89

Figure 4.3 (a) The solubilizations of naphthalene and phenanthrene in C12-2-12/C12-2-16

mixed surfactants ............................................................................................ 91

Figure 4.3 (b) The solubilizations of pyrene in C12-2-12/C12-2-16 mixed surfactants .......... 92

Figure 4.4 (a) The MSR values of various single and mixed Gemini surfactants on

naphthalene ..................................................................................................... 98

Figure 4.4 (b) The MSR values of various single and mixed Gemini surfactants on

phenanthrene................................................................................................... 99

Figure 4.4 (c) The MSR values of various single and mixed Gemini surfactants on

pyrene ........................................................................................................... 100

Figure 5.1 (a) Variations of γ values of single surfactant systems over a wide

concentration range at 25 ◦C ......................................................................... 111

xii

Figure 5.1 (b) Variations of γ values of equimolar binary and ternary mixed surfactant

systems over a wide concentration range at 25 ◦C........................................ 112

Figure 5.2 (a) Variation of solubility of naphthalene with a total surfactant concentration

of single surfactant systems .......................................................................... 122

Figure 5.2 (b) Variation of solubility of naphthalene with a total surfactant concentration

of equimolar binary and ternary mixed surfactant systems .......................... 123

Figure 5.2 (c) Variation of solubility of pyrene with a total surfactant concentration of

single surfactant systems .............................................................................. 124

Figure 5.2 (d) Variation of solubility of pyrene with a total surfactant concentration of

equimolar binary and ternary mixed surfactant systems .............................. 125

Scheme 6.1 Molecular Structure of (a) selected Gemini surfactants, (b) naphthalene; (c)

pyrene ........................................................................................................... 137



Figure 6.1 (b) Variations of γ values of equimolar bi and ternary mixed surfactant


Figure 6.2 (a) Variations of K values of single surfactant systems over a wide


Figure 6.2 (b) Variations of K values of equimolar bi and ternary mixed surfactant


Figure 6.3 (a) Variations of solubilities of naphthalene with various surfactant

concentrations of single Gemini surfactant at 25 ◦C..................................... 156

Figure 6.3 (b) Variations of solubilities of naphthalene with various surfactant

concentrations of equimolar bi and ternary Gemini surfactant systems at 25

◦C................................................................................................................... 157

xiii

Figure 6.3 (c) Variations of solubilities of pyrene with various surfactant concentrations

of single Gemini surfactant at 25 ◦C ............................................................. 158

Figure 6.3 (d) Variations of solubilities of pyrene with various surfactant concentrations

of equimolar bi and ternary Gemini surfactant systems at 25 ◦C ................. 159

Figure 7.1 Surfactant sorption isotherms (Symbols: experimental data; dotted lines:

fitted Langmuir isotherms) ........................................................................... 176

Figure 7.2 Water solubility enhancement of naphthalene as a function of the

concentration of the selected Gemini surfactants at 25 ◦C ........................... 179

Figure 7.3 Amount of naphthalene solubilized as a function of amount of Gemini

surfactant in solution .................................................................................... 186

Figure 7.4 The apparent partition coefficients of naphthalene in relation to the Gemini

surfactants concentration .............................................................................. 190

Figure 8.1 Water solubility enhancement of PAHs as a function of the concentration of

C12-3-12 at 25 ◦C.............................................................................................. 202

Figure 8.2 Correlation of the micelle-water partition coefficient (Kmc) with the octanol–

water distribution coefficient (Kow) for PAHs .............................................. 205

Figure 8.3 The adsorption isotherm of C12-3-12 onto natural soil .................................... 208

Figure 8.4 Correlation of the organic-carbon normalized distribution coefficient (Koc)

with the octanol-water distribution coefficient (Kow) for PAHs ................... 210

Figure 8.5 The measured apparent sorption coefficients (*

dK ) of PAHs versus the

equilibrium concentration of C12-3-12 ............................................................ 212

Figure 8.6 Schematic representations of PAHs participation in a soil-water-Gemini

surfactant system .......................................................................................... 215

Figure 8.7 Correlation of the apparent HOCs soil-water distribution coefficient with

surfactant (*

dK ) with the octanol–water distribution coefficient (Kow) for

PAHs............................................................................................................. 217

xiv

Figure 8.8 The variation of Kss for PAHs with the sorbed C12-3-12 concentration .......... 221

Figure 8.9 Correlation of the carbon-normalized solute distribution coefficient (Kss) with

the octanol-water distribution coefficient (Kow) for PAHs ........................... 222

1

CHAPTER 1

INTRODUCTION

1.1 Background

For decades, persistent organic pollutants (POPs) have been recognized as important

toxic contaminants which are widely distributed in the environment. Studies in the

Canadian Arctic indicate the contaminants have been accumulating in the Arctic

environment from various sources (AMAP (Arctic Monitoring and Assessment Program),

1998). Although many different forms of POPs may exist, they primarily include two

types of unintentionally and intentionally produced compounds: polycyclic aromatic

hydrocarbons and halogenated hydrocarbons, which are characterized by their

hydrophobic nature, long environmental persistence and long-range transport (Çok et al.,

2012).

Polycyclic aromatic hydrocarbons or polynuclear aromatic hydrocarbons (PAHs)

occur in oil, coal and tar deposits, and are produced through incomplete combustion and

pyrolysis of materials containing carbon and hydrogen (Gan et al., 2009). They

originated mainly from both natural and anthropogenic sources. The former includes

industrial combustion of fossil fuels, incinerators, vehicular emissions, residential wood

burning and petroleum catalytic cracking which contributes to the release of PAHs into

the environment; the latter includes forest fires and diagenetic progress (Ding et al.,

2007). Owing to their bioaccumulation and toxicity on immune, nervous and

reproductive systems, the overall emission of PAHs has been restricted or banned since

the beginning of the 1970’s, in most countries (Rose and Rippey, 2002; Yunker et al.,

2

2002). However, evidence from the Greenland ice sheet implies that their residues still

exist and have been transported via a variety of global environmental media for many

decades and thus, continue to induce an adverse impact on the environment and its

ecosystems, even up to the early 1990s (Masclet et al., 1995). Owing to

chemical/microbiological stability, low water solubility, lipophilic properties and vapor

pressure, a large portion of PAHs are globally cycled in the atmosphere, accumulate in

terrestrial systems, act as a long-term source of groundwater contamination, slowly

dissolve and cause contaminant concentrations above safety levels.

Soil may contain significant amounts of harmful PAHs, especially if the sources

directly originated in soil and underground water systems, such as industrial system

discharges, the release of non-combusted petroleum products. Therefore, underground

storage tank fuel leaks and the associated piping have been a major source of soil and

groundwater contamination. The Province of Saskatchewan houses Canada’s second-

largest oil producer with an oil production of 157.8 million barrels in 2011 when the

price of oil exceeded $140 per barrel and is questing to overtake Alberta as Canada’s

largest conventional oil producer (The Regina Leader-post, 2012). The majority of the

attention has been focused upon the impetuous industrial development, and far less effort

has been devoted to the minimization of soil and groundwater contamination which

originated from petroleum exploration, production, storage, transportation and utilization

processes. Particularly, as one of the major storage facilities, underground storage tanks

are extensively employed. It was reported that 5 to 10 % of the over 200,000

underground storage tanks, used across Canada, are currently leaking, leading to various

negative impacts upon communities and industries (Chen et al., 2001). Dealing with

3

crude oil pollution is a very difficult and expensive situation. PAHs are one of the

harmful groups stemming from crude oil and their contribution can vary from 4 to 22%

depending upon substance type (Oleszczuk and Baran, 2003). In Alberta, the clean-up

cost for hydrocarbon-contaminated sites has reached approximately $5 billion CAD

(Chen et al., 2001).

More recently, growing public attention and updated legislation have appropriately

activated the development of cleaner production processes and remediation technologies.

The latter issue is of extreme significance since the presence of highly toxic

contaminants in the environment and food chain exerts profound adverse influence upon

human health. In view of the urgent need for environmental protection, the development

of remediation technologies for contaminated soil and groundwater have been the subject

of extensive study, such as absorption, thermal desorption, solvent extraction, and

incineration. Nevertheless, the application of any of these techniques must be evaluated

on an individual basis which is principally dependent upon the type of matrix and

contamination, accessibility and most importantly, cost effectiveness (Kasi et al., 1993).

Among them, surfactant-enhanced remediation (SER) has been developed to be a more

powerful and economical remedial method (Paria, 2008), which takes full advantage of

the surfactant’s high solub ilization and powerful interception capability towards organic

contaminants. Surfactants can promote microbial remediation of PAHs in soil and water

by affecting the accessibility of PAHs to microorganisms (Tiehm, 1994). Thus, studies

regarding the interaction of surfactant and contaminants and their effect upon the

behaviors of contaminants in the environment (soil, water systems) have come to the

world’s attention.

4

1.2 Challenges

1.2.1 Solubilization of PAHs by surfactants

Surfactants are chemicals possessing a hydrophobic core and a hydrophilic outer

surface. Surfactant molecules, when above their critical micelle concentration (CMC),

form aggregates in water, called “micelles”. An important property of micelles is their

capability to promote the solubility of solvophobic molecules by trapping them in

energetically favorable microenvironments via a process known as “solubilization” (Mir

et al., 2011). For example, in aqueous solutions containing surfactant micelles, the

solubility of poorly water-soluble hydrocarbons can be improved by many orders of

magnitude, similarly, in non-polar solvents, the presence of reverse micelles enhance the

solubility of polar substances (Nagarajan, 1996). Solubilization serves as the basis for a

wide variety of industrial, pharmaceutical and biological applications of surfactants.

Furthermore, high solubilization efficiency could potentially facilitate engineered

treatments, e.g., soil and water remediation, drug delivery, and separation procedures, etc.

Thus, research regarding the solubilization ability of surfactant is the core to developing

novel technology that predominantly depends upon surfactant.

Several factors which control the solubilization capacity of micelles include the type

and dose of surfactant, the compatibility between surfactant and solute, the presence of

other additive molecules and so on. Over the last decade, tremendous attention has been

paid to the enhanced solubility of contaminants via the addition of conventional

surfactants, which have a single hydrophilic head group and a single alkyl chain. An

improved strategy to SER is to reduce the dosage of surfactants and the cost level while

5

maintaining the efficiency of remediation. Thus, the above information led us to pursue

new, novel, and more efficient surfactant in the hope their superior properties may

compensate for their presently higher cost and vast dosage. Novel surfactants such as

Gemini surfactants, consist of two hydrocarbon chains and two polar groups linked with

a spacer and have attracted considerable attention. They have stronger surface activity, a

greater efficiency in reducing surface tension and unusual aggregation morphologies

compared with conventional surfactants (Wei et al., 2011a). Furthermore, they have been

generating a growing interest owing to their superior application performance. While the

majority of present research activities regarding Gemini surfactants are mainly focused

on their synthesis and many details regarding Gemini surfactants concerning aiding the

remediation of the environment are not very clear, further investigation of the system is

still required.

Moreover, the majority of earlier studies were performed with individual surfactant

solutions. In a practical work scenario, mixtures of surfactants, rather than individual

surfactants, are almost always employed. In many cases, when different types of

surfactants are purposely mixed, synergistic properties are exhibited. For instance,

mixing a nonionic surfactant with an anionic surfactant solution (Dubin et al., 1989)

appreciably reduced CMC and enlarged the size of mixed micelles as well, which would

give a boost to the degree of solubilization. Mixed surfactants could be applied over a

wide range of temperatures, salinity and hardness conditions rather than the individual

surfactants (Zhou and Zhu, 2005b). In addition, the interactions of coexisting mixed

surfactants affect not only the efficiency of solubilization but also the properties of

solubilizate, and thus, the transport and transformation of organic contaminants. Various

6

factors such as the structure, the composition, and the tendency to form mixed micelle

influence surfactant mixtures. Without adequate theoretical analysis, it is difficult to

understand and predict the solubilization behavior of mixed surfactants for practical

applications. Therefore, further understanding of the properties of mixed surfactant

systems, based on systematic and theoretical studies, is desirable, especially, the system

with novel Gemini surfactants and their mixtures.

Overall, in contrast to the numerous literature studies dealing with conventional

surfactants, limited information is available regarding Gemini surfactants and their

mixtures. It would be valuable to know details with respect to the aggregation of

different Gemini surfactants, (i.e., the size, composition and interaction of Gemini

mixtures, the effect of spacer and alkyl chain length, the locus of solubilizate in Gemini

aggregates, the influence of solubilizates, mixed behavior on CMC and so on), which are

essential for both the interpretation of experimental solubilization data and the

construction of theories of solubilization.

1.2.2 Gemini surfactant enhanced technology (GSER)

Sorption and desorption have been proven to be one of the most effective techniques

for the remediation of contaminated soil, groundwater, potable water, effluents and

aqueous solutions. Surfactants are considered to be potential agents with which to adjust

and control these processes via different solubilization modes. They involve the

enhancement of organic compound mobility by partitioning it into a mobile surfactant

phase and minimizing organic compound from a point of source of pollution through

7

sorption to permeable reactive barriers formed by the sorbed surfactant phase. The latter

process is also regarded as a type of solubilization, where natural and surfactant obtained

organic matter functions essentially as a bulk organic solvent phase and organic solutes

distribute themselves between water and this organic phase (Gao et al., 2001).

Depending upon the given site conditions, alternative remediation approaches can be

applied by using specific surfactants via different mechanisms.

In order to understand the behavior of contaminants in the presence of surfactant,

recognize the effect of surfactant upon coexisting compounds in environment, and apply

surfactant in a reasonable way to reduce the cost of industrial processes, achieving

insight into surfactant adsorption would be beneficial. It is a process of transferring

surfactant molecules from the bulk solution phase to the surface/interface as determined

by a number of forces (Paria and Khilar, 2004) . The adsorption of surfactant at the

solid–liquid interface plays a significant role, not only in the remediation method for soil

and water contamination, but also in many technological and industrial applications such

as mineral flotation, corrosion inhibition and oil recovery.

The equilibrium of surfactant adsorption is significant because it directly determines

the interfacial properties of solids, which is the key point in most industrial applications.

It is a measure of the extent of surface of the adsorbent that is covered with adsorbent

molecules at a given condition, and hence, determines the interfacial properties (Paria

and Khilar, 2004). Studies with respect to conventional surfactants showed that anionic

surfactants tend to sorb the least to soil; nonionic surfactants are a good candidate to

enhance in situ remediation, based upon intermediate sorption and low biotoxicity; and

cationic surfactants are apt to change the surface property of a solid. Additionally, an

8

organic compound behaves differently in the absence and presence of surfactant in solid-

water-surfactant systems. It is reported that the sorption of organic pollutants on

modified clays or soils was ten- to a hundred-fold higher than that on natural clays or

soils (Meng et al., 2007). The factors include solid types, polarity of organic pollutants,

pH, sorption forms of modifying agents on the solid surface, surface charge density of

solids and competitive sorption of multi organic pollutants and so on.

Gemini surfactant possesses twin headgroups and hydrocarbon chains which exhibit

an unusual property when it comes to solubilization, sorption and desorption aspects.

Since a Gemini surfactant consists of a spacer and twice as much organic carbon per

molecule than the conventional surfactant molecule of the same species, for a given

number of carbon atoms per chain, there is interest in determining whether Gemini will

form a better microenvironment for the solubilization of an organic compound either in

the aqueous phase or solid phase and whether unusual behavior will occur during the

industrial process by using Gemini rather than conventional surfactant. As mentioned

above, the majority of recent studies regarding synthesis of novel Gemini surfactants, a

systematically investigation on their behaviors in environmental systems and interaction

with other coexisting surfactant or contaminant, are still scarce. Additionally, the high

concentration of surfactant, studied herein, has more practical significance to controlling,

as well as remediation contamination of soil and water systems.

1.3 Objectives

Based on the aforementioned problems, the objectives of this dissertation are:

9

(a) To investigate the micellar and interfacial aspects of selected Gemini surfactants

with different spacer and alkyl chain lengths, and to evaluate their enhanced

solubilization capabilities for representative PAHs when compared with conventional

surfactants with the same hydrophobic chain length.

(b) To correlate the interaction parameter with respect to developed multi-component

surfactant systems including single Gemini surfactants with different spacer and alkyl

chain lengths, binary and ternary Gemini/conventional cationic, nonionic surfactant

systems, and to elucidate the cause of their mixing effect for finding a synergism benefit

for enhanced solubilization abilities towards representative PAHs.

(c) To examine the sorption phenomena of selected symmetric and dissymmetric

Gemini surfactants and the subsequent conformation of Gemini surfactants on the soil

surface and to determine the effect of the hydrocarbon chain on the sorption mechanism

of Gemini surfactant on the soil surface, which is crucial to understanding the interaction

of the sorbed surfactants with organic compounds in solution and predicting the stability

of the sorbed surfactant.

(d) To investigate the partitioning of representative PAHs to aqueous surfactant

micelles, soil organic matter and the sorbed Gemini surfactant and develop overall HOCs

distribution coefficients that consider soil sorbed surfactant amounts and the presence of

micelles as a function of the surfactant dose so the mechanism of PAHs on Gemini

surfactant-modified soil could be elucidated.

10

1.4 Organization

This dissertation is structured as follows; Chapter 2 provides a comprehensive

literature review of previous studies on surfactant, Gemini surfactant, solubilization,

sorption of surfactants and contaminants, surfactant enhanced remediation, etc. Chapter

3 presents the enhanced solubilization capabilities of Gemini surfactant compared with

conventional cationic and nonionic surfactant possessing with the same hydrocarbon

chain length. Chapter 4 develops an equimolar binary cationic Gemini surfactant system

with different hydrophobic chain lengths for improved solubilization of representative

PAHs. Chapter 5 develops an equimolar bi and ternary cationic Gemini

surfactant/conventional nonionic surfactant system for enhanced solubilization of

representative PAHs. Chapter 6 develops an equimolar bi and ternary cationic Gemini

surfactant system with different spacer lengths for enhanced solubilization of

representative PAHs. Chapter 7 presents the sorption of Gemini surfactants with

different hydrophobic chain lengths on the soil surface and their effect upon the behavior

of PAHs in soil-water-Gemini surfactant systems. Chapter 8 discloses the partitioning of

representative PAHs to aqueous Gemini surfactant micelles, soil organic matter and the

sorbed Gemini surfactant and develops overall PAHs distribution coefficients. The

conclusion of this dissertation, as well as recommendation for future research, is drawn

toward the end of Chapter 9.

11

CHAPTER 2

LITERATURE REVIEW

2.1 Research Regarding Surfactants

2.1.1 Conventional surfactants

The molecules of surface active agents (surfactants) are amphiphilic, constituted by

one oil- like (non-polar) and one water- like (polar) region (moieties), as illustrated in

Figure 2.1. The oil- like portion, which can be linear, branch, or aromatic, is hydrophobic

in nature and the water-like potion, usually termed as “hydrophilic moiety” or “head

group’’, interacts powerfully with water via dipole-dipole or ion-dipole interactions and

is solvated (Rosen, 1989b). Thus, when surfactants are placed in water-air or water-oil

systems, they are able to accumulate at the interface with their water- like moiety in the

polar water phase and the oil- like moiety in the non-polar oil or less polar air phase. In

this way, both surfactant moieties are in a preferred phase, and the free energy of the

system is minimized (Rosen, 1989b).

When the aqueous surfactant concentration is below a certain level, the surfactant

molecules are present as monomers and are likely to adsorb at interfaces in an oriented

fashion. The amount of surfactant adsorbed per unit area of the interface is determined

indirectly from surface tension measurement. As a result, a plot of surface tension, as a

function of the logarithm of surfactant concentration in the aqueous phases can be drawn

to find a certain breakpoint, called the critical micelle concentration (CMC) (Zhang,

12

2007). As the aqueous surfactant concentration increases (until the CMC is first

exceeded), monomers begin to self-aggregate into spherical colloidal aggregates, known

as “micelles” (Rosen, 1989a). As a consequence, surface tension reduces dramatically

with an increased surfactant concentration, indicating surface saturation adsorption has

been reached. CMC is unique to surfactant molecules, as well as micelle formation

(Zhang, 2007). Moreover, the activity of surfactant remains constant above CMC even

though additional surfactant is added to the solution. Salinity, hydrocarbon chain length

and surfactant type will influence the concentration. Figure 2.2 shows a diagrammatic

representation of monomers and micelles in aqueous surfactant solutions. Surfactant

micelles with polar moieties at the exterior (making micelles highly soluble in water),

and non-polar moieties in the interior (providing a hydrophobic sink for organic

compounds) (Sabatini et al., 2000), effectively reduce the surface and interfacial tensions

and enhance the solubility of organic compounds (Figure 2.3), which are fundamental

reasons for the widespread use of remediation for soil and water contamination.

In general, surfactants are classified depending upon the nature of the polar head

group. It is the hydrophilic head group of the surfactant which gives it distinctive

chemistry. If a head group carries a positive charge, the surfactant is specifically called

“cationic”; if the charge is negative, it is called “anionic”; if a surfactant contains a head

with both positive and negative charges, it is termed “zwitterionic”; and nonionic has no

charge groups in its head. They have been widely used in various industrial processes

according to their respective characteristics.

13

Figure 2.1 Schematic diagram of a surfactant molecule

14

Figure 2.2 Surfactant molecules exist as monomer and micelle in aqueous solution

15

Figure 2.3 Schematic diagram of the variation of solute solubility, surface and interfacial

tension with surfactant concentration (Sabatini et al., 2000)

16

2.1.2 Biosurfactants

Biosurfactants are a structurally diverse group of surface-active substances,

biologically generated as metabolic byproducts by yeast or bacteria from various

substrates including sugars, oils, alkanes and wastes (Lin, 1996). Their synthesis

principally depend upon the fermentor design, pH, nutrient composition, substrate and

temperature used (Mulligan et al., 2001). They possess distinct advantages over the

highly used synthetic surfactants such as high specificity, biodegradability and

biocompatibility (Cooper, 1986). Biosurfactants are classified as glycolipids,

lipopeptides, phospholipids, fatty acids and neutral lipids (Biermann et al., 1987) and

most are either anionic or neutral, which limit their wide application. Their molecular

weights generally range from 500 to 1500 amu and their CMCs from 1 to 200 mg L-1

(Lang and Wagner, 1987). Although biosurfactants have many interesting properties,

they have not as yet been employed extensively because of their obvious disadvantages

which include expense, critical usage situation, risk of secondary environmental

pollution and toxicity to the environment under certain circumstances.

2.1.3 Gemini surfactants

Gemini (dimeric) surfactants represent a new class of surfactants. They are made up

of two amphiphilic moieties connected at the level of the head groups or very close to the

head groups by a spacer group (Song et al., 2012), as schematically represented in Figure

2.4. Gemini surfactant also can be classified into different groups according to the

charges of their head groups, such as anionic Gemini surfactant (Gautam et al., 2008),

17

cationic Gemini surfactant (Grosmaire et al., 2001) and nonionic Gemini surfactant

(Zhou et al., 2008), etc.

Gemini surfactants have structures and properties that are unique to the world of

surfactants (Song et al., 2012). Current interest in such surfactants mainly arises from the

following essential properties. First, the CMC values of Gemini surfactants are much

lower than those of conventional (monomeric) surfactants with the same hydrophobic

chain length. For example, the CMC value of the dimeric surfactant dimethylene-1,2-bis

(dodecyldimethylammonium bromide) is approximately 0.8 mM, whereas that of the

corresponding conventional surfactant “dodecyltrime-thylammonium bromide” (DTAB)

is 15 mM. Second, Gemini surfactants are much more efficient at decreasing the surface

tension of water than the corresponding conventional surfactants. For instance, the

concentration required to lower the surface tension of water by 0.02 N m-1 is only 0.12

mM for dimethylene-1,2-bis (dodecyldimethylammonium bromide) while being 6.3 mM

for DTAB (Zana, 2002a). In addition, Gemini surfactants have more charges than

conventional ionic surfactants and can strongly interact with counterions. These

discrepancies arise because two alkyl chains, rather than one, are transferred at a time

from water to the micelle pseudo-phase (Camesano and Nagarajan, 2000). The potential

applications suggested by these properties led to a renewed interest of the academic, as

well as industrial communities, in Gemini surfactants.

18

Figure 2.4 Schematic diagram of a Gemini surfactant molecule

19

2.1.4 Mixed surfactants

With the growing demands of industrial technology, the search for high performance

surface active compounds is increasing. With the exception of synthesizing new novel

surfactants, one approach is the mixing of surfactant, which may reduce cost and

environmental impact or be superior on the application front compared to the pure

systems. This practice has become increasingly popular in various commercial

applications in recent years. Surfactant mixtures often display a number of synergistic

effects on aggregates, which would manifest as enhanced surface activity, spreading,

wetting, foaming, detergency, solubilization and a host of other phenomena (Sheikh et al.,

2011).

Keeping this in mind, studies regarding the significant effects of mixing surfactants

on surfactant physicochemical property and performance have become a topic of interest

in the area of the self-assembly of amphiphiles. For example, the formation of mixed

anionic-nonionic micelles has higher HOC desorption efficiencies (Zhu and Feng, 2003).

Recent research by S.K. Mehta and Bhawna has observed a profound effect of DDAB on

the solubility and conformational behavior of Zein in the presence of SDS (Mehta and

Bhawna, 2010). Although the mixing of conventional surfactants has been studied

intensively, far fewer studies exist with respect to a case of mixed micellization of

Gemini/conventional or Gemini/Gemini surfactants. Particularly, Gemini surfactants

with a different spacer and hydrophobic alkyl lengths have complicated effects upon the

behavior of mixtures and have not been fully clarified, as yet.

20

2.2 Surfactant Solubilization

The apparent water solubility of an organic compound can be enhanced, substantially,

by surfactants due to their distinctive structure and characteristics, which ha ve potential

application prospects in water and soil remediation. As for water-surfactant-organic

compound systems, solubility enhancement has been convincingly correlated to the type

and concentration of surfactant, micelle structure, characteristic of organic compound

and other co-compounds, etc.

2.2.1 Solubilization and partition

Partition represents a process wherein organic compounds, especially a nonionic

organic compound is distributed from one phase to two phases and then reaches

equilibrium over a period of time (Leo et al., 1971). In the physical sciences, a partition

or distribution coefficient is the ratio of concentrations of a compound in the two phases

of a mixture of two immiscible liquids at equilibrium (Leo et al., 1971). Generally, one

of the chosen solvents, such as water, is hydrophilic and the other, possibly octanol, is

hydrophobic (Sangster, 1997). Hence, the partition and distribution coefficients are a

measure of how hydrophilic or hydrophobic a chemical substance is.

As for organic compound, the partition coefficient, n-octanol/water partition

coefficient (Kow) and water solubility (Sw), plays a crucial role in determining the

distribution and fate of organic contaminants in the global environment and have been

used to assess environmental partition and the transport of organic substances. The Kow

http://en.wikipedia.org/wiki/Physical_sciences

21

describes the tendency of the distribution between an aqueous phase and organic

constituents of environmental compartments. It basically gives direct information

regarding the hydrophobicity of a compound and is frequently used to predict

nonreactive toxicity, water solubility, bioconcentration factor and soil/water partition

coefficient (Zeng et al., 2012).

Solubilization, according to an IUPAC definition (Everett, 1972), is an abbreviation

for “micellar solubilization”, the process of incorporation of the solubilizate into or onto

the micelles, as one type of partition. The partitions theory was first introduced into

surfactant solubilization of organic contaminant by Chiou (Chiou et al., 1979; Chiou et

al., 1983) in the 1980’s. The studies on solute solubility enhancement by dissolved

organic matter showed that the apparent water solubility of some extremely water-

insoluble organic solute could be significantly enhanced by a low concentration of some

fractionated humic and fulvic acids which have relatively low molecular weights (<1000

daltons) and high polar group contents (Chiou et al., 1979; Chiou et al., 1986). Since

many surfactants have molecular weights and nonpolar group contents comparable to

those of humic and fulvic acids, it was postulated that some selected surfactants have

solubilization abilities.

2.2.2 Enhanced solubilization

As for a surfactant, a general expression of surfactant solubilization ability towards

an organic compound, in terms of concentrations for monomers and micelles, can be

evaluated by corresponding solute partition coefficients (Kile and Chiou, 1989),

22

*

1wmn mn mc mc

w

SX K X K

S (2.1)

where *

wS is the apparent solute solubility at a total stoichiometric surfactant

concentration of solute; Sw is the intrinsic solute solubility in “pure water”; Xmn is the

concentration of the surfactant as monomers in water (Xmn = X, if X ≤ CMC; Xmn = CMC,

if X > CMC); Xmc is the concentration of the surfactant as micelle in water (Xmc = X-

CMC); Kmn is the partition coefficient of the solute between surfactant monomer and

water, and Kmc term is the solute partition coefficient between micelles and water (Zhu et

al., 2003). The plot of the apparent solute solubility versus the total surfactant

concentration follows a bilinear relationship below and above CMC.

In 1989, Kile et al. (Kile and Chiou, 1989) studied the solubility enhancement of

highly water- insoluble compounds, p,p'-DDT and 1,2,3-trichlorobenzene by Triton-X

series, Brij 35, SDS, CTAB. They found the selected surfactants can produce significant

water solubility improvements even at concentrations below the CMC for extremely

water-insoluble organic compounds, which is speculated to result from the successive

micellization of the heterogeneous monomers. This effect is ascribed to a partition-like

interaction with a nonpolar part of the dilute surfactant, similar to the enhancement effect

caused by dissolved humic material (Chiou et al., 1987). Thus, the calculated Kmn are

comparable with the partition coefficient of solute between dissolved organic matter and

water (Koc), while the calculated Kmc values are nearly in proportion to the nonpolar

group content of the surfactant and are comparable with Kow. Since the effect of the

surfactant structure on the improved solubility is far smaller than that of the

characteristic of organic compound itself, i.e., the Kmc value is comparable to Kow for a

23

certain organic compound by a different surfactant. Thus, the partition coefficient is not

suitable to measure the enhanced solubilization capability of surfactant herein.

Subsequently, the molar solubilization ratio (MSR) and the micelle-water partition

coefficient (Kmic) were proposed to describe the solubilization capability of surfactant.

The molar solubilization ratio, which is the ratio of moles of solute solubilized to the

moles of surfactant present as micelles (Edwards et al., 1991) can be obtained from the

slope of the solubility curve above the critical mice lle concentration. The Kmic, which

represents the distribution of solute between surfactant micelles and the aqueous phase,

is present as (Edwards et al., 1991):

,

,

m i

mic

a i

XK

X (2.2)

where Xm,i is the mole fraction of i-th solute in the micellar phase and Xa,i is the mole

fraction of i-th solute in the micelle-free aqueous phase.

Solubilization efficiency has a significant relationship with their own structure and

physicochemical properties, such as surface tension, CMC and ionization. Generally, the

solubilization power of surfactant with the same hydrophobic chain length follows the

order of nonionic > cationic > anionic, especially the nonionic’s solubilization ability is

positively proportional to the content of the nonpolar portion (Kile and Chiou, 1989). A

study found that as far as the nonionic polyoxyethylene (POE) surfactants, the

solubilization abilities were inversely related to the number of hydrophilic ethoxylate (E)

groups and surfactants with straight-chain tails (C12E10) caused a larger increase in

dissolution than those with branched tails (Triton X-100) (Yeom et al., 1996). The

anionic surfactant, sodium dodecyl polyoxyethylenesulphates (C12H22(OC2H4)xSO4Na,

24

where x=1-10), shows increased solubilization by increasing the polyoxyethylene chain

(Tokiwa, 1968). A study of a homologues series of cationic surfactant (alkylpyridinium

bromide) shows MSR and Km increases by increasing the tail length (Paria and Yeuet,

2006). Very little literature regarding solubilization studies of mixed conventional

surfactant systems is available (Zhao et al., 2005; Paria and Yeuet, 2006; Prak, 2007).

2.3 Surfactant Sorption

2.3.1 Mechanisms of surfactant adsorption

Surfactant adsorption is a process of transferring surfactant molecules from the bulk

solution phase to the surface/interface (Paria and Khilar, 2004), which is probably the

most versatile phenomenon involved in surfactant applications such as detergency,

mineral flotation, corrosion inhibition, soil and water remediation, and oil recovery.

Explaining the mechanisms of sorption of surfactant molecules onto a solid is essential

(Paria and Khilar, 2004): 1) to understanding and predicting the coverage of

surface/interface by the surfactant, which in turn determines the performance of

surfactant in many industrial processes; 2) to envisaging the configuration of surfactant

molecules at the surface/interface to determine the characteristic of modified

surface/interface; 3) and in maximizing their sorptive capabilities in order to design new

families of surface-active agents. The sorption of surfactants onto a solid is controlled by

multiple mechanisms including (Paria and Khilar, 2004):

1) Ion paring: The attraction takes place between surfactant ions and oppositely

charged sites unoccupied by counter ions;

25

2) Ion exchange: The counter ions, adsorbed onto the substrate, are replaced by

similarly charged surfactant ions;

3) Hydrophobic bonding: The adsorption occurs when there is an attraction between

a hydrophobic group of adsorbed molecule and a molecule present in the solution;

4) Polarization of π-electrons: The interaction between the electron rich aromatic

nuclei of surfactant and positive sites on the adsorbent causes adsorption of surfactant;

5) Dispersion forces: Adsorption by London-van der Waals force between adsorbate

and adsorbent increases by increasing the molecular weight of the adsorbate.

2.3.2 Adsorption isotherm

The study of surfactant adsorption equilibrium plays a significant role in determining

the depletion of surfactant due to adsorption (Zhang and Somasundaran, 2006) and

ascertaining the adsorption isotherm for further research. The adsorption isotherm is a

plot of the amount of surfactant adsorbed per unit mass or unit area of the solid by

varying the equilibrium concentration of the adsorbate, which is a measurement of

surface coverage by surfactant molecules at a given condition and hence, determines the

interfacial properties of modified solids (Paria and Khilar, 2004). Therefore, the plotting

and explanation of surfactant adsorption isotherms have been an eagerly pursued

occupation of surface and colloid scientists for many years.

2.3.2.1 Adsorption isotherm of ionic surfactant

26

There have been considerable studies conducted regarding the adsorption of anionic

(Rodríguez-Cruz et al., 2005; Schouten et al., 2009) and cationic (Günister et al., 2006;

Para and Warszynski, 2007) surfactants on a solid–liquid interface. The solid surfaces

are either positively or negatively charged in the aqueous medium by

ionization/dissociation of surface groups or the adsorption of ions from solution onto a

previously uncharged surface. Therefore, the electrica l double layer at the solid–liquid

interface is usually a significant phenomenon for the adsorption of ionic surfactants

(Paria and Khilar, 2004), and the isotherm called the “Somasundaran-Fuerstenau”

isotherm, is plotted on a log- log scale (Rosen, 1989b).

Figure 2.5 schematically depicts the general form of isotherms plotted in this manner,

and the morphology of adsorbed structures associated with each region. In general, the

isotherm can be subdivided into four regions. In Region 1, at low surfactant

concentrations, the adsorption follows Henry’s law and increases linearly with

concentration by electrostatic interaction. In Region 2, a sharp increase in the adsorption

is exhibited due to lateral interaction between adsorbed monomers, resulting in the

formation of surface aggregates. Region 3 shows a slower rate of adsorption than Region

2 due to the neutralization of the surface charge. Finally, Region 4 is a plateau region

above the CMC. Further increases of surfactant concentration do not change the

adsorption density. Sometimes, Region 3 is ambiguously identified in the case of

cationic surfactants adsorption attributed to loose packing of cationic surface aggregates

(Fan et al., 1997).

Considerable studies have been conducted regarding the adsorption of ionic

surfactant and the effect of molecular structure, pH, coexistent, etc. For example, the

27

adsorption of 2,4-metaxylene sulfonate is much lower than those of 3,5-paraxylene and

2,5-paraxylene sulfonates (Sivakumar and Somasundaran, 1994). A study by Fan et al.

(Fan et al., 1997) looked specifically at the pH effect and found that the adsorption of

cationic surfactants is higher on negatively charged surfaces than a positively charged

surface (pH below isoelectric point (IEP)) while the anionic surfactants act oppositely.

The medium and long chain alcohols enhance the adsorption of sulfonate while propanol

decreases its adsorption (Fu et al., 1996). Surfactant adsorption is also affected by the

presence of soluble minerals owing to interactions between surfactant and dissolved

mineral species in bulk, bringing about its precipitation (Somasundaran et al., 1983).

2.3.2.2 Adsorption isotherm of nonionic surfactants

Nonionic surfactants are physically adsorbed rather than electrostatically or

chemisorbed (Paria and Khilar, 2004). Most nonionic surfactants contain polar groups,

which can form hydrogen bonds with the hydroxyl groups on a solid surface (Zhang and

Somasundaran, 2006). The adsorption of nonionic surfactant onto solids is always less

than that of ionic surfactant because the hydrogen bonding is weaker than the

electrostatic interaction. Nonionic surfactants adsorption isotherms are close to those of

ionic surfactants which increase sharply until a plateau is reached. It is clear Region 3

(Figure 2.6) does not exhibit in a nonionic surfactant isotherm due to the absence

electrostatic interactions. Only hydrogen bonding interactions (at low surfactant

concentrations) and lateral associative interaction (at high concentrations) are the

primary cause of nonionic adsorption (Somasundaran and Krishnakumar, 1997).

28

Figure 2.5 Schematic presentation of typical four-region adsorption isotherm (Atkin

et al., 2003)

29

Figure 2.6 The sorption isotherm of TX 100 onto natural soil (Zhou and Zhu, 2005a)

30

2.4 Applications of Surfactants for Soil and Water Remediation

Contaminated soil and water (groundwater or surface water) with organic compounds

poses potential risks to human and ecological health, and the removal of contaminant is

becoming a major concern. Various methods are available to the remediation of soil and

water with organic and inorganic pollutants. The U.S. Environmental Protection Agency

(EPA) has reported some appropriate technologies for the remediation of RCRA-listed

(Resource Conservation and Recovery Act) organic and inorganic hazardous waste

contaminated sites (Paria, 2008), as shown in Figure 2.7. Among the technologies,

Cement-based solidification/stabilization (S/S) technology has been identified by the U.S.

EPA as the most promising technology for RCRA-listed hazardous wastes occupying

24%. Surfactant based technologies account for 2-3% of the superfund sites. However,

S/S technology is not highly useful when addressing organic pollutants unless the soil is

treated with surfactants (U.S. EPA, 2003). Actually, surfactants are used to aid the

remediation process, which in turn, may not only decrease the treatment time but also

reduce the remedy cost of a site. Thus, surfactant based remediation technologies, with

respect to contaminated soil and water, are of increasing importance. As a matter of fact,

among the various available remediation technologies for organic and inorganic

contaminated sites, the surfactant based process is one of the most innovative

technologies (Paria, 2008).

31

Figure 2.7 Technologies selected for source control at superfund remedial action sites

(Fiscal year 1982-2002) (U.S. EPA, 2003; Paria, 2008)

32

2.4.1 Polycyclic aromatic hydrocarbons

Polycyclic aromatic hydrocarbons (also known as “polynuclear aromatic

hydrocarbons”) (PAHs) are organic compounds that consist of fused aromatic rings in a

linear or cluster arrangement, usually containing only carbon (C) and hydrogen (H)

atoms, although nitrogen (N), sulphur (S) and oxygen (O) atoms may readily substitute

in the benzene ring to form heterocyclic aromatic compounds (Dong and Lee, 2009; Gan

et al., 2009). The U.S. EPA has listed 32 priority PAH compounds.

PAHs can enter the environment primarily through human activities such as the

combustion of organic material containing fuels, various industrial processes, biomass

burning, waste incineration, and oils pills (Chang et al., 2006) and have raised severe

environmental concerns, owing to their carcinogenic properties. Although natural

sources from volcanoes and forest fires etc. could contribute to the PAHs burden (Wang

et al., 2010), the anthropogenic sources seemed to be the major source of PAHs. In

general, the anthropogenic sources of PAHs can be classified into petrogenic and

pyrogenic sources. The combustion of crude oil and petroleum products (including

kerosene, gasoline, diesel fuel, lubricating oil and asphalt) generates petrogenic PAHs,

while the combustion of fossil fuels (coal and petroleum) and biomass generate

pyrogenic PAHs (Baek et al., 1991; Dong and Lee, 2009). The following PAHs were

reported as being produced or manufactured in Canada in 1986: naphthalene [more than

1000 tonnes (t) produced, 10 to 100 t imported]; anthracene (0.1 to 1 t produced, 1 to l0 t

imported); fluorene (0.1 to 1 t produced, <1 t imported); and acenaphthene,

33

benzo[a]pyrene, chrysene, and pyrene (<0.1 t imported for each) (Environment Canada,

1992).

Owing to their toxic effects, PAHs have been listed as priority pollutants by many

international environmental protection agencies. The aqueous solubility and vapor

pressure of a PAH reduces as its molecular weight increases, which is especially low

when PAHs consist of three or more rings (Dong and Lee, 2009). Due to these

characteristics, PAHs could contaminate water, soil and sediment over a long period of

time, subsequently entering into food chains, and finally exerting harmful effects upon

humans (Wang et al., 2010). Most of the environmental burden of PAHs is attributed to

their presence in soil (approximately 95%) (Smith et al., 1995). The source, occurrence,

transportation, and fate of PAHs in a natural environment have been studied extensively

(Li et al., 2012). Based on these studies, remediation technology of contaminated soil by

PAHs has attracted strong interest from scientists. Surfactant-enhanced remediation

(SER) is one of the promising technologies for organic contaminants remediation.

2.4.2 Soil

Soil is a natural body which can be defined as loose material composed of weathered

rock, minerals and partly decayed organic matter that covers large portions of land

surface (Yong et al., 1992). Knowledge of the physiochemical characteristics of soil is of

importance in designing the remediation process since soil is the source and end product

of contamination. Soil is composed of 50% by volume mineral particles, 25% water, 20%

34

air and 5% organic matter (Paria, 2008). Thus, soil can be regarded as a mixture of solid,

liquid and gas.

Mineral and organic matter account for major and minor portions of soil which have

been derived from the solid geological deposits and the addition of organic debris (Paria,

2008), which play a vital role in pollutant migration and transformation. The

representative minerals in soil, which can be classified into three groups - sand, silt and

clay – having a common use. The different size limits of the three constituents are given

in Table 2.1. Additionally, soil usually bears a negative charge attributing to the

dissociation of protons from the surfaces and edges of the clay minerals and from acidic

groups in humus (Paria, 2008), which also has a potential influence upon the

transportation of contaminants and surfactant and the selection of remediation

technology.

35

Table 2.1 Size limits of different soil constituents (Paria, 2008)

Fraction Size limits expressed as particle diameters (mm)

Coarse sand 2.0 to 0.2

Fine sand 0.2 to 0.02

Silt 0.02 to 0.002

Clay <0.002

36

2.4.3 Improved surfactant-aided soil washing

In situ flushing is the injection or infiltration of solutions into soil using surface

trenches, horizontal drains or vertical drains (Mulligan et al., 2001). The solubility of a

contaminant is the controlling removing mechanism. Water is always used as the

flushing regent; however, due to its minor solubilization power for organic compounds,

additives are employed to enhance the efficiency. Anionic, cationic, and nonionic

surfactants can be employed as additives to counter the low aqueous solubility of PAHs

and enhance the efficiency of soil washing or flushing using water. The presence of

surfactant micelles is useful in decreasing surface and interfacial tensions and enhancing

the water solubility of organic compounds by partioning them into hydrophobic cores of

surfactant micelles, above CMC (Mulligan et al., 2001).

The mobilized contaminant can then be recovered in an extraction well (Figure 2.8).

Numerous batch and column studies have indicated that surfactants enhance soil washing

via solubility enhancement or desorption. There have been indications that pretreatment

of a soil with surfactant washing to mobilize PAHs enhanced biodegradation of these

contaminants (Mulligan et al., 2001). The desirable surfactants for soil washing include

high solubilization efficiency, low toxicity, low adsorption to soil, low surface tensions,

low CMC, biodegradability and so on. Generally, anionic and nonionic surfactants have

a higher washing efficiency than cationic, owing to the fact they have less absorption

onto negative charged soil. A variety of novel surfactants bearing such properties have

been synthesized or discovered, such as Gemini surfactants, biosurfactants and plant

surfactants. Studies regarding the removal efficiency of organic compounds from the soil

37

matrix to improve surfactant-aided washing technology by these surfactants are still

scarce.

2.4.4 Enhanced surfactant-aided surface and groundwater treatment

Surfactants are known to alter the surface characteristics of solids by conglomerating

on the solid surfaces and interlayer regions creating sorbents that control the sorption of

compounds such as HOCs, inorganic cations, anions and inorganic oxyanions

(Karapanagioti et al., 2005). Therefore, surfactants are used industrially as adhesives to

modify material which is considered to be potential sorbents in wastewater treatment,

appropriate landfill liner and an effective barrier to prevent the down-gradient pollution

of groundwater and aquifer from organic pollutants since the original adsorbents do not

have adequate adsorption capacities or require long adsorption equilibrium time. These

systems demonstrated chemical, biological and long-term stability (Li et al., 1998; Altare

et al., 2007). During the last decade, a wide range of novel modifications have been

exploited to attribute absorbent with new properties by different surfactants (Adak et al.,

2005; Rawajfih and Nsour, 2006; Brum et al., 2010; Neupane and Donahoe, 2012).

However, few studies regarding Gemini surfactant remediation technology have been

undertaken.

38

Figure 2.8 Typical in situ soil flushing installation (Mulligan et al., 2001)

39

Numerous developed surfactant-modified materials were also pilot tested as a

permeable barrier (Juang et al., 2004; Li and Hong, 2009) for groundwater remediation

and demonstrated that the surfactant-aided immobilization represents a valid alternative

to the conventional pump-and-treat approach. An in situ reactive barrier or permeable

wall can be made into a treatment zone with such material or by directly modifying soil

using surfactant and installing it perpendicularly into the flux of the polluted

groundwater to intercept the contamination (Figure 2.9). Physical barriers not only

decrease leakage but also inhibit the spreading of external intruding contaminant plumes

into uncontaminated regions (Vo et al., 2008). Cationic surfactant is always used as an

amendment for soil retardation since the soil possess a net negative structural charge

resulting from isomorphic substitution of cations in the crystal lattice (Zeng et al., 2010).

Additionally, barriers made of organic material are able to stimulate biodegradation

processes if some effective biomaterial is added to the barrier to allow in situ

biodegradation.

The main advantage of such barriers is no external source of energy for pumping

wastewater is required because after their installation, the barrier works passively; no

additional fee for storage, treatment, transportation or the elimination of pollutants

because after the barrier is installed, the surface of the treated zone can be easily reused

and contaminants can be in situ biodegraded; and both management and maintenance

costs are very low or negligible.

40

Figure 2.9 In situ modification of aquifer material to create a pollutant sorptive zone

coupled with the biodegradation of pollutants for groundwater remediation

41

2.5 Literature Review Summary

(1) Surfactant molecules are surface-active agents with a polar head group and a

non-polar chain. Micellar solubilization is one of its outstanding properties, which allows

improved aqueous solubility of organic contaminants otherwise present as a separate

phase. Gemini surfactant, as a new generation of surfactant, has been drawing growing

attention worldwide. They consist of two hydrophobic chains and two hydrophilic

headgroups connected through a relatively short spacer group at or near the headgroups,

which endow them with properties that are superior to those of corresponding

conventional surfactants with an equal chain length such as enhanced surface activity,

much lower CMC, and better wetting properties and mildness to skin. They appropriately

possess various characteristics which cannot only be different in their hydrophobic chain

length but also in their spacer length. The potential applications suggested by these

unique properties led to a renewed interest within the academic, as well as industrial

communities, in Gemini surfactants. Surfactant based solubilization studies have always

been important research subjects because solubilization is the center technology in many

applications such as soil and water remediation, drug delivery, and separation procedures.

On one hand, as a new family of surfactants, Gemini surfactants have not been

extensively popularized and applied due to scarce studies on their solubilization

properties for different organic compounds caused by their different structures. On the

other hand, as new novel surface-active agents in the market, Gemini surfactants are also

expected to be mixed by themselves or with conventional surfactants to enhance their

solubilization capabilities as a cost reduction drive. Along with the demand for industrial

42

and social development, it is necessary and urgent to carry out systematic studies on

Gemini surfactants in order to provide scientific evidence for technological innovation.

(2) Surfactant-enhanced remediation (SER) has been suggested as one of the most

promising techniques to remediate contaminated water and soil due to organic or

inorganic pollutants. It has two aspects to it application depending upon the properties of

used surfactants, i.e., solubilization and adsorption. Surfactant also has two existing form

types, i.e., monomers (below CMC) and micelles (above CMC). Monomers have less or

no solubilization ability but are the main adsorbed species and could form single, bi or

multi layers on solid surfaces which facilitate the retention of pollutants from an aqueous

phase; while, micelles only have solubilization power, which have been extensively used

in the fields of detergent, drug, food and the environment etc. The rational and

professional design of SER technology is not only beneficial to different remediation

requirements in the water phase or soil phase, but also favorable to cost and energy

savings. As a promising surfactant, Gemini surfactant possesses all the properties that a

surface-active agent should have. Besides, it has outstanding properties such as enhanced

surface activity, much lower critical micelle concentration, etc., which offer a sufficient

comparable to conventional surfactant, even biosurfactant. It would be of interest to

know whether Gemini surfactant also performs well in the SER process and does it affect

the partition of organic compounds in a soil-water-Gemini surfactant system; How to

choose the right Gemini surfactant for different SER remediation technologies, in terms

of efficiency and cost-saving is the question. Future and more thorough studies into this

research are still rare and should be conducted immediately.

43

CHAPTER 3

EFFICIENCY OF GEMINI SURFACTANT ON THE

SOLUBILIZATION OF PAHS IN COMPARISON WITH

CONVENTIONAL SURFACTANTS

3.1 Introduction

The contamination of soils and water by polycyclic aromatic hydrocarbons (PAHs)

which are bio-accumulative, resistant to degradation, and possess toxic properties has

become a widespread environmental concern (Barra et al., 2005). Various ways such as

chemical (Ferrarese et al., 2008), physical (Dhenain et al., 2009), biological (Hansen et

al., 2004), and their combined methods (Zheng et al., 2007) have been made to develop

efficient technologies with which to remedy PAHs contaminated soils and groundwater.

The main principle of such remedial strategies is their significant capabilities of

desorbing organic compounds from a contaminated subsurface. Among them, in situ

surfactant-enhanced remediation (SER) has been developed to be a more powerful

remediation method (Paria, 2008) due to the outstanding solubility enhancements of

organic compounds by partitioning them into the hydrophobic cores of surfactant

micelles. Numerous peer-reviewed studies have shown that the enhanced solubility of a

contaminant, in the presence of surfactants, occurs above their CMC (Rouse et al., 2008).

Notably, surfactants can promote the microbial remediation of PAHs in soil and water by

affecting the accessibility of PAHs to microorganisms (Tiehm, 1994).

44

However, most previous studies related to the partitioning of PAHs in micellar

dispersion reported the use of conventional surfactants having single monomers with a

single hydrophobic tail and a single hydrophilic head group. A more recent and

innovative surfactant to aid the removal of contaminants from so il is Gemini surfactant.

It is a family of surfactant molecules consisting of two hydrophobic chains and two

hydrophilic headgroups connected by a relatively short (rigid or flexible) spacer group

(Hait and Moulik, 2002). Gemini surfactants have attracted considerable attention since

they have lower CMC values, a much greater efficiency in reducing surface tension and

unusual aggregation morphologies (Misra et al., 2010). They have been generating a

growing interest owing to their superior performance in applications.

What is noteworthy is that the solubilization capabilities of surfactants are selective.

The amount and structure of surfactants, even the type of contaminants could also be

expected to influence the efficient solubility of pollutants in surface and ground water. It

is meaningful to explore new surfactant systems and develop more practical soil and

water remediation methods. However, few reports of solubilization with respect to

organic compounds by single or mixed Gemini surfactant systems are available (Kabir-

ud-Din et al., 2009a) and most present research activities regarding Gemini surfactant are

mainly focused on their synthesis and characterizations (Li et al., 2010).

This work has investigated the solubilization of phenanthrene in selected single and

equimolar Gemini/nonionic and Gemini/cationic mixed surfactant systems. The theories

of Clint (Clint, 1975), Rubingh (Rubingh, 1979), Edwards (Edwards et al., 1991) and

Treiner (Treiner et al., 1990) were used to analyze and compare the experimental results

in order to understand the synergism and antagonism in solubilization capabilities of

45

multi component surfactant systems. The experimental results of this study will be useful

in finding and predicting the solubilization properties of mixed Gemini/conventional

surfactant systems.

3.2 Materials and Methods

3.2.1 Materials

The nonionic surfactants (Brij35 and TX-100) and cationic surfactants (CPC and

DTAB) with a purity of >99% were all purchased from Sigma Aldrich. Gemini

surfactant N1-dodecyl-N1,N1,N2,N2-tetramethyl-N2-octylethane-1,2-diaminium

bromide (C12-2-12) was obtained from Chengdo Organic Chemicals Co., LTD., Chinese

Academy of Science, with a purity of 95%. The molecular structures and properties of

selected surfactants are given in Table 3.1. Phenanthrene, the three- ring PAH, was

selected as the representative contaminant in this study and purchased from Sigma

Aldrich. The purity of the compound is greater than 98%. The water solubility of

phenanthrene is 5.61×10-6 M at 25 ◦C, and the octanol-water coefficients (Log Kow) is

4.46. The stock solutions of surfactant were prepared by dissolving the weighed amount

of the relevant surfactants in deionized water. Different concentration levels of surfactant

solutions were made by diluting the stock solution in the appropriate amount of

deionized water.

46

Tab

le 3

.1 T

he f

orm

ula

and

phys

icoche

mic

al pro

per

ties

of th

e se

lect

ed s

urf

acta

nts

Sur

fact

ant

Str

uctu

re

MW

(g M

-1)

CM

Cex

pa (m

M)

C12

-2-1

2

C12H

25N

+(C

H3) 2

(C

H2) 2

-N+(C

H3) 2

C

12H

25·2

Br-

614.6

7

0.8

(0.8

1)

(Sun

et

al.,

20

07)

TX

-100

C

8H

17C

6H

4(O

CH

2C

H2) 9

.5O

H

628

0.2

(0.2

1)

(Zho

u a

nd Z

hu, 2

004

)

Bri

j35

C

12H

25(O

CH

2C

H2) 2

3O

H

1200

0.0

65 (

0.0

61)

(Pra

k a

nd P

ritc

har

d, 2

002

)

CP

C

ClC

H2(C

H3) 2

CO

Cl

155.0

2

1.0

2 (

0.9

9)

(Mai

ti e

t al

., 2

010

)

DT

AB

C

12H

25(C

H3) 3

NB

r 308.4

15.0

(14.0

) (B

ahri

et

al.,

20

06

)

a E

rro

r lim

its

of

CM

Cex

ps

are

± 4

%

47

3.2.2 Methods

3.2.2.1 Surface tension measurement

Surface tension measurement was performed by a Model 70545 surface tensiometer,

manufactured by CSC Scientific Company, INC. This instrument operates on the

DuNouy principle, in which a platinum ring is suspended from a torsion balance, and the

force (in mN m-1) necessary to pull the ring free from the surface film is measured. A

concentrated stock solution of the single, as well as mixed surfactants was added to a

known amount of water and allowed to equilibrate for approximately 3 hours. Then, the

surface tension values were measured until constant surface tension values indicated that

equilibrium had been reached. The accuracy of the measurement was within ±0.1 mN m-

1. The values of CMC were determined as the concentration at sharp breaks in the plot of

surface tension (γ) versus the logarithm values of surfactant solutions (Log Ct) over a

wide concentration range, as shown in Figure 3.1.

3.2.2.2 Solubilization measurement

Batch experiments for solubilization of phenanthrene in nine different single and

equimolar mixed surfactant systems were subsequently carried out. Each contaminant-

surfactant system involved seven to twelve batch experiments with surfactant solutions

having a range of concentrations above the CMC. For each batch test, an excess amount

of phenanthrene was separately added to each vial containing a series of 10 ml single or

48

equimolar mixed surfactant solutions to ensure maximum solubility. The sample vials

were sealed with a screw cap fitted with a Teflon lined septum to prevent any

volatilization loss of phenanthrene from surfactant. Three duplicate samples were

prepared for each surfactant concentration. These samples were then equilibrated for a

period of 48 hours, on a reciprocating shaker, maintained at a temperature of 25±0.5 ◦C.

(Previous experimental results showed 48 hours to be sufficient in reaching

solubilization and partitioning equilibrium under mixing.) After this, the samples were

subsequently centrifuged with a Model Durafuge100 (Thermo Electron Corporation) at

5000 rpm for 30 minutes to separate the undissolved phenanthrene. An appropriate

aliquot of the supernatant was then carefully withdrawn with a volumetric pipette and

diluted to 10 ml in flasks with 1 ml methanol and the remainder with the corresponding

surfactant-water solution. The concentration of dissolved phenanthrene was analyzed

spectrophoto-metrically with a Unico spectrophotometer (Model UV–2110) at a 254 nm

wavelength. Because surfactants exhibit broad UV adsorption, the surfactant

concentration was kept the same in both the reference and the measurement cells to

eliminate the effect of surfactant on determining solubility. All experiments were

performed at room temperature (22-26 ◦C). The concentration of dissolved phenanthrene

was taken as the mean of the triplicate readings, and the typical error in the measurement

was less than 5%.

49

-1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

32

36

40

44

/

mN

m-1

Log (Ct /mM)

C12-2-12

CPC

C12-2-12/CPC

Figure 3.1 (a) Plots of surface tension (γ) vs. the total surfactant concentration (Ct) of

single and equimolar binary C12-2-12 and CPC

50

-1.0 -0.5 0.0 0.5 1.0 1.5

30

35

40

45

50

55

/

mN

m-1

Log ([Ct] /mM)

C12-2-12

DTAB

C12-2-12/DTAB

Figure 3.1 (b) Plots of surface tension (γ) vs. the total surfactant concentration (Ct) of

single and equimolar binary C12-2-12 and DTAB

51

-3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5

30

35

40

45

50

55

60

/m

N m

-1

Log (Ct /mM)

C12-2-12

Brij 35

C12-2-12/Brij 35

Figure 3.1 (c) Plots of surface tension (γ) vs. the total surfactant concentration (Ct) of

single and equimolar binary C12-2-12 and Brij 35

52

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.525

30

35

40

45

50

55

60

/m

N m

-1

Log (Ct /mM)

C12-2-12

TX-100

C12-2-12/TX-100

Figure 3.1 (d) Plots of surface tension (γ) vs. the total surfactant concentration (Ct) of

single and equimolar binary C12-2-12 and TX-100

53

3.3 Results and Discussion

3.3.1 Surface properties of the individual and mixed surfactants

This study focused mainly upon the solubilization of phenanthrene in the presence of

single surfactants (C12-2-12 and conventional surfactants) and equimolar mixtures of C12-2-

12 with conventional surfactants. Solubilization is closely associated with the solution

properties of surfactant micelles, particularly the behaviors of the mixed surfactant

systems which are complex in nature and mostly follow a nonideal path. Hence, to better

understand solubilization abilities, the micellar properties of selected single surfactants

and their equimolar combinations were studied.

The CMC values of the studied single, as well as their equimolar binary mixed

surfactant systems, are presented in Tables 3.1 and 3.2. The values for single surfactants

are in good agreement with the literature values. The mixed CMCs of four equimolar

binary surfactant systems are intermediate between the respective individual surfactants

studied in this combination. To know whether the mixed solution follows ideal or

nonideal behavior, the ideal critical micelle concentration values for mixed surfactant

systems, CMCmi, were calculated using the Clint’s equation (Clint, 1975).

1 2

1 2

1

miCMC CMC CMC

(3.1)

where CMC1, CMC2, α1 and α2 are the critical micelle concentrations and the mole

fraction of components 1 and 2 in mixed surfactant solutions. Table 3.2 clearly indicates

that all the experimental CMCexp values were less than CMCmi, as predicted by the above

54

equation, demonstrating that the formation of mixed micelles exhibit a negative

deviation with respect to an ideal mixture.

In light of the regular solution theory, the deviation of experimental CMCexp values

for mixed surfactant systems from CMCmi can be measured by the interaction parameter

β which can be evaluated according to Rubingh’s equation (Rubingh, 1979):

12 1 12 2

1 1 2 2

2 2

1 2

ln ln

1 1

CMC CMC

CMC X CMC X

X X

（）（）

(3.2)

where X1 and X2 are the micellar mole fraction of surfactants 1 and 2 in the mixed

micelles; CMC1, CMC2 and CMC12 are critical micelle concentrations for surfactant 1,

surfactant 2 and their mixture respectively; α1 and α2 are their corresponding bulk mole

fractions. The micellar mole fraction Xi can be calculated from the equation below for a

nonideal binary mixture of surfactants by solving iteratively (Rubingh, 1979).

2 1 121

1

2 1 121

1 2

X ln

11

1- ln1

CMC

X

CMCX

X CMC

（）（）（

（）

(3.3)

A negative value of β indicates negative deviation of CMCexp’s from CMCmi

demonstrating a reduction in free energy of micellization over that predicted by the ideal

solution theory (Rao and Paria, 2009). A positive value signifies antagonism between

components of the surfactant combination.

55

Tab

le 3

.2 E

xperi

ment

al

crit

ical

mic

elle

conc

ent

ratio

n (C

MC

exp),

idea

l cr

itic

al

mic

elle

conc

entr

atio

n (C

MC

mi),

inte

ract

ion

par

am

ete

r (β

), m

icellar

mo

le f

ract

ion

(i

X)

and

act

ivit

y c

oeff

icie

nts

(fi) v

alu

es o

f eq

uim

ola

r su

rfac

tant

mix

ture

s at

25

◦C

a

a E

rro

r lim

its

of para

mete

rs C

MC

exp, β

, i

Xan

d f

i ar

e ±

4%

.

Sur

fact

ant

syst

em

C

MC

exp

(m

M)

CM

Cm

i (m

M)

β

X1/X

2

f 1/f

2

C12

-2-1

2/T

X-1

00

0.3

0

0.3

252

-0

.3792

0.2

342/0

.7658

0.8

006/0

.9794

C12

-2-1

2/B

rij3

5

0.1

0

0.1

202

-0

.9425

0.1

384/0

.8615

0.3

897/0

.9821

C12

-2-1

2/C

PC

0.8

3

0.8

979

-0

.3189

0.5

529/0

.4471

0.9

382/0

.9071

C12

-2-1

2/D

TA

B

1.5

0

1.5

189

-0

.2241

0.9

383/0

.6165

0.9

991/0

.9209

56

The activity coefficient, f i, of i-th surfactant within the mixed micelles derived from

Rubingh equations is expressed by the interaction parameter of β and 1X as follows

(Rubingh, 1979):

1

1

2

1

2

2

exp (1 )

exp ( )

f X

f X

(3.4)

It is clearly indicated from Table 3.2 that the values of β, studied herein, are all

negative, thus, presenting a synergistic effect for all mixed surfactant systems. The larger

negative value of β denotes the greater negative deviation of CMCexp’s from CMCmi. The

descending deviation order exhibited through β values follows C12-2-12/Brij 35 > C12-2-

12/TX-100 > C12-2-12/CPC > C12-2-12/DTAB. The stronger synergism effect of C12-2-12 and

nonionic conventional surfactants is due to (Zhou and Rosen, 2003) the momentous

electrostatic self-repulsion between cationic head groups is replaced by the attractive

ion-dipole interaction between hydrophilic groups of cationic and nonionic micelles. It

also may be a consequence of the fact that the electrostatic self-repulsion of cationics and

weak steric self-repulsion of nonionics, before mixing, are weakened by dilution effects.

Whereas, minor negative values of β, and minute deviations of 1X and fi values o f

C12-2-12 and cationic conventional mixed surfactant systems signify less attractive

interactions of individual components due to the combined effect of hydrophobic

interaction among their tails and intensive electrostatic repulsion among their head

groups. It is worth noting that the equimolar combinations of Gemini and conventional

surfactants with the same chain length have strong interaction. For example, the

synergism effect of C12-2-12/Brij35 is stronger than that of C12-2-12/TX-100, while the

57

antagonism effect of C12-2-12/DTAB is stronger than that of C12-2-12/CPC. Thus, the

strength of the interaction of mixtures containing two surfactants not only depends on the

variation of their CMC values, but also relies on the relevant properties of their structure.

3.3.2 Solubilization by surfactants

Before investigating the potent solubilization of surfactant mixtures, single

surfactants were studied first to obtain an idea about the efficiency of given Gemini

surfactant in comparison with conventional surfactants. As shown in the plot of the

solubility of a poorly soluble phenanthrene, as a function of the concentration of

surfactant (Figure 3.2), it is obvious that solubility increases linearly with the increasing

surfactant concentrations above CMC, indicating that solubilization is related to

micellization. However, the reduced CMC value does not exactly represent the increased

solubilization ability. In other words, the CMC values fail to measure the solubilization

capabilities of surfactants efficaciously. Thus, water solubility enhancement of

phenanthrene by selected single and equimolar binary surfactant systems requires further

evaluation and comparison.

3.3.2.1 Solubilization by Single Surfactants

The molar solubilization MSR is characterized as the number of moles of compound

solubilized by one mole of micellized surfactant (Edwards et al., 1991). It denotes the

effectiveness of a particular surfactant in solubilizing a given solute and can be

expressed as follows (Edwards et al., 1991):

http://www.ualberta.ca/~csps/JPPS8(2)/C.Rangel-Yagui/solubilization.htm#Figure 4

58

-

-

ac cmc

ac

S SMSR

C CMC (3.5)

where Sac is the total apparent solubility of phenanthrene in a given surfactant solution at

a specified surfactant concentration Cac (the surfactant concentration above CMC at

which Sac is evaluated) and Scmc is the apparent solubility of phenanthrene at CMC.

In the presence of excess phenanthrene, the MSR values of both single and mixed

surfactants could be obtained from the slope of the linear fitted line in which the

concentration of phenanthrene is plotted against a surfactant concentration above the

CMC (surfactant concentration in mM vs. phenanthrene concentration in mM), as given

in Figure 3.2.

To further characterize the effectiveness of solubilization, the partition coefficient Km

(Edwards et al., 1991) is defined as the distribution of the mole fraction of phenanthrene

between surfactant micelles and the aqueous phase may be calculated from an

experimental measurement by using the following formula (Edwards et al., 1991):

mm

a

XK

X (3.6)

where Xm and Xa are the mole fractions of solute in micelles and the aqueous phase,

respectively. The value of Xm can be calculated in terms of the MSR as:

1m

MSRX

MSR

(3.7)

The mole fraction of solute in the aqueous phase is approximated for dilute solutions

by:

a cmc WX S V (3.8)

59

where Scmc is the total apparent solubility of solute at the CMC, and Vw is the molar

volume of water (1.805×10-2 L M-1 at 25 ◦C).

Thus, an expression for Km can be re-arranged by substituting MSR to produce:

55.4

(1 )m

cmc

MSRK

S MSR

(3.9)

The solubilization capacities of the selected single surfactants for phenanthrene

quantified by MSR and Log Km follow the order of C12-2-12 > Brij35 > CPC > TX-100 >

DTAB, as shown in Table 3.3. The MSR values of Brij 35 and TX-100 are in agreement

with the literature values (Zhu and Feng, 2003). All the chosen surfactants studied

followed the CMC rule, the lower the CMC values, the higher the solubilization

efficiency of the surfactants (except C12-2-12 and CPC). It is also in accord with the report

that the solubilizing power for organic solutes by an inner nonpolar core of micelle is to

be nonionic > cationic surfactants due to their low CMC values (Saito, 1967).

However, the higher solubilization power of C12-2-12 surfactant, as in the present case,

may be attributed to the two head group with a positive charge which has a wider spread

interaction with π-electrons of phenanthrene than that of conventional cationic micelle

which has more hydrophobic content than conventional nonionic micelle, thus, assisting

in excessive micellar core solubilization. In addition, the higher solubilization ability of

CPC may be due to the aromatic ring present in its head group having delocalized

positive charge. This may strengthen the aromatic hydrophobic solutes in solubilization

owing to a wide spread interaction of π-electrons of arenes with a positive charge. The

aforementioned results established that the lower CMC is not the only criteria for higher

solubilization power towards PAHs.

60

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.00

0.05

0.10

0.15

0.20

0.25

C12-2-12

TX-100

C12-2-12/TX-100

Ph

enan

thre

ne

/mM

Surfactant concentration /mM

Figure 3.2 (a) The solubilization of phenanthrene in single C12-2-12, TX-100 and

equimolar mixed C12-2-12/TX-100 surfactants

61

0.0 0.5 1.0 1.5 2.0 2.5 3.0

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

C12-2-12

Brij35

C12-2-12/Brij35

Ph

en

an

thre

ne /

mM


Figure 3.2 (b) The solubilization of phenanthrene in single C12-2-12, Brij 35 and equimolar

mixed C12-2-12/Brij 35 surfactants

62

0.5 1.0 1.5 2.0 2.5 3.0

0.00

0.05

0.10

0.15

0.20

0.25

C12-2-12

CPC

C12-2-12/CPC

Ph

enan

thre

ne

/mM


Figure 3.2 (c) The solubilization of phenanthrene in single C12-2-12, CPC and equimolar

mixed C12-2-12/CPC surfactants

63

0 5 10 15 20 25

0.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

C12-2-12

DTAB

C12-2-12/DTAB

Ph

enan

thre

ne

/mM


Figure 3.2 (d) The solubilization of phenanthrene in single C12-2-12, DTAB and equimolar

mixed C12-2-12/DTAB surfactants

64

3.3.2.2 Solubilization by equimolar binary mixed surfactant systems

Water solubility enhancements of phenanthrene by equimolar binary surfactant

systems of C12-2-12/Brij35, C12-2-12/TX-100, C12-2-12/CPC and C12-2-12/DTAB were

measured and compared with those of individual surfactants. The apparent solubility of

phenanthrene increases linearly with equimolar binary mixed surfactant concentrations

(Figure 3.2), the same as single surfactants, thus, demonstrating the formation of mixed

micelles and their potential capacity to enhance the solubility of phenanthrene in water.

In mixed surfactant systems, the following order of MSR and Log Km values is

observed: C12-2-12/Brij35 > C12-2-12/TX-100 > C12-2-12/CPC > C12-2-12/DTAB, which

parallels that of the negative deviation of CMCexp from an ideal mixture. In tune with the

previous report by Mehta (Mehta et al., 2009), MSR and Log Km values are found to be

higher for the equimolar mixture of cationic-nonionic than for cationic-cationic mixed

surfactant solutions. Nonionic micelle processes high micellar core solubilization

characteristic intercalates into the micelle-water interface, creating superior

solubilization power towards PAHs. In addition, the solubilization of phenanthrene by

mixed C12-2-12/nonionic surfactant solutions is also higher than those in single nonionic

surfactant solutions. For example, synergistic solubilization of phenanthrene in C12-2-

12/Brij35 and C12-2-12/TX-100 are 1.51 and 1.45 times that of a single Brij35 and TX-100.

However, the weak effect of mixed micelles leads to a lower solubility even though

the mixed system has a lower CMC value than their corresponding single component.

For instance, a decrease in MSR and Km are coupled with a decrease in CMC with respect

to conventional cationic surfactant for C12-2-12/cationic surfactant systems. In accordance

65

with earlier findings (Zhou and Zhu, 2004), the mixing effect of surfactants for organic

compounds can be attributed to the relationship of the CMCs and Km values. Thus, when

the solubilizing power towards phenanthrene was measured by mixed surfactant systems,

the “complement-counteract” effect of two important factors Km and CMC should be

taken into account, simultaneously. Regarding the solubilization of phenanthrene by

selected mixed surfactant systems studied herein, Km is apparently more dominant than

CMC.

In the interest of ascertaining the mixing effect of mixed surfactants on solubilization

for phenanthrene and observing the nature of deviation, the deviation ratio (R) between

the MSRexp and the MSRideal can be determined by the following equation (Zhou and Zhu,

2004):

exp

ideal

MSRR

MSR (3.10)

MSRideal is the MSR for organic compounds in a mixed surfactant system at the ideal

mixed state and can be estimated using the MSR of single surfactant solutions, based on

the ideal mixing rule (Zhou and Zhu, 2004):

MSRideal = MSR1X1 + MSR2X2 (3.11)

where X1, X2, MSR1 and MSR2 are the mole fraction and the molar solubilization ratio for

solute of components 1 and 2 in mixed surfactant solutions, respectively. The data re

parameter R, from Table 3.3, obviously indicate that the MSRexps have a positive

deviation from the ideal mixture for the C12-2-12/nonionic surfactant systems, meaning

they have a positive mixing effect upon solubilization of phenanthrene. However, the

opposite results are found in C12-2-12/cationic surfactant systems.

66

Another parameter, Km12, the partition coefficient of a neutral organic solute between

micelles and the aqueous phase in a mixed surfactant has been put forward by Treiner et

al. (Treiner et al., 1990) to gain a better understanding of the mixing effect of mixed

surfactant systems on solubilization of solutes. This partition coefficient is expressed

based upon the regular solution approximation (RSA) as follows (Treiner et al., 1990):

12 1 1 1 2 1 1ln ln (1 )ln (1 )m m m m

m m mK X K X K BX X (3.12)

where Km1, Km2 are the micelle-water partition coefficients of individual surfactant

solutes constituting the mixed micelles and representing the micellar mole fraction of a

surfactant. B has the same origin as β in Equation (3.2) and is an empirical parameter

involving both the surfactant-surfactant and surfactant-solute interactions. B=0 indicates

there would be no mixing effect on partitioning of a solute between the aqueous and

micellar phase (Zhou and Zhu, 2004); B > 0 (< 0) implies that Km12 in the mixed

surfactant system is larger (smaller) than predicted by the ideal mixing rule (Dar et al.,

2007).

As presented in Table 3.3, the B values are found to be positive for all C12-2-

12/nonionic equimolar mixed surfactants and negative for all C12-2-12/cationic equimolar

mixed surfactants, in accordance with the deviation of MSRs from an ideal mixture. In

particular, the C12-2-12/Brij35 and C12-2-12/DTAB have the largest and the smallest B

values, respectively. The B values of the selected mixed systems are also consistent with

the variation tendencies of β values, indicating the attractive interactions between

individual surfactants in mixed micellar systems through large negative β values lead to

greater solubilization efficiency towards phenanthrene, denoted by impressive positive B

67

values. In viewing the present study, the B value of a mixed surfactant system, as

estimated from Equation (3.12) can be regarded as a useful and pertinent parameter in

explaining experimental observations such as the R and β values. As proven by the above

reference parameters, the higher solubilization power towards phenanthrene by C12-2-

12/nonionic equimolar surfactant systems may be attributed to the Gemini micelles with a

positive charge intercalate into the nonionic micelles generating a larger dimension of

mixed micelles, in addition to the electrostatic interaction of π-electrons of phenanthrene

with mixed micelles at the micelle-water interface (Saito, 1967). In C12-2-12/cationic

surfactant systems, low solubilizing power can therefore, be attributed to limited

solubilization at the micelle-water interface and micellar core. Besides, learning from the

solubility of single surfactant, the efficiency of nonionic micellar core solubilization is

larger than the extent of phenanthrene adsorbed at the cationic micelle-water interface.

From this point of view, in the process of phenanthrene solubilized by C12-2-12/cationic

micelle, the decreased micellar core solubilization offse ts the increased interface

absorption capability bringing about the reduced solubilization towards phenanthrene

with respect to C12-2-12/nonionic surfactant systems.

In the present study, the solubilization of phenanthrene by all selected mixed

micelles also can be deemed as a normal partitioning of phenanthrene between the

micelles and aqueous phases. From the thermodynamic point of view, all the

solubilization behavior of both single and mixed micelles can consequently be measured

by the standard free energy of solubilization, as given by the following expression

(Rangel-Yagui et al., 2005):

lns mG RT K (3.13)

68

Tab

le 3

.3 M

SR

, L

og

Km

,0 s

G

, R

and

B o

f p

hena

nthre

ne i

n var

ious

single

and

eq

uim

ola

r m

ixed

mic

ellar

sys

tem

s at

25

◦C

a

Sur

fact

ant sy

stem

M

SR

L

og

Km

0 sG

(

kJ

M-1

) R

B

C12

-2-1

2

0.1

097

5.7

75

-3

2.9

6

Bri

j35

0.0

729

5.6

15

-3

2.0

5

TX

100

0.0

549

5.5

32

-3

1.5

7

CP

C

0.0

616

5.5

81

-3

1.8

5

DT

AB

0.0

362

4.8

83

-2

7.8

7

C12

-2-1

2/B

rij3

5

0.1

073

5.7

39

-3

2.7

6 1.1

54

1.7

99

C12

-2-1

2/T

X-1

00

0.0

872

5.6

39

-3

2.1

9 1.0

28

0.7

617

C12

-2-1

2/C

PC

0.0

605

5.5

70

-3

1.8

0 0.7

063

-1

.058

C12

-2-1

2/D

TA

B

0.0

202

4.8

15

-2

7.4

8 0.2

832

-1

7.5

9

a E

rro

r lim

its

in t

he m

easu

rem

ent

of

MS

R,

Lo

g K

m,

0 sG

a

re ±

3.1

%, ±

2.8

% a

nd ±

2.4

%, re

spec

tively

.

69

The standard free energy 0

sG of solubilization, presented in Table 3.3, was negative,

indicating spontaneous solubilization for all systems. The largest negative value of the

C12-2-12/Brij35 micellar system indicates that the phenanthrene solubilizing behavior is

energetically more complementary in the cationic-nonionic system attributed to intense

micellar core solubilization and additional micelle-water interface adsorption.

3.4 Summary

This present study investigated the role of the solution composition and category of

mixed surfactants in the solubilization of phenanthrene via the CMC and solubilizing

approach. Although the studied Gemini surfactant has a little higher CMC value, the

excellent solubilization towards phenanthrene was exhibited due to its more micellar

core solubilization and intense attraction with phenanthrene at the micelle-water

interface. In addition, within the four equimolar mixed surfactant systems, the

solubilizing capacities towards phenanthrene by C12-2-12/nonionic surfactants are higher

than their corresponding conventional surfactant which is in contradiction to the

increased CMC values of mixed micelles from single nonionic micelle. According to the

aforementioned information, the conclusion reached is that CMC and Km are two

important factors in measuring the potential solubilization capacity of surfactant. In this

study, Km should be more dominant than CMC for Gemini surfactant and its combination

with conventional surfactant. The estimated parameters, according to the Rubingh

equations, are well correlated with the solubilization power of equimolar binary

surfactants in terms of MSR and Km: C12-2-12/Brij35 > C12-2-12/TX-100 > C12-2-12/CPC >

70

C12-2-12/DTAB. The superior solubilization of Gemini/nonionic surfactant for

phenanthrene to Gemini/cationic surfactant may be due to the joint effect of micellar

core solubilization and micelle-water interface attraction for organic compounds.

71

CHAPTER 4

INVESTIGATION ON THE SOLUBILIZATION OF PAHS IN THE

PRESENCE OF SINGLE AND BINARY GEMINI SURFACTANT

SYSTEMS WITH VARIOUS HYDROPOBIC CHAIN LENGTHS

4.1 Introduction

Contamination of soil, sludge and water by persistent organic pollutants (POPs) has

been recognized as a major widespread environmental waste concern. A small amount of

released organic compounds can easily contaminate large volumes of soil and water,

many times exceeding the safe levels defined by law (Orecchio, 2010). The situation is

far more serious when the pollutants are toxic or even carcinogenic. What is worse, they

can strongly adsorb to soil or sediments making them persist in the soil for extremely

long periods of time. PAHs are made of two or more fused benzene rings in linear,

angular, or clustered arrangements, each with a different toxicity. For example,

naphthalene, a PAH with a low molecular mass is known to be the most acutely toxic,

whereas, many PAHs with high molecular mass are considered as a potential

carcinogenic such as benzo(a)pyrene (Atkins et al., 2010). Owing to their exceptionally

high adsorption capacity and low water solubility, their removal from the soil-water

system is often dependent upon desorption of the contaminant from the soil surface and

its subsequent incorporation into the bulk aqueous phase (Mohamed and Mahfoodh,

2006).

72

Based on this, solubilization becomes very important industrially, as well as quite

interesting, chemically. Surfactants are not only capable of incorporating insoluble

contaminants into the bulk aqueous phase via their high-efficiency solubilization abilities,

but are also potential candidates to improve microbial remediation for PAHs in soil by

affecting the accessibility of PAHs to microorganisms (Edwards et al., 1994).

Considerable studies have been devoted to the solubilization of PAHs by conventional

surfactants, which are made up of one hydrophilic head group and one hydrophobic

chain (Ahn et al., 2010; Chatterjee et al., 2010). Particularly now, that mixed surfactant

systems for the remediation of organic contaminants are of great interest in scientific and

industrial applications due to their efficient solubilization, suspension, dispersion and

transportation capabilities (Dar et al., 2007). However, reported investigations into the

solubilization performances of mixed surfactants are mostly on a laboratory scale and

limited as far as single surfactants.

Gemini surfactant have lower CMC values and a much greater efficiency in reducing

surface tension and are better at solubilizing when compared to corresponding

conventional surfactants with equal chain lengths (Misra et al., 2010). Kabir-ud-Din et al.

(Kabir-ud-Din et al., 2009a) have reported the potential capacity of cationic Gemini

surfactant to enhance the solubility of anthracene and pyrene in water. It is also observed

(Wei et al., 2011a) that the chosen cationic Gemini surfactant has a better solubilization

capacity for phenanthrene than conventional cationic and nonionic surfactants.

Gemini surfactants have been generating a growing interest owing to their superior

performance in solubilization. In this study, a series of n–s–m type dissymmetric

quaternary ammonium Gemini surfactants, having a similar spacer length but dissimilar

73

hydrophobic chain lengths, were selected. It is reported (Liu and Ding, 2004) that

quaternary ammonium surfactant have been widely used in industrial applications and

pharmaceutical/cosmetic/household product preparations, owing to their excellent cell

membrane penetration properties, low toxicity, good environmental stability, non-

irritation, low corrosivity and extended residence time and biological activity. Until very

recently, only sporadic attempts have so far been made to explain the solubilization of

organic compounds by Gemini surfactants, not to mention mixed Gemini surfactant

systems.

In the study reported here, an investigation is proposed regarding the solubilization

aspects of three PAH model compounds in a series of single and equimolar mixed

Gemini surfactant systems which has not been reported to the best of our knowledge.

Based on the aqueous solubility data, parameters associated with the solubilization and

partition process were correlated through several theories to discover synergism in

solubilization capabilities of an equimolar binary component Gemini surfactant system.

The experimental results of this study will be useful in assessing and predicting the

solubilization properties of mixed Gemini surfactant systems and meaningful in

providing statistical proof with which to explore new surfactant systems for practical soil

and water remediation.

74


4.2.1 Materials

Four Gemini surfactants were obtained from Chengdo Organic Chemicals Co. Ltd.,

Chinese Academy of Science, with a purity of 95%. The molecular structures and

properties of the selected surfactants are given in Table 4.1. Naphthalene, phenanthrene

and pyrene, the two, three and four-ring PAHs, were selected as solutes in this study and

purchased from Sigma-Aldrich. The purities were greater than 98%. Their formulas and

properties are listed in Tables 4.1 and 4.2. The stock solutions of the surfactants were

prepared by dissolving the weighed amounts of the relevant surfactants in deionized

water. Different concentration levels of surfactant solutions were made by diluting the

stock solution in the appropriate amount of deionized water.

4.2.2 Methods

4.2.2.1 Solubilization experiment

The solubilization experiment of the PAHs via a series of single aqueous surfactants

and equimolar mixtures C12-2-12/C12-2-4, C12-2-12/C12-2-8, C12-2-12/C12-2-16, composed of

symmetric Gemini surfactant C12-2-12, with different dissymmetric Gemini surfactants

(C12-2-4, C12-2-8 and C12-2-16), were subsequently performed.

75

Table 4.1 The formula, molecular properties and experimental critical micelle

concentrations (CMCexp) of the selected Gemini surfactants

Surfactant Molecular structure Molecular Weight

(g M-1)

CMCexp

(mM)

C2-12-4

501.43 2.6

C2-12-8

558.67 1.5

C12-2-12

614.79 0.87

C12-2-16

672.03 0.2

a Error limits of CMCexpS are ± 4%.

76

Table 4.2 The formula and physicochemical properties of the selected PAHsa

Compound Molecular structure

Molecular Weight

(g M-1)

Water solubility

(M)

Log Kow

Naphthalene 128.2 2.5×10-4 3.36

Phenanthrene 178.2 6.6×10-6 4.57

Pyrene 202.3 6.7×10-7 5.18

a Data for PAHs reported by Verchueren (Verschueren, 1996).

77

Each contaminant-surfactant system involved seven to twelve batch experiments

with surfactant solutions having a range of concentrations below and above the CMC. As

for each batch test, excess amounts of PAHs were added separately to 10 ml of the

different single or mixed micellar solutions in vials to ensure maximum solubility. The

sample vials were sealed with a Teflon-coated cap to prevent any volatilization loss of

PAHs from surfactant. The samples were then equilibrated for a period of 48 hours on a

reciprocating shaker and maintained at a temperature of 25±0.5 ◦C. (Previous

experimental results showed 48 hours of mixing time was sufficient to reach

solubilization and partitioning equilibrium under mixing.) Following this step, excess

amounts of respective poorly soluble PAHs were separated via a 30 minute

centrifugation at 5000 rpm. An appropriate aliquot of the supernatant was carefully

withdrawn with a volumetric pipette and diluted to 10 ml in flasks with 1 ml methanol

and the remaining with the corresponding surfactant-water solution. Naphthalene,

phenanthrene and pyrene in the solutions were analyzed at wavelengths of 220 nm, 254

nm and 334 nm, respectively by a Model UV-2110 spectrophotometer. The surfactant

concentration was maintained in the reference and measurement cells to eliminate the

effect of surfactant on determining solubility. All experiments were performed at room

temperature, 22-26 ◦C. All data reported are the average of at least three independent

samples and the typical error in the measurement was less than 5%.

78

4.2.2.2 Surface tension measurement

Surface tension (γ) measurement was conducted by a model 70545 surface

tensiometer, manufactured by CSC Scientific Company, INC. The total stock solution

surfactant concentration was kept constant at 5.0 mM or 10.0 mM. Based on these

solutions, a geometrical dilution series was prepared, and every dilution was allowed to

equilibrate for approximately 5 hours before the CMC measurement. Then, the surface

tension values were measured until constant surface tension values indicated that

equilibrium had been reached. The accuracy of measurement was within ±0.1 mN m-1.

The values of CMC were determined by noting distinct breaks in the plot of surface

tension versus the logarithm values of surfactant solutions (Log Ct), over a wide

concentration range, as shown in Figure 4.1.

79

-2.0 -1.6 -1.2 -0.8 -0.4 0.0 0.4 0.820

40

60

80

C12-2-4

C12-2-8

C12-2-12

C12-2-16

/

mN

m-1

Log (Ct /mM)

Figure 4.1 (a) Plots of the surface tension (γ) vs. the total surfactant concentration (Ct) of

the single Gemini surfactant combinations

80

-2.4 -2.0 -1.6 -1.2 -0.8 -0.4 0.0 0.420

30

40

50

60

70

80

C12-2-12/C12-2-4

C12-2-12/C12-2-8

C12-2-12/C12-2-16

/m

Nm

-1

Log (Ct /mM)

Figure 4.1 (b) Plots of the surface tension (γ) vs. the total surfactant concentration (Ct) of

the equimolar binary Gemini surfactant combinations

81


4.3.1 Critical micelle concentration properties of the single and equimolar binary

Gemini surfactants

The CMC values were determined experimentally from the plots in Figure 4.1 and

tabulated in Tables 4.1 and 4.3 for all systems. The ascending order of CMC values for

four single Gemini surfactants is C12-2-16 < C12-2-12 < C12-2-8 < C12-2-4. The chain length of

surfactant is the main driving factor for micellization and hydrophobic interactions. The

increase in Entropy is generally attributed to the release of structured water molecules in

hydration layers surrounding the hydrophobic parts of monomeric amphiphiles during

micelle formation (Rosen, 1998). As the hydrophobic chain length of Gemini surfactant

increases, more water molecules are released increasing the entropy. Therefore,

micellization of Gemini surfactant with a longer hydrophobic tail occurs at lower

concentrations.

In the equimolar combinations of C12-2-12/C12-2-4, C12-2-12/C12-2-8 and C12-2-12/C12-2-16,

the CMC values determined from the surface tension (γ)-Log Ct plots (Figure 4.1), are

intermediate between the respective component’s CMC. This indicates the Gemini

surfactants can be easily incorporated into each other, strengthening the hydrophobic

environment of a mixed state in comparison to an individual state. The observed CMC

values are less than ideal critical micelle concentration values (CMCmi) calculated with

Clint’s equation (Clint, 1975), thus demonstrating that the formation of mixed micelle

exhibits a negative deviation with respect to an ideal mixture for mixed surfactant

systems. The mixing effect of the Gemini surfactant system is favorable over the ideal

82

state because the two tails of Gemini surfactant make the micellar molecules even more

hydrophobic.

In view of the regular solution theory which has proven to be commendable in

modeling the nonideal behavior of mixed surfactant system, the deviation of CMCexp

values from CMCmi can be measured by the interaction parameter β, the micellar mole

fraction iX , ideal micellar mole fraction Xideal and activity coefficient fi, of the i- th

surfactant within mixed micelles. The parameters can be estimated with Rubingh’s

equations (Rubingh, 1979). β is an indicator of degree of interaction between two

components in mixed surfactants relative to the self- interaction of individual surfactants

under comparable conditions before mixing and accounts for deviation from ideality

(Kabir-ud-Din et al., 2009a). A negative value of β implies negative deviation of CMCexp

from CMCmi demonstrating a reduction in free energy of micellization over that

predicted by the ideal solution theory (Clint, 1975); a positive value signifies antagonism

between components of a surfactant combination.

It is clearly indicated from Table 4.3 that the values of β studied herein are all

negative presenting a synergistic effect for all mixed surfactant systems. There are

various deviations between the ideal and experimental CMC values for each studied

mixture. The descending deviation order exhibited through β values follows C12-2-12/C12-

2-16 > C12-2-12/C12-2-8 > C12-2-12/C12-2-4. In the case of a C12-2-12/C12-2-4 mixture, small

deviations of CMC are observed indicating their almost ideal behavior. Moreover, C12-2-

16 has more interaction with C12-2-12 in the mixed micelle which means more closely

packed surfactant molecules in the mixed micelle than the pure micelle (Rao and Paria,

2009).

83

Tab

le 4

.3 E

xperi

ment

al cr

itic

al m

icelle

conc

ent

ratio

n (C

MC

exp),

idea

l cr

itic

al m

icelle

conc

entr

atio

n (C

MC

mi), in

tera

ctio

n

par

am

ete

r (β

), m

icellar

mo

le f

ract

ion

(i

X),

idea

l m

icellar

mo

le f

ract

ion

(Xid

eal)

and

act

ivit

y c

oeff

icie

nts

(fi) v

alu

es o

f eq

uim

ola

r

Gem

ini

mix

ture

s at 25

◦C

a

a E

rro

r lim

its

of para

mete

rs C

MC

exp, β,

Xi a

nd f

i ar

e ±

4%

.

Sur

fact

ant

syst

em

C

MC

exp

(mM

) C

MC

mi

(mM

) β

X1/X

2

Xid

eal

f 1/f

2

C12

-2-1

2/ C

12

-2-4

1

1.3

037

-0

.0823

0.6

848/0

.3674

0.2

501

0.9

838/0

.9385

C12

-2-1

2/C

12

-2-8

0.9

1.1

271

-0

.1396

0.5

947/0

.4053

0.3

522

0.9

773/0

.9518

C12

-2-1

2/C

12

-2-1

6

0.3

0.3

252

-0

.2895

0.2

303/0

.7697

0.8

131

0.8

424/0

.9848

84

X1s are found to be larger than Xideals in the case of C12-2-12/C12-2-4 and C12-2-12/C12-2-8

indicating the level of richness of C12-2-12 in the two surfactant systems which are in tune

with the comparison of X1s and X2s. Each Gemini surfactant involved in the equimolar

mixed binary surfactant system does not contribute equally to the CMCmi. The system

with a lower CMC value is more active in forming a mixed micelle. The coefficients f1 of

C12-2-12 are higher in the C12-2-12/C12-2-4 and C12-2-12/C12-2-8 mixtures, indicating its

predominant effect in mixed micelles.

4.3.2 Solubilization of PAHs by single Gemini surfactants

Water solubility enhancements are completed by measuring the apparent solubility of

naphthalene, phenanthrene and pyrene in various solutions of single Gemini surfactants.

Earlier research has reported the solubility of naphthalene, phenanthrene and pyrene in

pure water as 2.44×10-4, 5.61×10-6 and 6.57×10-7 M, respectively, at 25 ◦C (Zhu and

Feng, 2003). Qs is adopted herein as a solubility enhancement factor which equals the

ratio of the apparent solubility of solute in the studied surfactant concentration and

intrinsic solute solubility in pure water. Plots for the variations of Qs with single Gemini

surfactant concentrations are shown in Figure 4.2. The solubility of PAHs in water is

remarkably enhanced by all single Gemini surfactants following the order of

phenanthrene > pyrene > naphthalene except for C12-2-8 which has a superior solubilizing

ability for pyrene. This result is different from Zhu et al.’s (Zhu and Feng, 2003)

research where the solubility of different PAHs by conventional single and mixed

surfactants increases with increasing Log Kow of the PAHs.

85

The order of the extent of solubility enhancements for all PAHs at surfactant

concentrations above their CMC is observed: C12-2-16 > C12-2-12 > C12-2-8 > C12-2-4. This

could be interrelated to the hydrocarbon chain length of different Gemini surfactants.

Rosen et al. (Rosen, 1989b) suggested that solubilization power increased with

increasing hydrocarbon chain length. In the long run, the Gemini surfactant has a longer

alkyl chain and, hence, stronger solubilizing power. However, this rule is not applicable

to mixed Gemini surfactants which will be discussed in the next section.

It is apparent in Figure 4.2 that the solubility of PAHs remains practically constant or

shows a minor increase below the CMC indicating weak interactions between the solutes

and micelles. The equilibrium solubility of PAHs increase linearly with increasing

surfactant concentrations above CMC, which indicates that solubilization is related to

micellization. Moreover, it is worth noting that the CMC values of each surfactant vary

with the presence of different PAHs, as evidenced from Figure 4.2. Take C12-2-8 for

example, the CMC values reach 2.8, 2.0 and 4.3 mM in the presence of naphthalene,

phenanthrene and pyrene, respectively. It coincides with Rosen’s research (Rosen, 1989b)

that the CMC of a surfactant in the aqueous phase, in equilibrium with solubilizates,

should change because the activity of the surfactant changes with the introduction of

solubilizates. Theoretically and practically speaking, the effects of the chemical nature of

solutes on the CMC values of surfactants are therefore of great importance. The same

results are observed in mixed surfactant systems.

86

0 2 4 6 8 10

0

10

20

30

40

50

Naphthalene

Phenanthrene

Pyrene

Qs

Concentration of C12-2-4 /mM

(a)

Figure 4.2 (a) Water solubility enhancements (Qs) of PAHs by C12-2-4

87

0 2 4 6 8 10

0

15

30

45

60

75

Naphthalene

Phenanthren

Pyrene

Qs


(b)

Figure 4.2 (b) Water solubility enhancements (Qs) of PAHs by C12-2-8

88

0 1 2 3 4 5

0

20

40

60

80

100

Naphthalene

Phenanthrene

Pyrene

Qs


(c)

Figure 4.2 (c) Water solubility enhancements (Qs) of PAHs by C12-2-12

89

0.0 0.5 1.0 1.5 2.0

0

15

30

45

60

75 Naphthalene

Phenanthrene

Pyrene

Q

s


(d)

Figure 4.2 (d) Water solubility enhancements (Qs) of PAHs by C12-2-16

90

4.3.3 Solubilization of PAHs by equimolar binary Gemini surfactants

Water solubility enhancements of naphthalene, phenanthrene and pyrene by aqueous

surfactant mixtures (C12-2-12/C12-2-4, C12-2-12/C12-2-8 and C12-2-12/C12-2-16) are carried out

respectively by taking equimolar proportions of two composed surfactants and

comparing them with individual surfactants. The aqueous solubility of PAHs enlarges

linearly with surfactant concentration (Figure 4.3) in the same manner as a single

surfactant, demonstrating the formation of mixed micelles and their potential capacity to

enhance the solubility of PAHs in water.

The molar solubilization MSR is characterized as the number of moles of compound

solubilized by one mole of micellized surfactant (Edwards et al., 1991). It denotes the

effectiveness of a particular surfactant for solubility enhancement in a given solute and

can be expressed as follows:

*

*

SMSR

C (4.1)

Where S* is the discrepancy of the total apparent solubility of PAHs in a given surfactant

solution at a specified surfactant concentration Cac above CMC and the apparent

solubility of the solute at CMC; C* is the discrepancy of Cac and CMC. In the presence

of excess PAHs, MSR values of mixed surfactants could be obtained from the slopes of

the linearly fitted lines in which the concentration of PAHs are plotted against surfactant

concentrations above the CMC (surfactant concentration in mM vs. PAH concentration

in mM) as Figure 4.3 illustrates.

91

0.4 0.8 1.2 1.6 2.00.0

0.4

0.8

1.2

[P

AH

s] /

mM

[C12-2-12/C12-2-16] /mM

Naphthalene

Phenanthrene

Figure 4.3 (a) The solubilization of naphthalene and phenanthrene in C12-2-12/C12-2-16

mixed surfactants

92

1 2 3 4 5

0.00

0.02

0.04

0.06

0.08

[PA

H]

/mM

[C12-2-12/C12-2-16] /mM

Pyrene

Figure 4.3 (b) The solubilization of pyrene in C12-2-12/C12-2-16 mixed surfactants

93

According to the CMC analysis, the solubilizing efficiencies of mixed systems are

generally between the single surfactants. However, given the MSR values to evaluate the

solubilizing effectiveness of surfactants, the apparent water solubility of different PAHs

in equimolar mixed Gemini surfactant solutions are not expected to conform with their

surface properties. Here, though the CMC of C12-2-12/C12-2-16 is small, it is a poor

solubilization aid for tested naphthalene and phenanthrene, in comparison with those

from corresponding individual surfactants. This can be ascribed to the effect of mixing

surfactants in the microenvironment of naphthalene and phenanthrene. It is reported that

looser packing of surfactant molecules in the mixed micelle may increase the

solubilization of organics in the micellar phase (Roy et al., 1995). In this study, a similar

observation was observed that the interaction parameter (β) of C12-2-12/C12-2-16 is larger

demonstrating the formation of more closely packed micelles and, in turn, resistance to

naphthalene and phenanthrene molecules entering the micellar phase from the aqueous

phase by diffusion, which may ultimately reduce the solubilization abilities.

In addition, the solubilization of aromatic molecules such as benzene, toluene and

naphthalene is believed to occur at the micelle-water interface (Moroi, 1992). Along with

the formation of large micelles through Gemini surfactants with long hydrophobic chain

lengths, the specific surface area of micelles reduce resulting in the depressed interaction

of organic compounds and micelle-water interfaces. However, the C12-2-12/C12-2-16 is

found to be superior in pyrene solubilization in comparison to all other investigated

solubilizing systems.

The increase in MSR can be explained, in part, by the decrease in the CMC of the

C12-2-12/C12-2-16 mixed system. Another reason is related to the interaction of mixed

94

surfactants and the microenvironment of pyrene. The locus of solubilization of a solute

in micelles is influenced by the interaction between the solute and the surfactant micelle

(Prak and Pritchard, 2002). Compared to a relatively polar PAH (naphthalene), pyrene is

more hydrophobic and, therefore, preferable to incorporate in the palisade layers of the

mixed micelles and interact with the head group through cation-π interaction. Generally,

the micelles formed are sphere like and the micelle volume can be approximately

reckoned by the length of the hydrophobic chain as the radius or the spherical core

(Zheng and Zhao, 2006). C12-2-16 micelle, with long hydrophobic chain length,

intercalates into the C12-2-12 micelle generating a larger dimension palisade layer resulting

in increased Py molecule solubilization.

C12-2-12/C12-2-4 and C12-2-12/C12-2-8 have relatively comparable solubilization capacities

of phenanthrene over that of their individual components. This may be because of the

conjoint effect of solubilization in the micellar core and the adsorption of organic

compounds at the micelle-water interface. However, the exact location in the micelle at

which solubilization occurs varies with the structure and chemical nature of the solutes

and surfactants and is hardly defined.

In order to fully quantify the effect of a mixed surfactant system on PAH

solubilization and observe the nature of deviation, the deviation ratio (R) between the

MSRexp and the MSRideal can be determined. MSRideal is the MSR for organic compounds

in a mixed surfactant system at the ideal mixed state which can be estimated using the

MSR of a single surfactant solution based on the ideal mixing rule (Zhou and Zhu, 2004):

MSRideal= MSR1X1 + MSR2X2 (4.2)

where X1, X2, MSR1 and MSR2 are the mole fraction and molar solubilization ratio for

95

solute of components 1 and 2 in mixed surfactant solutions, respectively. It is visibly

indicated from Table 4.4 that the deviation ratio (R) is close to 1 for most combinations

of surfactant systems demonstrating solubilization follows almost ideal behavior with a

small deviation. A value of R greater than 1 is the embodiment of the positive mixing

effect of C12-2-12/C12-2-4 and C12-2-12/C12-2-8 on phenanthrene as well as the synergistic

effect of C12-2-12/C12-2-16 on pyrene indicating the MSRexps of the three surfactant systems

have a positive deviation from ideal mixing rules regarding different PAHs. Such a

difference on the deviation ratio depends on the altered surfactant molecular

microstructures (packing of surfactant molecules at the micelle-water interface as well as

in the mixed micelle core) when mixed surfactant micelles formed and the interaction

between the solute and the mixed micelles. Additionally, the MSR values amplify

exponentially with the increase in hydrophilicity of PAHs (pyrene < phenanthrene <

naphthalene) for single and mixed Gemini surfactants in accordance with the early

findings (Kabir-ud-Din et al., 2009b), which are negatively proportional to their octanol-

water coefficients (Log Kow).

Beyond the structure of surfactants and solutes, the surfactant-solute interaction and

CMC properties may also affect the solubilization capacities of the mixed micelles. The

micelle-water partition coefficient (Km) which is defined as a distribution of the mole

fraction of PAHs between surfactant micelles and the aqueous phase ha ve been

calculated from experimental measurement by using the following formula (Edwards et

al., 1991):

-

- -

ac cmcm

ac ac cmc w cmc

S SK

C CMC S S V S

(4.3)

96

where Vw is the molar volume of water (1.805×10-2 L M-1 at 25 ◦C).

It is observed that the C12-2-12/C12-2-16 demonstrates a higher Km value and is still

higher than those of single surfactants C12-2-12 and C12-2-16 indicating synergism in pyrene

solubility improvement over the corresponding single surfactant. In addition, although

the CMC values of C12-2-12/C12-2-4 and C12-2-12/C12-2-8 are higher than C12-2-12/C12-2-16,

major partition coefficients Km contribute more to the increasing MSR values resulting in

solubilizing higher amounts of phenanthrene over that of C12-2-12/C12-2-16 and their

individual components. Thus, the solubilization abilities do not only depend on the CMC

properties and the interactions between the surfactant and solute, but also rely on the

relevant partition or incorporation properties of solutes.

Another empirical parameter B involving the surfactant-surfactant and surfactant-

solute interactions has been put forward to gain a better understanding of the mixing

effect of surfactant systems on solubilization of PAHs. It is expressed based on the

regular solution approximation (RSA) as follows (Treiner et al., 1990):

12 1 1 1 2 1 1ln ln (1- ) ln (1- )m m m m

m m mK X K X K BX X (4.4)

where Km12, Km1 and Km2 are the micelle-water partition coefficients of the solute in

mixed and individual surfactant systems, respectively, and1

mX represents the micellar

mole fraction of a surfactant having the value of Km1. B=0 indicates there would be no

mixing effect on partitioning of a solute between the aqueous and micellar phases (Zhou

and Zhu, 2004); for B > 0 (< 0) implies that Km12 in the mixed surfactant system is larger

(smaller) than predicted by the ideal mixing rule (Dar et al., 2007).

There is no appreciable relationship between the values of B and β, because the value

97

of B is supposed to depend on surfactant/solute and on surfactant/surfactant interactions

in the mixed micelles (Kabir-ud-Din et al., 2009a). According to the values of B

calculated from the above relation for Gemini surfactant mixtures and recorded in Table

4.4, the mixing effect of C12-2-12/C12-2-4 and C12-2-12/C12-2-8 on the partition of

phenanthrene and C12-2-12/C12-2-16 on pyrene is positive. The results are consistent with

their respective positive deviations of MSRs from ideal mixtures, which seemingly can

be used to interpret the synergistic (antagonistic) mixing effect of selected mixed

surfactants on the solubilization of PAHs studied herein. However, the B values are

found to disagree with the R values for the C12-2-12/C12-2-8 regarding the solubility of

naphthalene and phenanthrene in comparison with C12-2-12/C12-2-4 and C12-2-12/C12-2-16.

Similar contradictory results have been found in related research (Tokuota et al., 1994;

Sun et al., 1995) involving solubilization of organic compounds in mixed surfactant

systems. Hence, the results indicate the mixing effect of surfactants on the Km12 cannot be

exactly utilized as the unique factor to account for the solubilization of organic

compounds in the mixed surfactants. The B values do not reveal the degree of mixing

effect, but only denote the presence (absence) of the synergistic mixing effect of Gemini

surfactant.

The solubilization capacities of all the mixed micelles can be regarded as a normal

partitioning of the PAHs between the micellar and aqueous phases. The standard free

energy 0

sG of solubilization presented in Table 4.4 is negative indicating spontaneous

solubilization for all systems. The larger absolute values of the micellar systems indicate

the PAHs solubilizing behaviors are energetically more complementary and attributed to

intense conjoint effects caused by the factors discussed above.

98

Naphthalene0.0

0.1

0.2

0.3

C12-2-4

C12-2-8

C12-2-12

C12-2-16

C12-2-12/C12-2-4

C12-2-12/C12-2-8

C12-2-12/C12-2-16

MS

R

Figure 4.4 (a) The MSR values of various single and mixed Gemini surfactants on

naphthalene

99

Phenanthrene0.0

0.1

0.2

0.3

C12-2-4

C12-2-8

C12-2-12

C12-2-16

C12-2-12/C12-2-4

C12-2-12/C12-2-8

C12-2-12/C12-2-16

MS

R

Figure 4.4 (b) The MSR values of various single and mixed Gemini surfactants on

phenanthrene

100

Pyrene0.000

0.005

0.010

0.015

0.020

C12-2-4

C12-2-8

C12-2-12

C12-2-16

C12-2-12/C12-2-4

C12-2-12/C12-2-8

C12-2-12/C12-2-16

MS

R

Figure 4.4 (c) The MSR values of various single and mixed Gemini surfactants on pyrene

101

Tab

le 4

.4 L

og

Km

, L

og

Km

12,

0 sG

, R

and

B o

f P

AH

s in

vari

ous

eq

uim

ola

r m

ixed

Gem

ini su

rfac

tant

sys

tem

s at

25

◦ C a

PA

Hs

Sin

gle

Sur

fact

ant

Lo

g K

m

Mix

ed

Sur

fact

ant

L

og

Km

12

R

B

0 sG

(kJ

M-1

)

Nap

htha

lene

C12

-2-4

5.3

856

C

12

-2-2

/C12-2

-4

5.4

33

0.8

002

-4

.7059

-31.0

1

C12

-2-8

5.4

807

C

12

-2-2

/C12-2

-8

5.4

71

0.8

561

-5

.9225

-33.2

3

C12

-2-1

2

5.5

739

C

12

-2-2

/C12-2

-16

5.3

72

0.8

412

-3

.6703

-29.9

1

C12

-2-1

6

5.5

884

Phe

nanth

rene

C12

-2-4

4.3

661

C

12

-2-2

/C12-2

-4

5.8

57

1.9

69

2.5

821

-3

3.4

3

C12

-2-8

4.9

811

C

12

-2-2

/C12-2

-8

5.8

90

2.0

77

0.2

462

-3

3.6

2

C12

-2-1

2

5.6

006

C

12

-2-2

/C12-2

-16

4.4

95

0.5

50

-8

.2234

-25.6

6

C12

-2-1

6

4.7

985

Pyr

ene

C12

-2-4

4.3

454

C

12

-2-2

/C12-2

-4

4.9

72

0.8

193

-2

.1367

-28.3

8

C12

-2-8

5.7

126

C

12

-2-2

/C12-2

-8

5.1

96

0.6

590

-4

.0123

-29.6

6

C12

-2-1

2

5.5

499

C

12

-2-2

/C12-2

-16

5.6

92

1.6

99

10.4

60

-3

2.4

9

C12

-2-1

6

4.6

887

a E

rro

r lim

its

in t

he m

easu

rem

ent

of

Lo

g K

m, L

og

Km

12 a

nd0 s

G

are

±3.3

%, ±

3.1

% a

nd ±

2.8

%,

resp

ecti

vely

.

102

4.4 Summary

The present study investigated the effect of the solution composition and category of

selected single and equimolar binary Gemini surfactant systems in the solubilization of

three model PAHs (naphthalene, phenanthrene and pyrene) via CMC and solubilization

approaches. The selected single Gemini surfactants observably enhance the water

solubility of PAHs following the order of phenanthrene > pyrene > naphthalene, except

for C12-2-8 which has superior solubilizing ability for pyrene. For the same PAH,

solubilization with the single Gemini surfactant is in tune with the order of the variation

tendencies for the CMC values. The PAH solubilities increase linearly over the range of

mixed surfactant concentrations above the CMC, which illustrate the potential capacities

of the equimolar mixed Gemini surfactants to facilitate the solubilization of PAHs in

water. However, the chosen mixed Gemini surfactants have relatively selective

solubilization capacities; they are not all as high as their corresponding individual

surfactant solutions at comparable surfactant concentrations and are not just related to

molar capacity. Therefore, it is possible to assume that the dissymmetric Gemini

surfactant possessing a longer hydrophobic tail (C12-2-16), mixed with a symmetric

Gemini surfactant (C12-2-12), bears superior solubilization for a relatively hydrophobic

PAH (pyrene). The dissymmetric Gemini surfactants possessing a shorter hydrophobic

tail (C12-2-4 and C12-2-8) and mixed with a symmetric Gemini surfactant (C12-2-12) has a

relatively comparable solubilization capacity for relatively polar PAHs (naphthalene and

phenanthrene). Thus, the structure of surfactants and polarity of solutes must be

considered simultaneously when analyzing the mixing effect of surfactants on the

dict://key.0895DFE8DB67F9409DB285590D870EDD/to%20possess

dict://key.0895DFE8DB67F9409DB285590D870EDD/to%20possess

dict://key.0895DFE8DB67F9409DB285590D870EDD/polarity

103

solubilization for different PAHs, such as CMC and Km, since they are all important

factors influencing the solubilization of mixed surfactant solutions for organic

compounds. The results studied herein have yielded interesting results and provided

valuable information regarding the selection of mixed Gemini surfactants for the SER of

contaminated soils and water.

104

CHAPTER 5

ENHANCED SOLUBILIZATION CAPABILITIES OF BI AND

TERNARY CATIONIC GEMINI AND CONVENTIONAL

NONIONIC SURFACTANTS TOWARDS PAHS

5.1 Introduction

PAHs are ubiquitous pollutants formed by natural and anthropogenic pyrolysis of

organic matter during forest fires, fossil fuel utilization, and chemical manufacturing

(Atkins et al., 2010). The contamination of soils and water by carcinogenic pollutants has

been a widespread environmental problem for several decades. Poor solubility and

toxicity of such compounds has motivated the development of various technologies with

which to overcome obstacles to remedy contaminated soil and water (Wu et al., 2008;

Gan et al., 2009; Petitgirard et al., 2009).

The In situ SER process (Mulligan et al., 2001) is one of the most promising

techniques for the removal of sorbed contaminant in subsurfaces involving the use of

aqueous surfactant solutions above their CMC, which are able to efficiently amplify the

aqueous phase concentrations of sparingly water-soluble substances. Numerous efforts

(Menger and Littau, 1993; Kabir-ud-Din et al., 2010a; Kabir-ud-Din et al., 2010b;

Chavda et al., 2011; Guo et al., 2011) have been undertaken to synthesize and select

efficient surfactant to facilitate the solubilization of PAHs.

A few studies (Zhou and Zhu, 2008; Madni et al., 2010; Vilasau et al., 2011) about

mixed micelle formation of known composition are of great interest to scientific and

industrial applications since the surfactants are almost always prepared commercially

105

and used as mixtures rather than in pure forms. In view of this, investigation into the

behavior of multi-component systems, with two or more species, which are much more

efficient in a working scenario, has generally been performed. There are numerous

reports regarding the solubilization of PAHs by conventional single/mixed surfactants

systems. Zhou and Zhu (Zhou and Zhu, 2004) have reported the potential capacity of

anionic–nonionic mixed surfactants to enhance the solubility of pyrene in water. Aijaz

Ahmad Dar et al. (Dar et al., 2007) have concluded that cationic-nonionic mixtures have

better solubilization capacities for PAHs than pure cationic, nonionic and cationic-

cationic mixtures. Paria et al. (Paria and Yeuet, 2006) found a negative deviation

solubilization of naphthalene from the ideal value via various ionic-nonionic mixed

surfactant systems. Although considerable work has been carried out, to date, on

examining the solubilization of PAHs by mixed conventional surfactant systems, there

are sporadic reports of the same in Gemini and its mixed systems.

Geminis are third generation surfactants which have drawn increased attention

resulting from their unique properties. The alkanediyl-α,ω-bis(alkyl-dimethylammonium

bromide) type Gemini surfactants have been the most investigated surfactant systems.

They are often referred to as “m-s-n surfactants”, where m and n are the carbon numbers

of the alkyl chain and s represents the carbon numbers present in the polymethylene

group in spacer, respectively (Sharma et al., 2003). The surfactants show very high

bactericidal activity and have found a manifold application in the detergent and cosmetic

industries (Khan et al., 2011). It is found (Mehta et al., 2009; Kabir-ud-Din et al., 2010a;

Sheikh et al., 2011; Wei et al., 2011a) that a mixed system involving Gemini and

conventional surfactants often exhibits superior characteristic properties regarding the

106

application front when contrasted to the pure system due to the presence of two charged

sites in a Gemini which propose a strong interaction with other surfactants (Gemini or

conventional). Practically, the detailed fundamental studies regarding mixed

micellization of the Gemini and conventional surfactants are limited though these

mixtures are extensively used in a wide range. Moreover, to our knowledge, there is no

study available on the solubilization of PAHs in mixed ternary Gemini and conventiona l

surfactant combinations.

The current work attempts to carry out experimental and theoretical investigation to

study the effect of single, binary and ternary Gemini cationic C12-2-n (n=12, 16) and

conventional nonionic surfactant (C12E23) mixtures on the solubilization aspects of PAHs.

More specifically the detailed have been performed: (a) to correlate the interaction

parameter in the respect of equimolar Gemini cationic-cationic, Gemini cationic-

conventional nonionic and Gemini cationic-cationic- conventional nonionic multi-

component micelle formation and elucidate the cause of their mixing effect for finding

synergism, and (b) to evaluate solubilization capabilities of the selected equimolar multi-

component surfactant systems and compare with that predic ted according to the ideal

mixing rule. The analysis gave sufficient valuable information to understand the mixing

and solubilizing behavior of mixed Gemini cationic and conventional nonionic multi-

component surfactant systems which can contribute to their potential application in the

selection of surfactant mixtures to enhance the recovery of hydrophobic organic

compounds (HOC’s) contaminated soils and aquifers.

107


5.2.1 Materials

Naphthalene and pyrene, the two and four-ring PAHs, were selected as the

hydrophobic solute in this study (all > 98% Aldrich products) for their varying physico-

chemical properties and their widespread presence in contaminated sites. Gemini

cationic surfactants C12-2-12 and C12-2-16 were obtained from Chengdo Organic Chemicals

Co., Ltd., Chinese Academy of Science, with a purity of 95%. The nonionic surfactants

C12E23 with purity >99% was purchased from Sigma-Aldrich. All chemicals were used

without further purification. The structure and properties of chosen surfactants and

organic compound are all included in Table 5.1. The stock solutions of Gemini and

nonionic surfactant were prepared by dissolving the relevant surfactant in deionized

water. Then desired mole fractions were obtained by mixing precalculated volumes of

the stock solutions and the following experimental procedures performed.

5.2.2 Tensiometric measurement

The tensiometric measurement was made with a Model K11MK3 surface

tensionmeter, manufactured by Krüss. The total surfactant concentration of the stock

solutions was kept constant at 5.0 mM. In an experimental run, different single,

equimolar binary and ternary dilution series were prepared with the help of an Eppendorf

microsyringe in small installments and every dilution was allowed to equilibrate for

approximately 3 hours before CMC measurement. Then the surface tension values were

108

measured until constant surface tension values indicating that equilibrium had been

reached. The accuracy of measurement was within ±0.1 mN m-1. A digital thermostated

bath from Polyscience was used for maintaining the temperature within 25±0.01 ◦C. The

values of CMC were determined by noting distinct breaks in the plot of surface tension (γ)

versus the logarithm values of surfactant solutions (Log Ct) over a wide concentration

range.

5.2.3 Solubilization measurement

Batch tests for solubilization of naphthalene and pyrene were performed

subsequently in single and equimolar bi and ternary combinations of symmetric Gemini

surfactant C12-2-12, dissymmetric Gemini surfactants C12-2-16 and conventional nonionic

surfactant C12E23 with one same carbon hydrophobic group. Each contaminant-surfactant

system involved five to seven batch experiments with surfactant solutions having a range

of concentrations above the CMC. For each batch test, excess amounts of naphthalene

and pyrene were separately added to 10 ml of surfactant solutions with different

concentrations and composition which were placed in borosilicate glass vials of 20 ml

capacity to saturate the solution. The sample vials were sealed with a screw cap fitted

with a Teflon lined septum to prevent any volatilization loss of PAHs from surfactant.

Three duplicate samples were prepared for each surfactant concentration. These samples

were then agitated for a period of 48 hours on a thermostated shaker maintained at a

temperature of 25±0.5 ◦C. (Previous experimental results showed 48 hours mixing time

to be sufficient for reaching solubilization and partitioning equilibria under mixing.)

109

After this, subsequent centrifugation for 30 minutes at 5000 rpm was performed by a

Model Durafuge100 centrifuge (Thermo Electron Corporation, US) to remove the

undissolved solid solute. An appropriate aliquot of the supernatant was then carefully

withdrawn with a volumetric pipette and diluted to 10 ml in flasks with 1 ml methanol

and rest with the corresponding surfactant-water solution. The concentration of dissolved

naphthalene and pyrene were detected spectrophotometrically with a Varian

spectrophotometer (Cary 300) at the wavelength of 220 nm and 334 nm. The surfactant

concentration was kept the same in both the reference and the measurement cells to

eliminate the effect of surfactant on determining solubility. All data reported are the

average of at least three independent samples and the typical error in the measurement

was less than 5%.

110

Tab

le 5

.1 T

he s

truc

ture

and

physi

coche

mic

al p

ropert

ies

of

the

sele

cted

sur

fact

ants

and

so

lute

sa i

n t

his

stu

dy

Mat

eria

l S

truc

ture

M

W

(g M

-1)

CM

Cex

pb

(mM

) W

ater

so

lub

ilit

y

(M)

Lo

g K

ow

C12

-2-1

2

C12H

25N

+(C

H3) 2

(C

H2) 2

-N+(C

H3) 2

C12H

25·2

Br-

614.7

0.8

C12

-2-1

6

C12H

25N

+(C

H3) 2

(C

H2) 2

-N+(C

H3) 2

C16H

33·2

Br-

670.8

0.2

C12E

23

C12H

25(O

CH

2C

H2) 2

3O

H

1198

0.0

61

Nap

htha

lene

128.2

2.5

×10

-4

3.3

6

Pyr

ene

202.3

6.7

×10

-7

5.1

8

a D

ata

for

PA

Hs

is r

epo

rted

by

Ver

chue

ren (

Ver

schue

ren,

1996

)

b E

rro

r lim

its

of

CM

Cex

ps

are

± 4

%

111

-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5

30

35

40

45

50

55

60

a

/m

N m

-1

Log (Ct /mM)

C12-2-12

C12-2-16

C12

E23


concentration range at 25 ◦C

112

-2.5 -2.0 -1.5 -1.0 -0.530

35

40

45

50

55

b

/m

N m

-1

C12-2-12

/C12

E23

C12-2-16

/C12

E23

C12-2-12

/C12-2-16

C12-2-12

/C12-2-16

/C12

E23

Log (Ct /mM)

Figure 5.1 (b) Variations of γ values of equimolar binary and ternary mixed surfactant

systems over a wide concentration range at 25 ◦C

113


5.3.1 Critical Micelle Concentration Properties of all Surfactant Systems

The CMC values of single as well as their equimolar bi and ternary surfactant

systems are presented in Tables 5.1 and 5.2. The values for pure surfactants are

complementary with literature values (Berthod et al., 2001; Sun et al., 2007), given in

parenthesis in the Table 5.1. Apparently, the chain length of Gemini surfactant is major

driving factor for micellization and hydrophobic interactions as same as conventional

surfactant, larger the chain length lower is the CMC values. The lowest CMC value of

nonionic surfactant C12E23 owing to a larger number of oxyethylene (OE) units in its

hydrophilic head (Dar et al., 2007).

Mixed systems comprising of the equimolar combinations of C12-2-12/C12E23, C12-2-

16/C12E23, C12-2-12/C12-2-16 and C12-2-12/C12-2-16/C12E23 in water undergo presumably physic-

chemical changes due to interactions and invariably yield enhanced micellar properties.

Usually the CMCs of mixed surfactants determined from the surface tension (γ )–Log Ct

plots (Figure 5.1) fall between those of individual pure components (Dar et al., 2007; Hu

et al., 2011), as was observed here (Table 5.2) which are different from the earlier

findings where Gemini-nonionic mixtures had lower CMC values than both of the

constituent surfactants (Sharma et al., 2006). The difference might be attributed to the

shorter spacer and hydrophobic tails of the Gemini surfactant of our systems, and the

longer polarized oxygen atoms (PEO chains) chain length of the studied nonionic

surfactants. Moreover, the reduced CMC values demonstrate that nonionic amphiphilles

can easily intercalate among the Gemini surfactant amphiphiles, PEO chains may coil

dict://key.0895DFE8DB67F9409DB285590D870EDD/attribute%20to

114

around the charged headgroups of Gemini surfactant (Hu et al., 2011), so the

electrostatic repulsion is reduced, which will reinforce the hydrophobic environment in

the mixed media in comparison to the pure Gemini state.

Additionally, the mixed CMC (CMCmix) for mixture systems obtained by mixing the

constituents is given by the following equation based on the Rubingh’s regular solution

theory (Rubingh, 1979):

1mix

1 ni

i i iCMC f CMC

(5.1)

where αi, CMCi and fi are the stoichiometric mole fraction, CMC value and activity

coefficient of the respective components in mixed micelle. In case of ideal behavior, fi

=1 (i=1, 2···n) and hence Eq. (1) reduces to the form:

1ideal

1 ni

i iCMC CMC

(5.2)

as proposed by Clint (Clint, 1975) for ideal mixed.

The obtained experimental CMC values (CMCexp) were observed to be less than ideal

critical micelle concentration values (CMCideal) for all equimolar bi and ternary systems

included in Table 5.1, demonstrating the existence of interactions leading to a distinction

between the constituent surfactants in nonideality of the multi-component systems, i.e.,

micellization takes place at lower concentration with respect to the ideal state.

5.3.2 Surfactant-surfactant Interactions in mixed micelles

As discussed above, CMCexp < CMCideal values indicate negative deviation from ideal

behavior for all studied mixed micelle formations. The degree of the negative deviation

115

and hence the extent of nonideality of binary mixed surfactant systems can be estimated

by the interaction parameter (β) as follows (Rubingh, 1979):

12 1

1 1

2

1

ln

1

CMC

CMC X

X

（）

(5.3)

2

1 1 12 1 1

2

1 1 12 1 2

( ) ln( / )1

(1- ) ln[(1- ) / (1- ) ]

X CMC X CMC

X CMC X CMC

(5.4)

where X1 is the micellar mole fraction of component in the mixed micelle; α1 and α2 are

their corresponding bulk mole fractions; CMC1, CMC2 and CMC12 are critical micelle

concentrations for two component surfactant and their mixture respectively. Eq. (5.4) is

solved iteratively for X1, which is then substituted into Eq. (5.3) to obtain β. The negative

value of β implies synergism demonstrating reduction in free energy of micellization

over that predicted by the ideal solution theory (Rao and Paria, 2009) whereas the

positive value signifies incompatible surfactants and repulsion while zero value states no

interaction, hence ideal mixing (Sheikh et al., 2011).

Furthermore, the regular solution theory will yield expressions for activity coefficient,

fi, of i-th surfactant within the mixed micelles related to the interaction parameter of β

and X1 through the relation (Rubingh, 1979):

2

1 1ln ( ) (1 )f X (5.5)

2

2 1ln ( ) ( )f X (5.6)

The comprehensive set of results for all binary surfactant system is tabularized in Table

5.2.

116

As evident from the results, the values of β in the present case are all negative

indicating a positive effect for all mixed surfactant systems which depends not only on

the interaction but also on relevant properties of individual surfactants of mixture

(Tanford, 1980). From data it is also observed that the activity coefficients, obtained

from the Eq. (5.5) and (5.6), are all less than unity, indicating nonideal behavior with

synergistic interaction between surfactants in the micelles, which is comparable with the

earlier findings for conventional cationic-cationic and cationic-nonionic mixed surfactant

systems (Mehta et al., 2009).

117

Tab

le 5

.2 E

xperi

ment

al

crit

ical

mic

elle

conc

ent

ratio

n (C

MC

exp),

idea

l cr

itic

al

mic

elle

conc

entr

atio

n (C

MC

mi),

inte

ract

ion

par

am

ete

r (β

), m

icellar

mo

le f

ract

ion

(i

X )

and

act

ivit

y c

oeff

icie

nts

(fi) v

alu

es o

f eq

uim

ola

r su

rfac

tant

mix

ture

s at

25

◦C

a

aE

rro

r lim

its

of para

mete

rs a

re ±

4%

.

Sur

fact

ant sy

stem

C

MC

exp

(m

M)

CM

Cm

i (m

M)

β

X1/X

2/X

3

f 1/f

2/f

3

C12

-2-1

2/ C

12E

23

0.1

0

0.1

13

-1

.2708

0.1

549/0

.8451

0.4

034/0

.9644

C12

-2-1

6/ C

12E

23

0.0

7

0.0

93

-1

.4163

0.3

303/0

.6697

0.5

298/0

.8568

C12

-2-1

2/C

12

-2-1

6

0.3

0

0.3

20

-0

.3792

0.2

342/0

.7658

0.8

005/0

.9744

C12

-2-1

2/C

12

-2-1

6/C

12E

23

0.0

994

0.1

325

-1

.1495

0.0

192/0

.1593/0

.8215

0.0

729/0

.1456/0

.8986

C12

-2-1

2/(

C1

2-2

-16/C

12E

23)

0.0

994

0.1

025

-2

.0478

0.2

176/0

.7823

0.2

855/0

.9076

C12

-2-1

6/(

C1

2-2

-12 /C

12E

23)

0.0

994

0.1

674

-1

.2637

0.6

052/0

.3947

0.6

296/0

.8214

C12E

23/(

C12-2

-16/C

12-2

-12)

0.0

994

0.1

447

-0

.1371

0.8

184/0

.1816

0.9

955/0

.9133

118

The minor value of β and small deviation of f i values with each other in the case of

Gemini cationic-cationic system signify less attractive interactions of individual

component owing to the conjoint effect of hydrophobic interaction among their carbon

tails and intensive electrostatic repulsion among their hydrophilic groups. Whereas, the

stronger synergism effect of Gemini cationic and conventional nonionic surfactants

would result in part from the momentous electrostatic self- repulsion of cationics is

decreased by intercalating the nonionic amphiphiles into cationic micelles, i.e. the

attractive ion-dipole interaction between positively charged head groups of cationic and

negatively PEO chains of nonionic amphiphille. Another reason is a consequence of the

fact that the electrostatic self- repulsion of cationics and weak steric self-repulsion of

nonionics are weakened by dilution effects (Kabir-ud-Din et al., 2009a).

Analysis show that mixed micelles of Gemini cationic-conventional nonionic

surfactants contain large fraction of nonionic as indicated by X1 values in Table 5.2 due

to its stronger tendency to micellize as also experimentally verified by other studies on

different cationic-nonionic mixed micelles (Dar et al., 2007). Because the nonionic

C12E23 has large numbers of PEO chains containing lone pair of electrons which will

have stronger tendency to interact with the hydrophilic groups of Gemini catatonics.

Furthermore, more negative β values of C12-2-16/C12E23 mixed systems over C12-2-

16/C12E23 systems is thus the result of stronger charge-dipole interaction and also may be

ascribed to the additional chain-chain interactions which play a major role in the stability

of mixed micelles, while in case of C12-2-12/C12E23, chain-chain interactions are negligible

due to similar chain lengths.

119

The β value of ternary mixtures C12-2-12/C12-2-16/C12E23 have been calculated using

pseudobinary treatment (Burman et al., 2000) which equals to the average value of three

possible combinations, i.e., one is regarded as one component and the other two

components are treated as the second component. In this study, Xi and β obviously

depend on the way of selection of the pairing of the three components (Moulik et al.,

1996). The calculated β values of different combinations are various which are not

concordant with the previous report (Dar et al., 2007) for conventional cationic and

nonionic ternary surfactant systems. This might because of the large CMC distinction of

Gemini cationic surfactant studied herein. Nevertheless, all β values of ternary mixture

and its three pseudobinary combinations are negative indicating mutual synergistic

interaction of three components in the ternary system which is higher than its

corresponding cationic/cationic parts and lower than cationic/nonionic part. Analysis

also presents the predominance of the nonionic surfactant in the mixed ternary micelles

demonstrating its intense inclination to micellize regardless of the constitutes of

surfactant mixtures.

The activity coefficients and micellar composition for ternary mixture have been

addressed using a generalized multi-component nonideal mixed micelle model on the

basis of pseudo-phase separation approach developed by Holland and Rubingh (Holland

and Rubingh, 1983). The model makes a good use of interaction parameters determined

from CMC measurement on binary system. The basic equations are presented below:

12

1 1 1( ) ( )

ln ( )jn n

i ij j ij ik jk j k

i j ki j i j k

f x x x

(5.7)

dict://key.0895DFE8DB67F9409DB285590D870EDD/constitute

120

where βij is the pairwise interaction between component i and j which can be obtained

independently from binary mixtures using the Rubingh method, and Xj is the mole

fraction of j-th component in the n-component mixture system. The results of the activity

coefficients for a three-component system, i.e., f1, f2 and f3 can be obtained by using the

method of successive substitutions subject to the constraint that sum of Xi’s equals unity

(Dar et al., 2007). These calculations were done using MATLAB.

As evident from the results, the ternary system also has attracted an interaction,

suggesting strong synergism as denoted by β. The CMCmix value is found to be lower

than the CMCideal, indicating the synergistic nonideal nature of the mixed ternary

micellar system. The mole fractions of individual amphiphiles in the ternary mixed

micelles are different from stoichiometric composition, i.e., the X value is much lower

than the α value of the cationic component, but is fairly higher than the α value and is

close to unity (Table 5.2). The activity coefficients of the individual surfactants, within

the ternary mixture, are in the order of C12-2-12 < C12-2-16 < C12E23 which is in conformity

with their mole fractions, showing the predominance of the nonionic surfactant and its

active effect in the mixed ternary micelles.

5.3.3 Solubilization of naphthalene and pyrene

The variation of solubilization capabilities were plotted as a function of single and

equimolar mixed surfactant solution concentrations for each data set as shown in Figure

5.2, respectively. The apparent solubility of naphthalene and pyrene elevated linearly

with increased concentrations of single surfactant systems above CMC. Similar trends

can be observed in all bi and ternary surfactant compositions studied, demonstrating the

121

formation of mixed micelles and their potential capacity to enhance the solubility of

PAHs in water. The behavior is generally associated to the incorporation or partitioning

of organic solutes within multi-component micelles (Wang et al., 2005).

As for single surfactants, their solubility enrichment being over that in water for both

naphthalene and pyrene, are in conformity with their surface tension characters, i.e., the

lower the CMC values, the superior the solubilizing abilities towards PAHs. The noted

difference between C12-2-12 and C12-2-16 could be associated with the hydrocarbon chain

length of different Gemini surfactants. According to Rosen et al. (Rosen, 1989a), the

solubilization power increased with an increasing hydrocarbon chain length. After all,

Gemini surfactant, which has a longer alkyl chain, would possess a stronger solubilizing

ability. The strongest solubilization power of C12E23 might be interrelated to its low

CMC and larger micellar size, as compared to that of C12-2-12 and C12-2-16, which states

that a large fraction of C12E23 is present in micellar form, thus, creating more locations

within the solution at which PAHs may be solubilized.

122

0.4 0.8 1.2 1.6 2.0

0.0

0.2

0.4

0.6

0.8

C12-2-12

C12-2-16

C12E23

[Nap

hth

ale

ne]

/mM

[Surfactant] /mM

Figure 5.2 (a) Variation of solubility of naphthalene with a total surfactant concentration

(Ct) of single surfactant systems

123

0.0 0.5 1.0 1.5 2.0

0.0

0.2

0.4

0.6

0.8

C12-2-12/C12E23

C12-2-16/C12E23

C12-2-12/C12-2-16

C12-2-12/C12-2-16/C12E23

[Nap

hth

ale

ne]

/mM

[Surfactant] /mM

Figure 5.2 (b) Variation of solubility of naphthalene with a total surfactant concentration

of equimolar binary and ternary mixed surfactant systems

124

0 1 2 3 4 5

0.00

0.02

0.04

0.06

0.08

0.10

C12-2-12

C12-2-16

C12E23

[Py

ren

e]

/mM

[Surfactant] /mM

Figure 5.2 (c) Variation of solubility of pyrene with a total surfactant concentration of

single surfactant systems

125

0 1 2 3 4 5

0.00

0.04

0.08

0.12

C12-2-12/C12E23

C12-2-16/C12E23

C12-2-12/C12-2-16

C12-2-12/C12-2-16/C12E23

[Py

ren

e]

/mM

[Surfactant] /mM

Figure 5.2 (d) Variation of solubility of pyrene with a total surfactant concentration of

equimolar binary and ternary mixed surfactant systems

126

It is also an obvious finding in Figure 5.2 that the solubilization capabilities of

cationic Gemini surfactant towards naphthalene and pyrene are all enhanced by adding

equimolar nonionic surfactant in both bi and ternary mixed systems. The nonionic

surfactant, with large POE groups which will have a stronger tendency to intercalate into

the Gemini cationics making large size micelles within the solution at PAHs, may be

solubilized. Among multi-component surfactant systems, Gemini cationic/cationic

system, has the least solubilizing power for both naphthalene and pyrene. However, it is

significantly improved upon adding nonionic surfactant, as observed in the case of

ternary systems.

What is worth noting is that C12-2-12/C12-2-16 is found to be a superior solubilization aid

for pyrene, relative to that of C12-2-12 Gemini surfactant, which is contrary for

naphthalene. This phenomenon is presumably attributed to the major role of

solubilizate’s chemical nature. It was suggested by Moroi (Moroi, 1992) that the

solubilization of naphthalene is believed to occur at the micelle-water interface.

Therefore, in the case of the present study, the specific surface area of micelle decreases

with the formation of large micelle by intercalating surfactant processing relatively long

carbon chain lengths, bringing about the depressed interact area of solute and micelle-

water interface. Thus the solubility of naphthalene becomes less. Pyrene, on the contrary,

is more hydrophobic in comparison with relatively polar naphthalene, and therefore,

preferable to incorporate in the palisade layers of the micelles and interact with the

hydrophilic group of Gemini cationic amphiphiles via the interaction between π-

electrons of arenes and the positive charges. As a consequence, C12-2-12 amphiphiles are

incorporated by long carbon chain length C12-2-16 amphiphiles generating a larger

127

dimension palisade layer within the micelles which are conducive to more pyrene

molecule solubilized.

A measure of the solubilization capacity of a particular surfactant, for a given

substance, is known as the molar solubilization ratio (MSR), which is characterized as

the number of moles of compound solubilized by one mole of micellized surfactant

(Edwards et al., 1991). MSR can be expressed via equation:

( - ) / ( - )ac cmc acMSR S S C CMC (5.8)

which is obtained from the slope of the linearly fitted line that is plotted when the

concentration of solute is against the surfactant concentration above the CMC in the

presence of excess solute. Sac is the total apparent solubility of solute in given surfactant

solutions, at a specified surfactant concentration Cac (a surfactant concentration greater

than the CMC at which Sac is evaluated), and Scmc is the apparent solubility of solute at

the CMC, which is taken as their water solubility, since it changes only very mildly up to

the CMC of the surfactant. Earlier researches gave the solubility of naphthalene and

pyrene in pure water as recorded in Table 5.1. All concentrations are expressed in

millimoles per liter. The effectiveness of solubilization can also be quantified in terms of

the micelle-water partition coefficient (Km) which is defined as the distribution of the

mole fraction of solute between surfactant micelles and aqueous phases and has been

also calculated

The solubility data of two PAHs from all the surfactant systems, studied herein, are

tabulated in Table 5.3. The difference in the Gemini cationic surfactant hydrophobic

chain length produces a distinction in MSR and Km values, (significant values were

observed in pyrene). Meanwhile, the two types of Gemini/nonionic values are found to

128

be higher than Gemini/cationic surfactants for both naphthalene and pyrene, indicating

that with the same hydrophobic chain length, nonionics bear a higher solubilizing power

for the given solutes due to its larger micellar size, assisting in more micellar core

solubilization (Mehta et al., 2009).

Here, the solubilizing efficiencies of mixed systems are mostly in between that of

single surfactant, which is in tune with CMC analysis. A comparison of C12-2-12/C12E23

and C12-2-16/C12E23 shows that the later exhibits larger values of MSR and Km , in contrast

with the former. The behavior is due to the low CMC, as well as the larger micellar size

of C12E23 intercalating into Gemini C12-2-16 with a long hydrophilic chain length, resulting

in a larger effective solubilization area in mixed micelles. Simultaneously, the two values

of C12-2-12/C12-2-16 for the studied PAHs are appreciably increased by adding C12E23 in the

case of a ternary system. This is directly related to the higher micellar core solubilization

characteristic of nonionic surfactant, in addition to the micelle-water interface adsorption.

Therefore, MSR and Km of multi-component surfactant systems, for both naphthalene and

pyrene, follows the order of C12-2-12/C12-2-16 < C12-2-12/C12-2-16/C12E23 < C12-2-12/C12E23 <

C12-2-16/C12E23, which is the same as that of β values for various surfactant systems.

Meanwhile, their rise is coupled with an increase Log Kow of PAHs (naphthalene <

pyrene), in accord with previous findings (Li and Chen, 2002). Nevertheless, there are

substantial differences in MSR values between naphthalene and pyrene, yet relatively

less deviation between Km values, as displayed. Presumably, the palisade layer of micelle

account for solubilizing pyrene, and thus Km might be approximately comparative to the

nonpolar part of the micelle. This has been found to support Kile, et al. (Kile and Chiou,

dict://key.0895DFE8DB67F9409DB285590D870EDD/simultaneously

129

1989) in that Km values could be moderately related to the nonpolar part of cationic,

anionic and nonionic surfactant.

To further evaluate the mixing effect of selected multi-component surfactant systems

on solubilization of naphthalene and pyrene and observing the nature of deviation, the

parameter R could be proposed via the following equation (Zhou and Zhu, 2004):

exp / idealR MSR MSR (5.9)

MSRideal is the MSR for solute in a mixed surfactant system at the ideal mixed state and

can be determined using the MSR of single surfactant solutions, based on the ideal

mixing rule:

ideal i i

i

MSR MSR X (5.10)

where MSRi and Xi are the i-th experimental MSR value of solubilizate and the mole

fraction in multi-component surfactant solutions, respectively. As for the majority of

mixed surfactant systems, it is visibly observed from Table 5.3 that R is close to 1,

demonstrating solubilization follows almost ideal behavior with a small deviation. The R

values of C12-2-16/C12E23 system are greater than 1, implying its favorable mixing effect

on solubilization and positive deviation of MSRexps from the ideal mixing rule regarding

both naphthalene and pyrene. Additionally, the largest R value (1.9114), is the

embodiment of the positive mixing effect of C12-2-12/C12-2-16 in the case of solubilizing

pyrene, further demonstrating that the solubilization abilities not only depend upon the

changed surfactant micellar microstructures (packing of surfactant amphiphiles in the

mixed micelle core as well as at the micelle-water interface) when mixed micelles

130

formed, but also upon the chemical nature of solubilizate and their interaction with

mixed micelles.

In the present study, the standard free energy change of solubilization, 0

sG (Rangel-

Yagui et al., 2005) during the process of PAH molecules incorporating into chosen

mixed micelles, was all negative, thus, demonstrating the solubilization capacities of

studied mixed micelles can be regarded as a normal partitioning of the PAHs between

the micellar and aqueous phases, (i.e. spontaneous solubilization). The larger 0

sG values

of the surfactant systems designate that the behaviors of solubilizing naphthalene and

pyrene are energetically more complementary owing to the combined effects discussed

above.

131

Tab

le 5

.3 M

ola

r so

lub

iliz

atio

n ra

tio (

MS

R),

mic

elle–

wat

er

par

titi

on

coeff

icie

nt (

Km

), d

evia

tio

n ra

tio (

R)

and f

ree

ene

rgy

of

solu

biliz

atio

n (

0 sG

)

of

nap

hth

ale

ne a

nd p

yrene

in v

ario

us

single

and

eq

uim

ola

r b

i and

ter

nar

y m

icellar

sys

tem

s at

25

◦C

a

Com

po

und

S

urfa

ctant sy

stem

M

SR

L

og

Km

0 sG

(k

J M

-1)

R

C

12

-2-1

2

0.1

677

5.9

97

-3

4.2

4

C

12

-2-1

6

0.2

907

6.1

93

-3

5.3

5

C

12E

23

0.3

953

6.2

93

-3

5.9

2

Nap

htha

lene

C

12

-2-1

2/C

12

-2-1

6

0.1

427

5.9

37

-3

3.8

9 0.8

193

C

12

-2-1

2/ C

12E

23

0.3

065

6.2

11

-3

5.4

5 0.8

310

C

12

-2-1

6/ C

12E

23

0.3

844

6.2

84

-3

5.8

7 1.1

798

C

12

-2-1

2/C

12

-2-1

6/C

12E

23

0.3

043

6.2

08

-3

5.4

4 0.8

197

Pyr

ene

C12

-2-1

2

0.0

065

4.6

51

-2

6.5

5

C12

-2-1

6

0.0

178

5.0

83

-2

9.0

2

132

C12E

23

0.0

304

5.3

10

-3

0.3

1

C12

-2-1

2/C

12

-2-1

6

0.0

184

5.0

97

-2

9.1

0 1.9

114

C12

-2-1

2/ C

12E

23

0.0

264

5.2

51

-2

9.9

7 0.9

675

C12

-2-1

6/ C

12E

23

0.0

297

5.3

00

-3

0.2

6 1.2

596

C12

-2-1

2/C

12

-2-1

6/C

12E

23

0.0

252

5.2

31

-2

9.8

6 0.9

130

a E

rro

r lim

its

in t

he m

easu

rem

ent

of

MS

R,

Lo

g K

m a

nd

0 sG

ar

e ±

2.8

%,

±3.4

% a

nd ±

1.9

%, re

spec

tively

.

133

5.4 Summary

The interfacial and micellar behavior in a mixture of equimolar bi and ternary

Gemini cationic and conventional nonionic surfactant systems were studied in the hope

of finding synergism with which to gain better insight into the mix effect of multi-

component surfactant in an aqueous solution. Then the results of solubilization assays

show that the solubilities of naphthalene and pyrene increased linearly over the range of

multi-component surfactant concentrations above the CMC, which illustrates the

potential capacity of cationic Gemini/conventional nonionic mixed surfactants to

enhance the solubility of PAHs in water. Meanwhile, the tensiometric and solubilization

data manifest that the greater the synergistic interaction among each component of bi and

ternary mixed surfactants, which results in a greater negative deviation of the CMC from

an ideal mixture, and then the mixing effect of multi-component surfactant systems on

solubilizing power towards naphthalene and pyrene becomes stronger. However, MSR

and Km values exhibit a notable difference between the solubilization for two different

PAHs, demonstrating the solubilization behavior is not only dependent upon the

structure and mixing effect of surfactant but also associates with solubilizing

microenvironment and the chemical nature of organic solutes. The experimental results

of this study demonstrate a clear understanding of the solubilization capabilities of

equimolar bi and ternary Gemini cationic and conventional nonionic surfactant systems

and extend the surfactant selection used in solubilization for soil and water

contamination treatment.

134

CHAPTER 6

IMPROVED SOLUBILITIES OF PAHS BY MULTI-COMPONENT

GEMINI SURFACTANT SYSTEMS WITH VERIOUS SPACER

LENGTHS

6.1 Introduction

Surfactant solutions, as used in practical working scenarios, usually contain more

than one species of amphiphiles, possess unusual physicochemical properties that deviate

strikingly from what would be expected of pure components, and show synergistic

interactions. The investigation of such properties, of a known molecular structure and

composition, is of importance since it not only provides detailed information with which

to predict the nature of complex mixed surfactant systems but also can be significantly

favorable to the natural environment because it allows minimal amounts of surfactants to

be released and their impact to be appreciably depressed (Tikariha et al., 2011). A great

deal of research has been conducted to investigate a variety of multi-component

surfactant systems and find synergism in scientific and industrial applications (Kazuyuki,

2003; Dar et al., 2008). An essential idea is to include mixed ionic surfactants with the

same charge (e.g., in cosmetic products, the mixing of an ionic surfactant with another

surfactant possessing the same charge, is common) (Kazuyuki, 2003). However, less

information regarding the micellization and solubilization aspects of Gemini mixed

systems exists, and a more detailed relationship of the structure and properties of these

micelles are not yet clear.

135

Now, new classes of amphiphilic molecules have been drawing considerable

attention owing to their wide applicability and increased commercial uses. One is the

Gemini or dimeric surfactant. The aforementioned surfactants are expected to have

manifold applications in the field of detergent, cosmetic, soil remediation, oil recovery

and drug entrapment, etc. (Bai et al., 2001). The alkanediyl-α,ω-bis (alkyl

dimethylammonium bromide) type Gemini surfactants, with the structure of

[CmHm+1(CH3)2N(CH2)sN(CH3)2-CmHm+1]Br2 (m and s indicate the numbers of carbons

in the side alkyl chain and the spacer) are being extensively investigated due to their

unusual properties such as lower CMC (critical micelle concentration), stronger

biological activity, wetting, foaming, and lime-soap dispersing, which are superior to

those of analogous single-chain surfactants (Zana, 2002b). As to practical purposes,

these surfactants are currently being studied for their possible combination with

conventional surfactants since the presence of two charged sites in a Gemini provides a

stronger interaction with other ionic surfactants than those of the conventional surfactant

combinations (Sheikh et al., 2011). However, relative studies of Gemini surfactant and

mixtures with the same charge are still scarce.

This study is concerned with the results of experimental investigations regarding

micellar and interfacial properties such as CMC, maximum surface excess at the

air/water interface (Гmax) and minimum area per surfactant molecule at the air/water

interface (Amin) of single, equimolar bi and ternary cationic Gemini surfactants, which

have identical tails but differ in the size of the headgroups. Polycyclic aromatic

hydrocarbons (PAHs), naphthalene and pyrene, which have low water solubility and

various physicochemical properties, were used as solute to evaluate the solubilization

136

aspects of selected surfactant systems via measurement of the molar solubilization ratio,

the micelle–water partition coefficient, the deviation ratio and the free energy of

solubilization. Such fundamental and extensive investigations will help with predicting

properties of mixed Gemini micelles with the same charge and mixing effect on the

solubilization capabilities that will extend the scope of surfactant-enhanced engineering

applications.


6.2.1 Materials

Cationic Gemini surfactants C12-2-12, C12-3-12, C12-4-12 were obtained from Chengdo

Organic Chemicals Co., Ltd., Chinese Academy of Science, with a purity of 95%.

Naphthalene and pyrene, the two and four-ring PAHs were all Aldrich products and used

as received (98%). The structure of the chosen surfactants and PAHs is illustrated in

Scheme1. The desired mole fractions were obtained by mixing pre-calculated volumes of

the stock solutions and the following experimental procedures were performed.

137

Scheme 6.1 Molecular structure of (a) selected Gemini surfactants, (b) naphthalene; (c)

pyrene

(a)

CMCexp

S=2 0.81

S=3 0.91

S=4 1.05

(b)

(c)

MW(g M-1): 128.2 202.3

Sw (M): 2.5×10-4 6.7×10-7

Log Kw: 3.36 5.18

N

NBr-

(CH2)s

Br-

138

6.2.2 Methods

6.2.2.1 Tensiometric measurement

The surface tension was measured employing the Wilhelmy plate method using a

Model K11MK3 surface tension meter, manufactured by Krüss. In each experimental

run, the concentration of varied dilution series were prepared by adding stock solutions

progressively to the known volume of water in the glass vessel with the help of a

microsyringe in small installments. The surfactant solutions were allowed to mix

thoroughly and equilibrate for approximately 3 hours at 25±0.5 ◦C and maintained by a

circulating thermostatic bath. The plate was cleaned with alcohol and deionized water

and subsequently, subjected to heating over an alcohol flame. Each experiment was

repeated several times until good reproducibility was achieved. The results were accurate

within ±0.1 mN m-1. The CMC values of all selected surfactant systems are illustrated in

Figure 6.1 and listed in Table 6.1.

6.2.2.2 Conductivity measurement

The conductivity of solutions was measured at 25±0.1 ◦C employing a Mettle Toledo

conductivity meter equipped with a dip-type cell of cell constant 0.57 cm-1. The

concentrated stock solution was kept higher than CMC values. Then, all the solutions

were made progressively by adding an appropriate volume of suitably prepared stock

solution to double distilled water having conductivity in the range of 2-3 μs. Then, they

were equilibrated at the desired temperature for at least 30 minutes before being

139

measured. Each experiment run was repeated three times after thorough mixing and

temperature equilibration, and the error was minimized to ±0.2 K.


Aqueous solutions of single and equimolar bi and ternary combinations of cationic

Gemini surfactants (C12-2-12/C12-3-12, C12-2-12/C12-4-12, C12-3-12/C12-4-12 and C12-2-12/C12-3-

12/CC12-4-12) were prepared to cover a suitable range of concentrations above their CMC.

Excess amounts of naphthalene and pyrene were separately added to 10 ml of these

surfactant solutions in borosilicate glass vials to ensure maximum solubility. Then, they

were shaken for 48 hours on a thermostatic shaker, maintained at a temperature of

25±0.5 ◦C (Previous rate study revealed that 48 hours was supposed to be sufficient for

the concentrations of PAHs to reach equilibrium). Triple vials were run for each

concentration, and five or more concentrations were tested for each surfactant. After this,

subsequent centrifugation at 5000 rpm for 30 minutes was conducted to remove the

undissolved solid solute. A concentration of solubilized naphthalene and pyrene was then

detected with a Varian spectrophotometer (Cary 300) by measuring the absorbance at

220 nm and 334 nm wavelength. The concentration of surfactant was kept the same in

both the reference and the measurement cells to eliminate the effect of surfactant when

determining solubility. The typical error in the determination was less than 5%.

140

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4

30

35

40

45

50

55

60

C12-2-12

C12-3-12

C12-4-12

/m

N m

-1

Log ([Surfactant] /mM)

(a)



141

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2

30

40

50

60

C12-2-12/C12-3-12

C12-2-12/C12-4-12

C12-3-12/C12-4-12

C12-2-12/C12-3-12/C12-4-12

/m

Nm

-1

Log ([Surfactant] /mM)

Figure 6.1(b) Variations of γ values of equimolar bi and ternary mixed surfactant systems

over a wide concentration range at 25 ◦C

142


6.3.1 Critical micelle concentration properties and surfactant-surfactant

interactions

The selected Gemini surfactants possess twin hydrophobic chains of 12 carbons and

ionic head groups connected with a flexible spacer of 2 to 4 carbons, as Scheme 1

illustrated. The hydrophobic/hydrophilic nature of the spacer groups can influence the

properties of the pure and mixed Gemini surfactants, presumably because of the

modification of mobility and packing of surfactant monomers with the aggregate

(Tikariha et al., 2011). Table 6.1 summarizes the values required to judge whether a

synergism or an antagonism exists. As shown, the obtained CMC values increase with

augmentation in the spacer length of the selected Gemini surfactant and are

complementary with the literature values (Atkin et al., 2003; Duval et al., 2006).

In the present study, the spacer length of Gemini surfactant is an inhibitory factor for

micellization and hydrophobic interactions, contrary to that of the alkyl chain length

(Wei et al., 2011a), i.e., the Gemini surfactant with a longer spacer length has less of a

tendency to form micelles with larger surface curvatures than that with a shorter spacer.

The effect of spacer length on the variations of CMC is seemingly on account of the cis

conformation of Gemini surfactant monomers in solution, allowing for the altering of the

intramolecular interactions between the alkyl chains of the molecule (Sheikh et al., 2011).

However, this unique behavior of Gemini surfactant is only observed up to a spacer

length of 5 (Zana et al., 1991).

143

The intramicellar interactions in the Gemini surfactant mixtures are also studied by

means of critical micelle concentration, which is used to find various micellization

parameters to elucidate the difference of certain physicochemical properties before and

after the micellization mixing process. The obtained CMC values (CMCexp) of equimolar

bi and ternary surfactant mixtures are intermediate between those of individual

components, as observed herein (Table 6.1). The components are dissimilar with the

previous findings which suggest Gemini mixtures had lower CMC values than the

constituent surfactants (Kabir-ud-Din et al., 2010b). The results might be due to the

shorter spacer and hydrophobic tails of the Gemini surfactants of studied systems. All

CMCexp values are lower than estimated ideal CMC’s (CMCideal) as predicted by the Clint

equation (Clint, 1975):

31 2

1 2 3

1+ +

idealCMC CMC CMC CMC

(6.1)

where αi, CMCi are the stoichiometric mole fraction, and the CMC value of the

respective components in mixed micelle. This equation is useful as a comparison

between ideal (predicted) and nonideal (experimental) mixtures. As presented in Table

6.1, the observed CMCexps are lower than CMCideals suggesting that the formation of

mixed micelles exhibits negative deviation from ideal behavior for all studied mixed

surfactant systems. The negative deviation signifies the presence of nonideality due to

mutual attractive interactions of components in the mixed micelles and superior tendency

for micellization over pure surfactants. However, this equation only indicates the

nonideality of multi-component surfactant systems, studied herein. Therefore, the

interaction parameter βm is proposed to estimate the degree of negative deviation

dict://key.0895DFE8DB67F9409DB285590D870EDD/attribute%20to

144

between the CMCexp and CMCideal. It also reveals the nature and strength of the

interaction between two surfactant molecules in the mixed micelle which can be created

according to the Rubingh’s equation (Rubingh, 1979):

12 1 12 2

1 1 2 2m

2 2

1 2

ln ln

(1 ) (1 )

m m

m m

CMC CMC

CMC X CMC X

X X

(6.2)

21 12

1

1 1

2 1 121

1 2

ln

1(1- )

(1- ) ln(1- )

m

m

m

m

CMCX

X CMC

CMCX

X CMC

(6.3)

where subscripts 1, 2 and 12 refer to surfactant 1, 2 and their binary mixture, respectively;

α denotes bulk mole fraction, and mX represents the micellar mole fraction of

components in the mixed system. The values of

mX are solved by an iterative calculation

method and are substituted into equation (2) to find βm.

Holland and Rubingh (Holland and Rubingh, 1983) also forwarded a multi-

component nonideal mixed micelle model on the basis of pseudo-phase separation

approach that deals with the mX value for ternary surfactant system:

-12

1 1 1( ) ( )

ln ( - )jn n

m m m m m m m m

i ij j ij ik jk j k

i j ki j i j k

f X X X

(6.4)

where m

ij , m

ik and m

jk designate the net (pairwise) interaction between two components

which can be obtained independently from binary mixtures. m

iX , m

jX and m

kX are the

mole fractions of i-, j- and k-th component in the multi-component mixture system. At

the CMC, the relation

145

m m

i j j jm

i m

i j i

C f XX

C f

(6.5)

holds where Ci and Cj denote CMCs of the i- and j-th components, respectively.

By applying the knowledge of the average interaction parameter ( 12

m , 23

m and 13

m ),

as given in Table 6.1, the results of the ternary system, calculated with the help of

MATLAB, are tabulated in Table 6.1. A negative value of βm implies a synergistic

interaction, whereas, a positive value means an antagonistic interaction and for ideal

mixing, βm assumes a value of zero. A larger negative βm value designates the greater

negative deviation of CMCexp from CMCideal.

In the case of all binary surfactant systems, the βm values, though small, are negative,

demonstrating synergism in between the two components in the mixed micelle formation

rather than the self- interaction of the individual components before mixing which

depends upon both the interaction and relevant properties of individual surfactant

mixtures. This negative deviation from ideal behavior for the positively charged

surfactant mixtures is consistent with the results for conventional cationic-cationic (Dar

et al., 2008). However, the values are relatively smaller as compared with previous

reported Gemini cationic-nonionic (Wei et al., 2011a), indicating their almost ideal

mixed state. As all the hydrophobic tails of component surfactants are identical,

synergism is expected in the hydrophilic-groups packed in the micelle solutions. In

addition to this, some water molecules may be shared as a bridge below the

water/micellar interface by head groups and hydrophobic chains, and thereby the

attractive interactions will ensue (Sheikh et al., 2011), which contributes to further

negative βm values.

146

Tab

le 6

.1 E

xperi

ment

al cr

itic

al m

icelle

conc

ent

ratio

n (C

MC

exp),

idea

l cr

itic

al m

icelle

conc

entr

atio

n (C

MC

mi), in

tera

ctio

n

par

am

ete

r (β

m)

and m

icellar

mo

le f

ract

ion

(m i

X )

of eq

uim

ola

r su

rfac

tant

mix

ture

s at

25

◦C

a

a T

he a

ver

age

err

ors

in t

he C

MC

exp,

βm

and

Xm

are

±4.2

%,

±3.4

% a

nd 3

.8%

, re

spec

tive

ly.

Sur

fact

ant sy

stem

C

MC

exp

(m

M)

CM

Cm

i (m

M)

βm

12

3/

/m

mm

XX

X

C12

-2-1

2/C

12

-3-1

2

0.8

3

0.8

6

-0.1

793

0.5

328/0

.4671

C12

-2-1

2/C

12

-4-1

2

0.8

6

0.9

1

-0.2

49

0.5

574/0

.4598

C12

-3-1

2/C

12

-4-1

2

0.9

0

0.9

3

-0.1

511

0.5

074/0

.4822

C12

-2-1

2/(

C1

2-3

-12/C

12-4

-12)

0.8

526

-0

.0840

0.5

464/0

.4744

C12

-3-1

2/(

C1

2-2

-12/C

12-4

-12)

0.8

890

-0

.0918

0.4

845/0

.5213

C12

-4-1

2/(

C1

2-2

-12/C

12-3

-12)

0.9

271

-0

.2742

0.4

501/0

.5311

C12

-2-1

2/C

12

-3-1

2/C

12

-4-1

2

0.8

7

0.9

163

-0

.1499

0.2

834/0

.3370/0

.3796

147

The data in Table 6.1 clearly show that the extent of deviation, as indicated by the βm

values, are in the following order: C12-2-12/C12-4-12 > C12-2-12/C12-3-12 > C12-3-12/C12-4-12,

demonstrating that the binary Gemini micelles having a greater difference in spacer

length, are found to be more stabilized than those having a minor difference in spacer

length. A smaller extent of deviation in the case of C12-3-12/C12-4-12 denotes that the

components have a tendency to form almost similar shaped mixed micelles, probably

due to some flexibility in their spacers (Kabir-ud-Din et al., 2010b).

The βm of ternary mixture C12-2-12/C12-3-12/C12-4-12 is estimated by the average value of

three designed combinations, as exhibited in Table 6.1. Two hypotheses are based on the

pseudobinary treatment of these three combinations: a) the first one is regarded as

component 1 and the other two components are treated as component 2; b) the CMCexp

value of binary surfactant system is designated as CMC value of component 2. In the

present case, Xm and βm are observably different produced by the way of pairing the three

components. The calculated βm values of different combinations are various which might

because of the distinction of CMC values and proportions of each cationic Gemini

surfactant studied herein. Apparently, all βm values of ternary and pseudobinary systems

are negative, demonstrating a synergistic interaction of the three components in mixed

micelles. Moreover, the least βm value of ternary mixtures suggests an inferior ability of

reduction in free energy of micellization over that predicted by the ideal solution theory

(Kabir-ud-Din et al., 2009a). Analyses of Xm values by Rubingh suggest the

predominance of cationic Gemini surfactant with a shorter spacer within the mixed

micelles, which would result from the CMC properties, i.e., the one with the lower CMC

value has a relatively higher capability to micellize in surfactant mixtures.

dict://key.0895DFE8DB67F9409DB285590D870EDD/hypothesis

148

6.3.2 Counterion binding

Counterion binding can be revealed from breaks in conductivity versus the

concentration of surfactant isotherms, indicating the onset of micellization. As

conductance-concentration profiles depict, the conductance of surfactant solution is

found in a clear increase trend at different rates below and above the CMC, both in pure

and all mixed media. Ionic surfactant micelles are known to behave as strong electrolytes

and dissociate entirely at low concentrations, whereas, a relatively low rate of increase of

conductance might be attributed to binding some of the counter ions to the micelles

causing a reduction in the effective charge on the micelles (Kabir-ud-Din et al., 2010b)

and thereby stabilizing the self-aggregate of surfactant systems and lower the

intermolecular repulsion potential. The fraction of counter ions (Br-) dissociated from

the micelles (g2) is evaluated from the ratio of post- (S2) and pre-micellar (S1) slopes,

which is obtained from the plots of specific conductance of the surfactant solution at

various concentrations (S2 is always higher than S1) (Evans, 1956). Thus, the fraction

bound would be g1 = (1-g2), termed as “apparent counter ion binding”.

The g1 values for both single and equimolar mixed media are given in Table 6.2.

Generally, there is a fair decrease of the counterion binding value with the increased

number of carbon in the spacer of pure Gemini surfactant. This may be caused by the

packing of head groups, i.e., the less spacer length resulting in the closer space of two

head groups so that charge density becomes higher, leading to greater counterion binding.

149

Tab

le 6

.2 S

urf

ace

exce

ss c

onc

entr

atio

n (Г

max),

min

imum

are

a per

mo

lecule

(A

min

), c

oun

ter

ion

bin

din

g (

g1)

and f

ree

ene

rgie

s o

f

mic

elliz

atio

n (

0 mG

)

of si

ngle

and

eq

uim

ola

r m

ixed

sur

fact

ant sy

stem

s in

aq

ueo

us

med

ium

at 25 ◦

C a

Sur

fact

ant sy

stem

10

6 Г

ma

x (M

m−

2)

Am

in (

Å2)

g1

0 mG

(

kJ

M−

1)

C12

-2-1

2

1.5

4

107.6

6

0.7

9

-31.5

7

C12

-3-1

2

1.3

4

123.2

0

0.6

9

-29.2

7

C12

-4-1

2

1.0

0

165.8

9

0.5

5

-26.7

2

C12

-2-1

2 /C

12

-3-1

2

1.4

9

110.7

6

0.7

5

-30

.76

C12

-2-1

2 /C

12

-4-1

2

1.3

4

123.3

5

0.7

7

-31

.06

C12

-3-1

2 /C

12

-4-1

2

1.1

0

150.2

8

0.7

1

-29.7

1

C12

-2-1

2/C

12

-3-1

2/C

12

-4-1

2

1.0

4

159.9

2

0.7

3

-30

.30

a.

The

ave

rage

err

ors

in

the

Гm

ax, A

min

, g 1

and

0 m

G

are

±1.8

%, ±

1.9

%, ±

1.5

% a

nd ±

2.8

%, re

spec

tive

ly.

150

In the case of an equimolar multi-component system, the counterion binding are

almost similar, but found to fall in between that of two components in the mixture which

is in tune with the CMC. This behavior could be attributed to bimodal distribution of

Gemini surfactants in their aggregates. Owing to the major proportion of C12-2-12 in the

C12-2-12/C12-4-12 mixed systems, its comparatively higher charge density contributes more

to the binary systems leading to a higher counterion binding value. Similarly, C12-3-12/C12-

4-12 possesses the lowest g1 value among the surfactant mixtures owing to the amplified

average equilibrium distance between surfactant head groups. Diminished surface charge

density of the micelles, with increased molecular heterogeneity of mixed surfactant

systems, might take place due to an increased micellar size by interamphiphile repulsive

interactions (Kabir-ud-Din et al., 2009b). Therefore, the variation in the counterion

binding behavior of bi and ternary mixtures can be ascribed to differences in spacer

length of components in the mixtures, which decide the head group’s distance and hence,

the charge density of mixed micelles.

The standard free energy of micellization per mole of ionic monomer unit (0

mG ) is

related to the g1 and CMC by applying equation (Murphy and Taggart, 2002):

0

1(1 ) lnmG g RT CMC (6.6)

This can be applied to both singular and mixed species of surfactant systems. According

to the results presented here, it would seem that 0

mG values become less negative with

an increased length of pure Gemini surfactant, indicating that micellization is less

spontaneous. As for the binaries, C12-2-12/C12-4-12 has major value due to the relatively

large distinction of spacer length of individual components.

151

0.5 1.0 1.540

80

120

160

200

240

(a)

C12-2-12

C12-3-12

C12-4-12

K /

s cm

-1

[Surfactant] /mM

Figure 6.2 (a) Variations of K values of single surfactant systems over a wide


152

0.5 1.0 1.5 2.050

100

150

200

250

(b)

C12-2-12/C12-3-12

C12-2-12/C12-4-12

C12-3-12/C12-4-12

C12-2-12/C12-3-12/C12-4-12

K /

s cm

-1

[Surfactant] /mM

Figure 6.2 (b) Variations of K values of equimolar bi and ternary mixed surfactant

systems over a wide concentration range at 25 ◦C

153

6.3.3 Properties at the air/water interface

Amphiphilic molecules are apt to orientate themselves at an air/water interface and

alter the surface properties of the water. To better understand the interaction behavior of

multi-component micelle systems, under the conditions of the present study, the Gibbs

adsorption equation is adopted in order to estimate the actual amounts of amphiphilic

adsorbed per unit area of surface area, at given concentrations. The maximum surface

excess (Гmax) and minimum area per molecule (Amin) of various single cationic Gemini

surfactants, as well as in their bi and ternary combinations, are calculated using the

Gibbs adsorption equation, and are listed in Table 6.2.

2

max

1 1- ( / ln ) - (mol/m )

2.303 lnT

dd d C

nRT nRT d C

(6.7)

220

min

max

10(A )

A

AN

(6.8)

where dγ/dlnC is the maximum slope of the surface tension versus the logarithm of a

different surfactant concentration below and above CMC; R, C, T and NA are gas constant,

concentration, temperature and Avogadro’s number, respectively. The correct prefactor n

is the number of chemical species at the interface whose concentration varies with the

change of surfactant concentrations. As for calculating Гmax in the present case, n=3

(Devínsky et al., 1986). The slope of the tangent at the given concentration near CMC of

γ vs. Log Ct is used to calculate Г by using differentiate in ORIGIN.

The maximum surface excess (Гmax) is an effective indicator of the altered

air/solution interface through surfactant adsorption that depends on the molecular

structures of surfactants. As Table 6.2 indicated, the Γmax values are in the following

154

order: C12-2-12 > C12-3-12 > C12-4-12, the same order as reported previously by others (Chen

et al., 2008). The Amin values of pure Gemini surfactants are in the reverse order C12-2-12 <

C12-3-12 < C12-4-12. The results indicate that Gemini surfactant with a shorter spacer has a

greater preference to be adsorbed at the air/water interface and is more compact. This

may be due to intramolecular headgroup distances.

The Γmax and Amin of equimolar bi and ternary surfactant systems fall between their

individual components. Such expansion behaviors are properly attributed to the

dissimilarity in the nature of interaction of hydrophobic groups in the mixed adsorbed

layer. The magnitude of Amin amplifies with the increased number of total carbon atoms

in spacer groups of mixed surfactant systems. This might be due to the increased

difficulty of incorporating Geminis with twin hydrophobic and hydrophilic groups into a

convex micelle relative to adsorption at a planar air/water interface (Li et al., 2001). The

greatest value of Amin, for the ternary mixtures, suggests that the air/water interface is

loosely packed, attributing to the difference in orientation of the monomeric tails at the

air-water interface caused by different spacer lengths.

6.3.4 Solubilization abilities of selected surfactant systems towards PAHs

The dependence of apparent solubilities on surfactant concentrations, in the presence

of single as well as equimolar bi and ternary mixed surfactants, is plotted in Figure 6.3,

respectively. The solubilities of PAHs are significantly enhanced by the selected single

surfactant systems where solubilities are increased linearly over the range of surfactant

concentrations above CMC. Similar trends can also be found in all mixed surfactant

155

systems, studied herein. These results demonstrate the formation of single, as well as

multi-component micelles and their potential capacity to elevate the solubility of PAHs

efficiently due to the adsorption at micellar water interface, in addition to the

solubilization inside the micelle.

The molar solubilization ratio (MSR) is known to be an effective assessment for

solubilization capacity equivalent to the number of moles of solute solubilized by one

mole of micellized surfactant (Edwards et al., 1991). It can be obtained from the slope of

a linearly fitted line in which the concentration of solute is plotted against the surfactant

concentration above CMC in the presence of excess solutes, using the linear regression

procedure. All concentrations, studied herein, are expressed in mM, as shown in Figure

6.3.

Another parameter of evaluating the effectiveness of solubilization is the micelle-

phase/aqueous-phase partition coefficient (Km), which is defined as Km=Xm/Xa, the ratio

of the mole fraction of solute in the micelle pseudophase (Xm) to that in aqueous

pseudophase (Xa). Km is a function of temperature and the nature of surfactant and

solubilizate (Kabir-ud-Din et al., 2009a), which can be calculated from

Km=MSR/[ScmcVw(1+MSR)] (Vw=1.805×10-2 L M-1).

The values of MSR and Km for naphthalene and pyrene, in all surfactant solutions, are

shown in Table 6.3, respectively. Usually, surfactants with less CMC values possess

advantageous solubilizing capacities. Here, due to spacer length differences, the MSR

and Km values of both naphthalene and pyrene are observed in order to amplify in the

following order: C12-2-12 < C12-3-12 < C12-4-12, appearing in contradiction to CMC orders.

156

1.5 2.0 2.5 3.0 3.50.00

0.01

0.02

0.03

0.04

0.05

0.06

Nap

hth

ale

ne /

mM

[Surfactant] / mM

C12-2-12

C12-3-12

C12-4-12

Figure 6.3 (a) Variations of solubilities of naphthalene with various surfactant

concentrations of single Gemini surfactant at 25 ◦C

157

1.5 2.0 2.5 3.0 3.50.00

0.01

0.02

0.03

0.04

0.05

0.06

Nap

hth

ale

ne /

mM

[Surfactant] /mM

C12-2-12/C12-3-12

C12-2-12/C12-4-12

C12-3-12/C12-4-12

C12-2-12/C12-3-12/C12-4-12

Figure 6.3 (b) Variations of solubilities of naphthalene with various surfactant

concentrations of equimolar bi and ternary Gemini surfactant systems at 25 ◦C

158

1.0 1.5 2.0 2.5 3.0 3.50.1

0.2

0.3

0.4

0.5

0.6

Py

ren

e /

mM

[Surfactant] /mM

C12-2-12

C12-3-12

C12-4-12

Figure 6.3 (c) Variations of solubilities of pyrene with various surfactant concentrations

of single Gemini surfactant at 25 ◦C

159

1.0 1.5 2.0 2.5 3.0 3.5

0.1

0.2

0.3

0.4

0.5

Py

ren

e /

mM

[Surfactant] /mM

C12-2-12/C12-3-12

C12-2-12/C12-4-12

C12-3-12/C12-4-12

C12-2-12/C12-3-12/C12-4-12

Figure 6.3 (d) Variations of solubilities of pyrene with various surfactant concentrations

of equimolar bi and ternary Gemini surfactant systems at 25 ◦C

160

Since the CMC values of C12-2-12, C12-3-12 and C12-4-12 are close, the solubilizing

phenomenon is presumably interpreted to the predominance and stability of micelles in

aqueous as compared with monomers absorbed at air/aqueous, i.e., the minor Γmax value

of a long-spacer surfactant indicates relatively enriched monomers in the micelles

facilitating greater solute solubilization.

Interestingly, it is also clearly defined that the longer the spacer length of constituted

surfactants, the higher the solubilization power of the binary systems. Therefore, the

sequence of synergistic solubilization of naphthalene and pyrene, by different equimolar

binary surfactant systems, is C12-2-12/C12-3-12 < C12-2-12/C12-4-12 < C12-3-12/C12-4-12, which is

also interpreted as the relationship between air/aqueous adsorption and aqueous micelle

formation. Particularly, C12-3-12/C12-4-12 exhibits significantly greater values of MSR and

Km. The observation is valid for both naphthalene and pyrene. Therefore, with the

increased carbon atoms of spacer groups for both single and mixed surfactants, the

air/water interface properties suppress the effect of CMC properties, which in turn,

increase the solubilization efficiency, studied herein. Hence, the solubilization properties

not only depend upon the CMC of surfactants, but also have a relationship with their

structures. As for a ternary surfactant system, no obvious relationship between the

air/water interface property and solubilization ability is found. It only shows intermediate

value between those of individual surfactant as same as binary surfactant systems.

Therefore, the ternary surfactant system may behave in a non-predicted way, requiring a

further evaluation.

161

Tab

le 6

.3 M

SR

, L

og

Km

, R

and

0 s

G

of P

AH

s in

var

ious

sin

gle

and

eq

uim

ola

r m

ixed

Gem

ini su

rfac

tant

syst

em

s at

25

◦C

a

Com

po

und

S

urfa

ctant sy

stem

M

SR

L

og

Km

R

0 sG

(

kJ

M-1

)

Nap

htha

lene

C12

-2-1

2

0.1

067

5.3

47

-30.5

2

C12

-3-1

2

0.1

411

5.4

48

-31.1

0

C12

-4-1

2

0.1

658

5.5

14

-31.4

7

C12

-2-1

2/C

12

-3-1

2

0.1

319

5.4

10

0.9

247

-3

0.8

8

C12

-2-1

2/C

12

-4-1

2

0.1

350

5.4

20

0.8

485

-3

0.9

4

C12

-3-1

2/C

12

-4-1

2

0.1

466

5.4

49

1.1

354

-3

1.1

1

C12

-2-1

2/C

12

-3-1

2/C

12

-4-1

2

0.1

077

5.3

34

0.7

559

-3

0.4

5

Pyr

ene

C12

-2-1

2

0.0

077

5.7

86

-33.0

3

C12

-3-1

2

0.0

189

6.1

94

-35.3

6

C12

-4-1

2

0.0

225

6.2

65

-35.7

6

162

C12

-2-1

2/C

12

-3-1

2

0.0

104

5.9

34

0.6

959

-3

3.8

7

C12

-2-1

2/C

12

-4-1

2

0.0

111

5.9

66

0.5

388

-3

4.0

6

C12

-3-1

2/C

12

-4-1

2

0.0

205

6.2

23

1.5

419

-3

5.5

2

C12

-2-1

2/C

12

-3-1

2/C

12

-4-1

2

0.0

096

5.9

13

0.6

062

-3

3.7

5

a T

he a

vera

ge e

rrors

in

the

MS

R,

Lo

g K

m a

nd

0 sG

a

re ±

4.6

%,

±3.9

% a

nd ±

3.2

%,

resp

ecti

vely

.

163

Meanwhile, the rise of MSR and Km values is coupled with an increase in the Log Kow

of PAHs (naphthalene < pyrene), in accord with previous findings (Li and Chen, 2002).

Additionally, there are substantial differences in MSR values between naphthalene and

pyrene, and yet relatively less deviation in Km values between them are displayed. Such

differences in the nature of the solubilization, probably reflects the location of the PAHs

in the micelles. It is assumed that, Km value is approximately comparative to the

nonpolar part of the micelle. Because pyrene is more hydrophobic when compared with

naphthalene, the palisade layer of micelle (nonpolar part of micelle) is accountable for

solubilizing pyrene, thus, resulting in the increasing value of Km. Also proven by Kile, et

al. (Kile and Chiou, 1989) Km values could be moderately related to the nonpolar part of

cationic, anionic and nonionic surfactant. However, the ternary mixed system presenting

less solubilization power is inconsistent with the above rules, indicating the micellar size

is not the main factor for observed variations in Km values for ionic surfactants.

According to the majority of the studied mixed systems, it can be visibly observed

that R values are close to 1 signifying solubilization follows almost ideal behavior with a

small deviation for naphthalene. The R value of C12-3-12/C12-4-12 system is greater than 1,

indicating a positive deviation of MSRexp from the ideal mixing rule and also implying it

is a favorable mixing effect in the case of solubilizing naphthalene and pyrene. In

comparing the two series, the Gemini cationic-cationic mixed systems has a relatively

large solubilization power towards pyrene, further indicating that the solubilization

capabilities not only depend upon the changed surfactant micellar microstructures

(packing pattern) as mixed micelles formed, but also on the chemical nature of organic

compound and their mutual interaction.

164

In the present case, the standard free energy change of solubilization, 0

sG (Rangel-

Yagui et al., 2005) was entirely negative, indicating the solubilization abilities of studied

multi-component micelles, could be considered a normal partitioning of the PAHs

between the micellar and aqueous phases, namely “spontaneous solubilization”.

6.4 Summary

A comprehensive and systematic study has been conducted regarding the

micellization, interfacial and solubilization properties of series single, equimolar bi and

ternary cationic Gemini surfactants with different spacer lengths. A thorough analysis of

such similarly charged mixed Gemini surfactant systems shows that each observed

CMCexp are intermediate between those of individual components and are lower than

CMCideal, suggesting synergism and the formation of mixed micelles exhibiting negative

deviation from ideal behavior. The synergism effect in terms of βm are in the following

order: C12-2-12/C12-4-12> C12-2-12/C12-3-12> C12-3-12/C12-4-12, indicating the greater the

difference in spacer length, the larger and more negative the value obtained for βm; and

hence, the nonideality is increased. Interestingly, the solubilization capacities of single

Gemini surfactant follow the revere order of CMCs, suggesting the monomers absorbed

at air/aqueous have a predominance effect since their CMC values are close. All studied

bi and ternary mixed surfactant systems showed intermediate solubilization values

between those of individual surfactants, expressed by MSR and Km values and coupled

with increased Log Kow of PAHs (naphthalene < pyrene) at the same time. Thus, the

solubilization power not only depends on surfactant micellar microstructures (packing

165

pattern) as mixed micelles formed by also has an association with

hydrophilic/hydrophobic properties of solutes. The results also illustrate the potential

capacity of mixed bi and ternary cationic Gemini surfactants to enhance of surfactant

assisted technologies.

166

CHAPTER 7

GEMINI SURFACTANT-ENHANCED ADSORPTION AND

DESORPTION OF PAHS IN A SOIL-WATER-SURFACTANT

SYSTEM

7.1 Introduction

Widespread contamination of soil and groundwater by polycyclic aromatic

hydrocarbons (PAHs) and other hydrophobic organic compounds (HOCs) has emerged

as one of the most concerning issues world-wide (El Nemr and Abd-Allah, 2003; Tang et

al., 2005; Zhang et al., 2007; Maliszewska-Kordybach et al., 2008). Knowledge of the

transport and fate of these contaminants and novel remediation strategies are therefore,

of great interest to related fields. Sorption and desorption are well known fundamental

processes in controlling the fate and transport of organic contaminants in the

environment (Huang et al., 2003). In some circumstances, contaminants are removed via

an enhanced desorption. In other circumstances, contaminants can be prevented from a

point source of pollution by improving the sorption ability of sorbate. Surfactants are

valuable products possessing both of the capabilities which have been widely used in the

fields of water, soil and sediment remediation technology (Rodríguez-Cruz et al., 2007;

Neupane and Donahoe, 2012; Uhmann and Aspray, 2012). At low concentrations,

contaminants are present as dispersed molecules (monomers). Above the critical micelle

concentration (CMC), surfactant monomers aggregate in solution to form micelles

(Rosen, 1989b; Wang and Keller, 2008)

167

Surfactants are known to alter the surface characteristics of solids to enhance the

water solubility of organic compounds or to create sorbents that immobilize organic

compounds. Over the last decades, surfactant-aided soil washing has been widely used to

treat HOCs contaminated soil and sediment ex situ (Peng et al., 2011; López-Vizcaíno et

al., 2012), because surfactant may enhance the solubility of HOCs in water even below

the CMC of the surfactant (Edwards et al., 1991), which contributes to the removal of

organic contaminants from soil or sediment. Recently, many researchers began to

employ surfactants to modify solids in order to remove contaminants from aqueous

media or inject them into an aquifer to create an in situ enhanced sorption zone which

would intercept a migrating HOCs plume, significantly retarding the transport of these

contaminants. For instance, conventional cationic surfactant can be retained by both the

outer and inner layer surfaces of clay via an ion-exchange process in aqueous systems

and are not easily displaced by smaller cations such as H+, Na+, or Ca2+ (Karapanagioti et

al., 2005). Cationic-surfactant-modified sorbent were assessed and proposed for the

retardation of a variety of compounds. The systems demonstrated chemical and

biological stability and were pilot tested as a permeable barrier for groundwater

remediation (Li et al., 1998). This suggests the potential utility of the modified soils for

treatment of contaminated waters and as components of containment barriers (i.e., in

slurry walls, hazardous waste landfills, and petroleum) (Rodríguez-Cruz et al., 2007; Seo

et al., 2009; Neupane and Donahoe, 2012). However, most investigated surfactants are

conventional surfactants, which consist of single monomers with a single hydrophobic

core and a single hydrophilic shell.

168

Presently, a new generation of surfactant, called “Gemini surfactants”, have been

synthesized and have attracted the attention of various industrial and academic research

groups (Shukla and Tyagi, 2006). They form micelles at much lower concentrations than

the conventional surfactants, and also tend to aggregate at inter faces far more readily

(Neupane and Park, 2000). They have been shown to be more surface active than

monomeric surfactants at the same molar mass concentration. The application of Gemini

surfactants has been extensively explored. The most notable application is their

modification function in the remediation of contaminated water, soil or sediment.

Neupane et al. (Neupane and Park, 2000) have proposed the use of anionic Gemini

surfactant to treat alumina for the removal of HOCs from the aqueous phase. Adsorption

of some cationic Gemini surfactants onto silica has been investigated (Meziani et al.,

2003; Zhou and Somasundaran, 2009; Zheng et al., 2012). Moreover, studies relating to

HOCs partitioning into modified soil particles, formed by cationic Gemini surfactant, are

rather few at this time.

This study focused on the use of Gemini cationic surfactants to immobilize

polycyclic aromatic hydrocarbons onto soil particles, and is used as an example of an

innovative application to remove HOC in situ using the surfactant-enhanced sorption

zone. In this endeavor, three symmetric and dissymmetric Gemini surfactants and one

organic contaminant naphthalene were used, and specific work was under taken in detail,

as follows: 1) The sorption behavior of selected surfactant was evaluated; 2) The

sorption capacity of modified soils and natural soils was compared; and 3) The PAH

sorption efficiency, in the absence and presence of Gemini surfactant, was investigated.

By evaluating Gemini surfactant with different alkyl chain lengths and by looking at

169

both solubilization and sorption behavior, this work attempts to provide a broader

analysis of Gemini surfactant-modified soil sorbent.


7.2.1 Materials

The PAH employed was naphthalene, with a purity >98% (Aldrich products). Its

octanol/water partition coefficient (Log Kow) and water solubility are 3.36 and 34 mg L-1,

respectively (Wei et al., 2012). Gemini cationic surfactants C12-2-8, C12-2-12 and C12-2-16

were obtained from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Science,

having a purity of 95%. Selected physicochemical properties of surfactant are included

in Table 7.1. Concentrated PAH stock solutions were prepared in HPLC-grade methanol

and stored in the dark at 4 ◦C in an amber borosilicate bottle to minimize

photodegradation and/or volatilization. Fresh Gemini surfactant stock solutions were

prepared by dissolving the relevant surfactants in deionized water at room temperature.

Then, desired mole fractions were obtained by mixing pre-calculated volumes of the

stock solutions and the following experimental procedures were performed.

170

Table 7.1 The formula and experimental critical micelle concentration (CMCexp) of the

selected Gemini surfactants

Surfactant Molecular formula Molecular Weight

(g M-1)

CMCexpa

(mM)

C12-2-8 C12H25N

+(CH3)2 (CH2)3-N+(CH3)2

C8H17·2Br- 558.67 1.5

C12-2-12 C12H25N

+(CH3)2 (CH2)3-N+(CH3)2 C12H25·2Br-

614.79 0.87

C12-2-16 C12H25N

+(CH3)2 (CH2)3-N+(CH3)2 C16H35·2Br-

672.03 0.2

a Data for Gemini surfactants reported by Wei et al. (Wei et al., 2011a).

171

Table 7.2 Selected properties of the soil sample

Solid CEC

(cM kg-1) foc (%) pH Sand (%) Silt (%) Clay (%)

Soil 15.8 13.3 8.8 39.96 27.26 32.79

172

The soil sample of this study was a natural soil and was collected from the top (0-25

cm) of a contamination free area in Saskatchewan, Canada. It was transported back to the

laboratory in coolers, air-dried, crushed and passed through 2 mm mesh to remove

surface plant remains and coarse materials and was stored in closed containers in the

darkness of refrigerator. The soil pH value was measured in slurries made up at a 1:2.5

soil/water ratio. The Cation exchange capacity (CEC) was analysed employing a method

reported by Gao, B. et al. (Gao et al., 2001). The organic carbon of soil was determined

using a LECO TruPec CN determinator at the condition of 50% relative humidity and 25

◦C. The main soil properties were tabularized in Table 7.2.

7.2.2 Methods


Solubilization of naphthalene by Gemini surfactants was subsequently carried out in

batch mode. For each test, an excess amount of naphthalene was separately spiked into

each vial containing a series of 10 ml surfactant solutions having a range of

concentrations below and above the CMC to ensure maximum solubility. The sample

vials were sealed with a screw cap fitted with a Teflon lined septum to prevent any

volatilization loss of naphthalene from surfactant. Triplicate samples were prepared for

each surfactant concentration. The samples were then mechanically shaken end-over-end

for a period of 24 hours in a thermostatic chamber maintained at a temperature of 25±0.5

◦C. Following this step, the samples were subsequently centrifuged at 5000 rpm for 30

minutes to fully separate the undissolved naphthalene, at the same temperature. An

173

appropriate aliquot of the supernatant was then carefully withdrawn with a volumetric

pipette and diluted to 10 ml in flasks with 1 ml methanol and the rest with the

corresponding surfactant-water solution for naphthalene analysis.

7.2.2.2 Surfactant and PAHs sorption

Sorption of Gemini surfactants, as well as naphthalene onto soil from water in the

presence or absence of surfactants, was conducted using a batch equilibration technique

at different aqueous surfactant concentrations. A soil sample of 0.1 grams were weighed

into 20 mL capped glass vials, to which 10 ml of distilled water or a given concentration

of Gemini solution was added (based upon the estimated adsorption amount of surfactant

on the soil). A 0.01 M CaCl2 background electrolyte was used to minimize an ionic

strength change. To inhibit microbial growth, 0.01M NaN3 was used in all cases. The

initial surfactant concentration spanned a large range of values below and above the

nominal CMC of Gemini surfactant. Then, the samples were spiked with a known mass

of naphthalene prepared in methanol, and ensured to be lower than their water

solubilities. The content in methanol was about 2% in volume so that it could not have

an effect upon adsorption (Lee et al., 2004). Next, the capped tubes were placed in a

reciprocating chamber for 24 hours at 25±0.5 ◦C. Preliminary experiments showed the

sorption equilibrium of both surfactant and naphthalene were reached less than 24 hours

later. The aqueous phase was separated by centrifugation at 5000 rpm for 30 minutes in

the HERAEUS Multifuge X1R High Speed centrifuge after equilibration. Subsequently,

174

an appropriate aliquot of the supernatant was sampled for naphthalene and Gemini

surfactants analysis.

7.2.2.3 Analytical method

Cationic Gemini surfactant analysis was carried out, at room temperature, using a

G20 automatic titrator furnished with a 20 ml autoburette, a stirrer, a surfactant sensitive

electrode and a reference electrode. A 1 ml surfactant sample was added to 20 ml

distilled water and then was placed in a 100 ml titration vessel. The solution was titrated

with 4 mM SDS dropwise added from the burette at a rate of 10 ml min−1. The

intersection point in a titration was automatically distinguished by the titrator. The

concentration of naphthalene was detected by a Varian UV spectrophotometer (Cary 300)

using the peak at 220 nm. The typical error was less than 5% for solubilization

determination and 10% for adsorption determination.

The equilibrium concentration of surfactant and solute in solution was analyzed. The

amount of surfactant adsorbed onto the soil solid or the amount of pollutant adsorbed

onto the surfactant-treated soil solid is determined based upon the difference between the

initial mass added and the mass remaining in the solution and is given by:

-i ss

C C M VQ

G

（）

(7.1)

where

Qs=concentration of sorbed adsorbate (surfactant or pollutant) per gram of adsorbant,

in mg g-1;

175

Ci=initial concentration of adsorbate, in M;

Cs=equilibrium concentration of adsorbate, in M;

M= molecular weight, in g M-1;

V=volume of solution, in L;

G=weight of soil solid, in g;

As controls, duplicate blank samples were analyzed for each surfactant concentration.

The sorption of surfactant and naphthalene on the tubes was examined and found to be

negligible, and the amount of surfactant and naphthalene blank (with no soils) did not

show any significant change before and after mixing. Owing to the method’s high

sensitivity, all solute samples had to be diluted in order to obtain readings within the

linear calibration range. All data in the figures are presented as an average of the two

replicates.


7.3.1 Sorption of Gemini surfactant

Adsorption of three Gemini surfactants onto soil from double distilled water solution

is shown in Figure 7.1. Langmuir isotherms were used to interpret surfactant sorption on

selected soil (Rosen, 1989b). The equation (Karapanagioti et al., 2005) used is:

-1

max 1e L e L eeQ Q K C K C （） (7.2)

where Qe is the amount of surfactant sorbed per unit mass of sorbent at equilibrium, Ce is

the surfactant equilibrium concentration in solution, Qemax is the amount of surfactant

required to saturate the sorbent, and KL is the Langmuir constant.

176

0 2000 4000 6000 8000

0

50

100

150

200

C12-2-8

C12-2-12

C12-2-16

[Ad

sorb

ed

Am

ou

nt]

/m

g g

-1

[Equilibrium Concentration of Gemini Surfactant] /mg L-1

Figure 7.1 Surfactant sorption isotherms (Symbols: experimental data; dotted lines: fitted

Langmuir isotherms)

177

The lines in Figure 7.1 present the fitted Langmuir surfactant isotherms (Eq. 7.2) for

all surfactant-sorbent systems studied in the present work. All three selected Gemini

surfactants exhibit similar sorption behavior with sorption on the soils increasing sharply

with aqueous surfactant concentrations, until a plateau value is reached, and then stays

constant with further increases of the surfactant concentration in solution. As for the

three Gemini surfactant systems, different levels of surfactant are loaded onto the soil

surface in terms of surfactant mass adsorbed to the external surface area due to their

different CMC and molecular structures.

Table 7.1 presents the plateau values for the amount of surfactant sorbed (Qemax) and

the surfactant concentration in solution at which this plateau value (Cemax) is first

observed. The sorption saturation of all studied Gemini surfactants occurs when the

aqueous concentrations are at or a little above their CMC, manifesting that Gemini

monomers are mainly sorbed components on a solid surface.

It is well known that cationic surfactants are sorbed onto soil particles predominantly

through cation exchange interactions (Li and Bowman, 1998; Zhu et al., 2003). In this

study, the maximum amount (in mg g-1) of surfactant at the soil/aqueous solution

interface, Cemax, increases with the decreased length of one hydrophobic chain. The

plateau values of three isotherms correspond to 154.83, 119.47, 50.62 meq kg-1,

respectively, all below the CEC of the soil (158 meq kg-1), indicating that not all CEC

sites of selected soil are available for cationic exchange. In this study, three Gemini

surfactants, C12-2-8, C12-2-12 and C12-2-16 monomers, occupied approximately 97.99%,

75.61% and 32.04% of the total CEC soil sites.

178

The equilibrium surfactant concentration values, Ce, in the aqueous phase, required to

reach Cemax of the three Gemini surfactants are, respectively, much higher than their

individual CMC, almost double, indicating that some adsorption via the hydrophobic

group interaction also exists except for cation exchange mechanisms. The maximum

adsorption value for the three selected Gemini surfactants is in the order of C12-2-8> C12-2-

12> C12-2-16 because more Qemax is reached when high Cemax is exhibited, as it has a greater

CMC value. Obviously, it is easier for C12-2-16 to attain equilibrium with a relatively

small amount of added surfactant, whilst the amount of adsorbed C12-2-16 is minute.

Although C12-2-12 reaches its Qemax at lower Ce compared with C12-2-8, the adsorbed

amount seems to occur at higher concentrations in comparison with C12-2-16. Therefore,

the amount of surfactants in both solid and solution significantly influences their

mobilization and immobilization effect on organic contaminant.

7.3.2 Solubilization of naphthalene by Gemini surfactants

As well known, an important property of surfactants is their ability to enhance the

solubility of water insoluble molecules by trapping them in energetically favorable

microenvironments which also decides the application of surfactant in soil and water

remediation. In order to better understand the effect of selected Gemini surfactants on the

sorption of contaminant in the soil-water system, their solubilization capacity towards

naphthalene must be examined.

179

0 2 4 6 8

0.0

0.4

0.8

1.2

1.6

C12-2-8

C12-2-12

C12-2-16

Sw

* /

mM

[Surfactant] /mM

Figure 7.2 Water solubility enhancement of naphthalene as a function of the

concentration of the selected Gemini surfactants at 25 ◦C

180

The apparent water solubility of a solute in surfactant solution has been expressed as

(Kile and Chiou, 1989):

*

1wmn mn mc mc

w

SK X K X

S *

wS (7.3)

where *

wS is the apparent solute solubility at a surfactant total concentration of X; Sw is

the intrinsic solute solubility in “pure water”; Xmn is the concentration of the surfactant as

monomers in water (if X ≤ CMC, Xmn = X; if X > CMC, Xmn = CMC); Xmc is the

concentration of the surfactant as micelle in water (Xmc = X-CMC); Kmn is the partition

coefficient of the solute between the surfactant monomer and water; and Kmc is the solute

partition coefficient between the aqueous micellar phase and water (Kile and Chiou,

1989).

Figure 7.2 shows the apparent solubility of naphthalene in the presence of three

selected Gemini surfactants. Clearly, there is little or no solubility enhancement for

naphthalene below the CMCs of surfactants because monomers exist in solution under

this concentration forming a weak organic environment to facilitate the partition of

HOCs. While the obviously increased solubility of naphthalene begins with each

Gemini’s CMC, respectively, indicating that formed Gemini micelle has strong

solubilizing power. The molar solubilization MSR is characterized as the number of

moles of compound solubilized by one mole of micellized surfactant. It denotes the

effectiveness of a particular surfactant in solubilizing a given solute and can be

expressed as follows (Edwards et al., 1991):

-

-

ac cmc

ac

S SMSR

C CMC (7.4)

181

where Sac is the total apparent solubility of solute in given surfactant solution at a

specified surfactant concentration Cac (the surfactant concentration above CMC at which

Sac is evaluated) and Scmc is the apparent solubility of solute at CMC. In the presence of

excess naphthalene, MSR values of three selected Gemini surfactants could be obtained

from the slope of the linear fitted line in which the concentration of solute is plotted

against surfactant concentration above the CMC (surfactant concentration in mM vs.

phenanthrene concentration in mM) given in Figure 7.2. MSR signifies the extent of

solubility enhancements for naphthalene at all Gemini surfactant concentrations above

their CMCs following the order of C12-2-16 > C12-2-12 > C12-2-8, which reverse the order of

their monomer adsorption abilities. It is interrelated to the hydrocarbon chain length of

different Gemini surfactants.

Simultaneously, the micelle-water partition coefficient (Kmc L M-1) (Jafvert et al.,

1994) also calculated from the Sw and the obtained MSR values using the following

formula:

mc

w

MSRK

S (7.5)

The calculated Kmc of naphthalene by three Gemini surfactants are presented in Table

7.3. The larger value of Kmc indicates that the surfactant has greater tendency to dissolve

organic compound in micellar phase. In this study, the C12-2-16 micelle possesses the

superior solubilization capability compared with other two studied Gemini surfactants as

demonstrated by Kmc values.

182

7.3.3 Mobility of naphthalene in soil-water-Gemini surfactant systems

Dependence of the naphthalene concentrations on the total concentration of the

selected Gemini surfactants in the mobile phase, e.g., aqueous phase, is presented in

Figure 7.3, describing the mobility of the naphthalene within the soil-water-Gemini

surfactant systems. It is clear that the amount of aqueous naphthalene decreased

dramatically with the increasing amount of aqueous surfactant before the surfactants

reach their CMCs, respectively. Within this range of surfactant concentrations, the

surfactants exist as monomers in solutions which have less solubilization capabilities for

solute and easy to adsorb onto solid surface. Therefore, the residual surfactant monomers

in solutions have weak interaction with naphthalene compared with increased organic

matter formed by sorbed surfactant onto soil particles, which act as strong sorption

media for naphthalene. Consequently, the increased added surfactant concentration

contributes to high sorbed surfactant concentration resulting in unfavorable mobility of

contaminant.

After the CMC, with the increasing added surfactant concentrations, the adsorption

reaches saturation and the excess monomers begin to form micelles in the aqueous

solution which has powerful solubilization ability towards naphthalene. In couple with

the gradually decreased surfactant sorption, thus more naphthalene molecules partition

into the micelles within the aqueous phase. Obviously, the higher CMC values of

surfactant, the more amount of surfactant need to improve the mobility of contaminant.

Therefore, the optimized surfactant for desorption of organic compounds should not only

183

have low CMC value but also possess inferior adsorption capability. Among all three

selected Gemini surfactant, C12-2-16 has been determined to be the most effective

surfactant to mobilize PAHs due to its powerful solubilizing capability towards solute

and the least dosage, which is also indicated by its solubilization data.

7.3.4 Immobilization of naphthalene with Gemini surfactants

As expected, both natural and modified soils displayed sorption of naphthalene from

water. Compared with the natural soil, sorption of naphthalene by Gemini-modified soil

is noticeably enhanced. The following equation can be employed to describe naphthalene

partitioning within the soil-water-Gemini surfactant system (Lee et al., 2000):

*

1

oc oc soc ssd

mn mn mc mc

K f f KK

K X K X

(7.6)

where *

dK is the ratio of sorbed naphthalene to naphthalene in aqueous solution (ml g-1);

foc is the natural organic-carbon fraction (NOC) in the soil (0.0133 g g-1); fsoc is the

surfactant-derived organic-carbon (SOC) fraction in the soil; Koc and Kss are the carbon-

normalized naphthalene distribution coefficient with the NOC and the SOC, respectively

(ml g-1); Xmn and Xmc are the surfactant monomer and micellar concentrations in water,

respectively (g L-1); and Kmn and Kmc are the naphthalene partitioning coefficients with

the surfactant monomer and micellar phases, respectively (ml g-1); Kd is the solute

sorption coefficient with the soil in absence of surfactant, which equals to focKoc in case

of XmnKmn + XmcKmc=0 (Zhu et al., 2003). This equation implies competitive interactions

of contaminants between total organic matter and aqueous surfactant micelles. Therefore,

184

the potential existing of surfactant with different dosage may either promote or impede

remediation efforts depending on the magnitude of interaction between contaminant and

surfactant monomers or micelles.

Based on the surfactant properties, the surfactant-derived organic carbon contents on

the solid (fsoc) could be calculated as listed in Table 7.4. After modification, the organic

matter content of soil increases dramatically.

The apparent solute soil-water distribution coefficients, *

dK as a function of Gemini

surfactant equilibrium concentration in the aqueous phase are shown in Figure 7.4. All

systems share a similar sorptive behavior pattern and *

dK values are significantly

affected by the addition of Gemini surfactants demonstrating a strong and non- linear

uptake of naphthalene from water. The amount sorbed by C12-2-8 modified soil is 1.4-17.3

times of the natural soil, respectively, at original naphthalene concentration of 30 mg L-1.

For C12-2-12, the highest amount sorbed by modified soil is 13.2 times of the natural soil.

For C12-2-16, modified soil sorbed as many as 7.2 times as the natural soil. This is

reflected in the dramatic increase in the slopes of isotherms shown in Figure 7.4. The

increase in *

dK is obviously caused by the strong sorption of Gemini monomers on soil.

These results show that Gemini surfactant modified soils can be used as good sorbents to

sorb naphthalene from water.

185

Table 7.3 Parameters of selected Gemini surfactants and the distribution of PAHs in the

presence or absence of Gemini surfactant systemsa

Surfactant CMC

(mg L-1)

Qemax

(mg g-1)

Cemax

(mg L-1)

Kmc

(L M-1)

*

dK

(ml g-1)

C12-2-8 838.01 197.26 1640.72 456.03 491.22

C12-2-12 535.37 150.04 1114.14 880.75 380.17

C12-2-16 134.13 67.82 177.06 1453.32 275.04

aThe average errors in the Qemax, Cemax, Kmc and *

dK are ±7.3%, ±5.9%, ±2.8% and ±8.1%,

respectively.

186

0 4000 8000 12000 16000

5

10

15

20

25

30

C12-2-8

C12-2-12

C12-2-16

[Aq

ueo

us

Nap

hth

ale

ne]

/mg

L-1

[Aqueous Surfactant] /mg L-1

Figure 7.3 Amount of naphthalene solubilized as a function of amount of Gemini

surfactant in solution

187

Hence, the adsorbed surfactant seemed to create a more hydrophobic environment for

the solutes than that created by surfactant micelle in solution. Several explanations have

been proposed for this observation. This may result from geometric differences between

sorbed and dissolved surfactant aggregate structures (Sun et al., 2009) and the

differences in surfactant packing density seem to support this hypothesis. Danzer and

Grathwohl (Danzer and Grathwohl, 1998) reported that there is a larger hydration layer

surrounding the hydrophilic exterior of the micelle, which will become thickness when

the surfactant is sorbed to the solid. This might favor the hydrophobic interactions of

sorbed surfactant with HOC molecules. Zhu et al. (Zhu et al., 2003) also proposed that

the conformation of surfactant onto solid may rearrange and have higher packing density

corresponding to a more hydrophobic phase for PAHs.

Both the increased extent in *

dK at lower Gemini surfactant concentration (before

CMC) and decreased extent in *

dK at higher Gemini surfactant concentration (after CMC)

appeared to be positively related to the hydrophobic chain length of Gemini surfactant,

i.e., that is in the order of C12-2-8 < C12-2-12 < C12-2-16. A possible explanation may be found

in the different nature of CMC values. As for C12-2-16, its minor CMC values make it

reach equilibrium saturation at a quite short range.

However, the adsorption of naphthalene onto soil in the presence of Gemini

surfactant is ranked as C12-2-16 < C12-2-12 < C12-2-8. The greatest retardation of naphthalene

is observed using C12-2-8 modified soil due partially to the highest organic matter derived

from C12-2-8 and almost 100% surface coverage in the form of a monolayer is formed as

demonstrated by displacing 97.99% soil exchange cations as mentioned before. The

188

more sorbed surfactant, the larger area of monolayer is formed and greater organic

matter is obtained. Consequently, more naphthalene molecular can be intercepted by

modified soil particles. Correspondingly, the dosage of C12-2-8 is as well the highest

among them in order to attain elevated immobilization effect. Along with the added

cationic surfactant, new surfactant molecules could be retained by a tail-to-tail

interaction mechanism leading to a more dense coverage than the monolayer named

admicelles which has inferior capability to retard contaminant.

Therefore, although C12-2-8 has the greatest immobilization capability for naphthalene,

its dosage is the highest among all three studies Gemini surfactants in order to achieve

the same retardation goal. Taking all factors into consideration, C12-2-12 is the optimized

Gemini surfactant for in situ barrier remediation, which is not only has relative high

retention ability but also low dosage.

189

Tab

le 7

.4 E

quil

ibri

um

Gem

ini su

rfac

tant co

ncentr

atio

ns a

nd c

alc

ula

ted s

urfa

cta

nt-d

eriv

ed o

rganic

car

bo

n co

nte

nts

(fso

c)

in s

oils

C12

-2-8

C12

-2-1

2

C12

-2-1

6

Init

ial

conc

n,

(mM

)

Eq

uiv

co

ncn,

(mM

)

f so

c,

%

Init

ial

conc

n,

(mM

)

Eq

uiv

co

ncn,

(mM

)

f so

c,

%

Init

ial c

onc

n,

(m

M)

Eq

uiv

co

ncn,

(mM

)

f so

c,

%

0.4

15

0.0

72

1.0

73

0.2

55

≈0

0.9

75

0.0

82

≈0

0.3

35

0.7

94

0.2

79

1.6

11

0.6

01

0.0

12

2.1

92

0.1

63

≈0

0.6

65

2.3

32

1.3

22

3.1

57

1.0

96

0.0

50

3.8

93

0.2

32

0.0

15

0.8

84

2.6

60

1.4

28

3.8

50

1.3

62

0.1

35

4.5

69

0.2

71

0.0

70

0.8

20

3.8

44

2.2

59

4.9

54

1.7

82

0.3

69

5.2

59

0.5

47

0.1

19

1.7

48

5.8

25

2.8

58

9.2

71

2.2

03

0.6

62

5.7

35

0.7

05

0.1

24

2.3

72

6.5

89

3.3

37

10.1

61

2.7

06

0.9

31

6.6

06

0.9

92

0.1

44

3.4

59

7.9

76

4.5

90

10.5

82

3.0

21

1.0

73

7.2

51

1.4

23

0.4

55

3.9

53

9.7

68

6.4

49

10.3

72

3.3

00

1.2

34

7.6

89

1.6

43

0.6

35

4.1

16

190

0 4000 8000 12000

0

100

200

300

400

500

C12-2-8

C12-2-12

C12-2-16

Kd*

/ m

l g

-1

[Aqueous surfactant] /mg L-1

Figure 7.4 The apparent partition coefficients of naphthalene in relation to the Gemini

surfactants concentration

191

7.4 Summary

The binding of Gemini surfactant onto soil particles and the distribution of

naphthalene between the solid phase and the aqueous phase were investigated.

Symmetric and dissymmetric Gemini surfactants, with different hydrophobic chain

lengths were studied for the purpose of comparison. The presence of all selected cationic

Gemini surfactant increased naphthalene retardation, with this increase being dependent

upon surfactant CMC and available ion exchange sites on soil. Due to the results of the

solubilization and partition experiment, a conclusion can be formed that C12-2-16 is no

doubt a perfect amendment for soil washing remediation technology, while C12-2-12 is

preferred to form an in-situ immobile zone within an aquifer to restrict contaminants due

to its relatively high retention ability but also its low dosage. Additional studies would be

required when various contaminants are present, since the adsorption and desorption

behavior may depend upon the characteristics of organic compounds.

192

CHAPTER 8

DISTRIBUTION OF PAHS IN A SOIL-WATER SYSTEM

CONTAINING A GEMINI SURFACTANT

8.1 Introduction

PAHs enter the environment via uncontrolled discharges from petroleum fields,

chemical factories, etc. On one hand, PAHs, as with other organic pollutants, can absorb

to some soils or sediments, allowing them to persist in the soil for extremely long periods

of time, which lay the potential for adverse effects upon the environment and upon

public health (Wei et al., 2011a). On the other hand, because of their limited capacity for

sorption of organic contaminants by some soils or clays, they are ineffective in retarding

them. The contaminants can transport to other environmental compartments such as a

deeper soil layer or groundwater (Qu et al., 2008), resulting in human health risks though

bio-accumulation and bio-concentration in the food-chain system. Therefore, surfactant

enhanced remediation (SER) has been suggested as an economically and technica lly

feasible technology for the remediation of contaminated soils and groundwater, using

two different methods designed for various contaminants and in-situ conditions, i.e.,

mobilization and immobilization (Sun et al., 2009). Strong interest has been shown in

these two opposite effects on contaminant behavior. In terms of mobilization, the ability

of anionic surfactants to improve the water solubility of organic compounds, thus,

providing a potential means of enhancing soil washing treatment efficiency for HOCs

contaminated soils or ex-situ washing (Uhmann and Aspray, 2012). Unlike anionic

193

surfactants, with the help of cationic surfactants, the pseudo-organic phase can be formed

on the solid substrate due to the positive charge of cationics which can be easily

combined with soil particles possessing unusually negative charges (Neupane and Park,

2000). The tightly sorbed cationic surfactants bring about high organic matter facilitating

the transport of solubilized HOCs and increasing retardation of HOCs partitioning to

immobile sorbed surfactants favoring the environmental remediation of HOCs at the

localized scale and enhanced accessibility of HOCs to microorganisms (Zhu and Aitken,

2010). Therefore, the distribution of pollutants in each phase, under the impact of

surfactants during SER processes, must be considered to maximize the performance

efficiency and minimize the remediation costs.

Presently, the majority of surfactants studied with regard to partitioning of HOCs, are

conventional surfactants that have single monomers with a single hydrophobic tail and a

single hydrophilic head group (Couto et al., 2009; Pan et al., 2009; Villa et al., 2010). A

more recent and innovative class of amphiphilic molecules to aid the sorption or

desorption of HOCs from solid is Gemini surfactants. They have beco me a topic of

scientific interest due in part to their effectiveness in the modification of interfacial

properties but also because their molecular geometries lead to interesting aggregate

structures, both in solution and at the solid-aqueous interface (Manne et al., 1997).

Gemini surfactant molecules consist of two hydrophobic chains and two hydrophilic

headgroups connected through a relatively short (rigid or flexible) spacer group (Wei et

al., 2012). The spacer group controls the separation between the headgroups that may be

greater or less than the average separation of the corresponding monomer in an aggregate,

which changes the mobility and the packing geometry of the surfactant within an

194

aggregate, whether in solution or at a surface (Chorro et al., 1998). It is also well known

that partitioning of organic compounds between a solid and an aqueous phase depend

upon the organic carbon content of the solid phase (Chiou et al., 1983). Since each

monomer of a Gemini surfactant possesses two hydrocarbon chains, a Gemini surfactant

has twice as much organic carbons per molecule than the conventional surfactant

molecule of the same species for a given number of carbon atoms per chain (Neupane

and Park, 2000). Therefore, for the same moles of surfactant sorbed onto a solid

substrate, the Gemini surfactant will form the better sorbent phase for HOCs in the

aqueous phase than the conventional surfactant. The excellent HOCs solubility

enhancement properties of Gemini surfactant have been well defined (Wei et al., 2011a;

Wei et al., 2011b; Wei et al., 2012). There is interest in determining whether it is

efficient and/or effective in removing pollutants from aqueous media. According to the

authors’ knowledge, there is no report on the cationic Gemini surfactant used in this field

and much less is known about the distribution of HOCs in the soil-water-Gemini

surfactant system. Their presence will potentially affect the remediation time scales, as

well as surfactant addition strategies. Therefore, this knowledge is needed for the

development of effective and safe environmental remediation technologies utilizing

surfactants to maximize efficiency and minimize remediation costs.

This study was undertaken to investigate the overall partitioning of three

representative polycyclic aromatic hydrocarbons (PAHs) in the soil-water-surfactant

system which takes into consideration the soil-sorbed cationic Gemini surfactant,

presence of Gemini micelles, and related developed coefficients. The experimental

results from this research will be used to gain an understanding of the effect of cationic

195

Gemini surfactant on the distribution of HOCs in a soil-water system, predict the activity

and mobility of the sorbed solutes, as well as provide some fundamental and valuable

information in designing SER applications for contaminated soils.


8.2.1 Materials

Three representative PAHs were considered: naphthalene, acenaphthene and

phenanthrene, all with a purity >98% (Aldrich products). Gemini cationic surfactant C12-

3-12 was obtained from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of

Science, with a purity of 95%. Selected physicochemical properties of surfactant and

PAHs are included in Table 8.1. Concentrated PAH stock solutions were prepared in

HPLC-grade methanol and stored in a dark place at 4 ◦C in an amber borosilicate bottle

to minimize photodegradation and/or volatilization. Fresh Gemini surfactant stock

solutions were prepared by dissolving the relevant surfactants in deionized water, at

room temperature. Then, desired mole fractions were obtained by mixing pre-calculated

volumes of the stock solutions and the following experimental procedures were

performed.

The soil sample of this study was a natural soil and was collected from the top (0-25

cm) of an area in the Province of Saskatchewan, Canada, not having contamination. It

was transported to the laboratory in coolers, air-dried, crushed and passed through 2 mm

mesh to remove surface plant remains and coarse materials and was stored in closed

containers in the dark, in a refrigerator. The soil pH value was measured in slurries made

196

up at a 1:2.5 soil/water ratio. The texture of the soil is loamy clay containing 1.33%

organic carbon (0.0133g g-1), 39.96% sand, 27.26% silt, and 32.79% clay, respectively.

The organic carbon of soil was determined by employing the LECO TruPec CN

determinator at the condition of 50% relative humidity and 25 ◦C.

8.2.2 Methods


The solubilization of PAHs by Gemini surfactant was carried out, subsequently, in

batch mode. In each test, excess amounts of PAHs was separately spiked into each vial

containing a series of 10 ml surfactant solutions, having a range of concentrations below

and above the CMC to ensure maximum solubility. The sample vials were sealed with a

screw cap fitted with a Teflon lined septum to prevent any volatilization loss of PAHs

from surfactant. Triplicate samples were prepared for each surfactant concentration.

These samples were then equilibrated for a period of 24 hours on a reciprocating shaker,

maintained at a temperature of 25±0.5 ◦C. Following this step, the samples were

subsequently centrifuged at 5000 rpm for 30 minutes to completely separate the

undissolved PAHs, at the same temperature. An appropriate aliquot of the supernatant

was then carefully withdrawn with a volumetric pipette and diluted to 10 ml in flasks

containing 1 ml methanol and the remainder with the corresponding surfactant–water

solution for PAHs analysis.

197

8.2.2.2 Surfactant and PAHs sorption

Batch experiments for sorption of Gemini surfactant, as well as PAHs sorption onto

soil from water, in the presence or absence of surfactant, were conducted in duplicate.

Soil sample of 0.1 grams were weighed into 20 mL standard sample vials with Teflon-

lined septa and screw caps, to which 10 ml of distilled water or a given concentration of

C12-3-12 solution was added with 0.01 M CaCl2 and 0.01 M NaN3. The CaCl2 was used as

electrolyte to poise the ionic strength, and the NaN3 was used as an inhibitor for bacterial

growth (Yu et al., 2011). The initial surfactant concentration spanned a large range of

values below and above the nominal CMC of Gemini surfactant. Then, the samples were

spiked with a known mass of PAHs, prepared in methanol and ensured as being lower

than their solubilities. The content in methanol was about 2% in volume so that it could

not have an effect upon adsorption (Lee et al., 2004). The initial surfactant concentration

spanned a large range below and above the surfactant CMCs. The sample vials were then

equilibrated in a reciprocating chamber at 25 ± 0.5 ◦C for 24 hours (Our preliminary test

showed apparent sorption equilibrium occurred in less than 20 hours) and then

centrifuged at 5000 rpm for 30 minutes. Following centrifugation, an appropriate aliquot

of the supernatant was sampled for PAHs and Gemini surfactant analysis. Duplicate

blank samples were analyzed as controls for each surfactant concentration. The loss of

compounds by photochemical decomposition, volatilization and sorption to the tube

were found to be negligible.

198

8.2.2.3 Analytical method

The concentration of three PAHs in solutions was detected with a Varian

spectrophotometer (Cary 300) by measuring the absorbance at 220 nm, 226 nm and 334

nm wavelength. The typical error in the determination was less than 5%. Cationic

Gemini surfactant analysis was carried out at room temperature using a G20 automatic

titrator, furnished with a 20 ml autoburette, a stirrer, a surfactant sensitive electrode and

a reference electrode. A 1ml surfactant sample was added to 20 ml of distilled water and

then was placed in a 100 ml titration vessel. The solution was titrated with 4 mM SDS

dropwise added from the burette at a rate of 10 ml min−1. The intersection point in a

titration was distinguished by the titrator, automatically. Thus, the sorbed concentrations

of PAHs and surfactants could be determined from the difference of the initially added

mass of the surfactant and PAHs and the final mass in the aqueous phase at equilibrium.

The data in the figures are all presented as an average of the two replicates. The typical

error was less than 5% for solubilization determination and 10% for adsorption

determination.

199

Tab

le 8

.1 S

ele

cte

d p

hys

ico

che

mic

al p

roper

ties

of

surf

acta

nt and

PA

Hs

in t

his

stu

dy

Com

po

und

M

ole

cula

r st

ruct

ure

M

ole

cula

r fo

rmula

(M

W)

CM

Ca

(mM

)

Sw

b

(μM

) L

og

Ko

wb

C12

-3-1

2

N

NB

r-

Br-

CH2

C12H

25N

+(C

H3) 2

(CH

2) 3

-N+(C

H3) 2

C12H

25·2

Br-

(628.7

) 0.8

Nap

htha

lene

C10H

8 (128.2

)

250

3.3

6

Ace

nap

hthe

ne

C12H

10 (

154.2

)

25.5

3.9

2

Phe

nanth

rene

C14H

10 (178.2

)

6.6

2

4.5

7

a s

urfa

ctant dat

a fr

om

Wei e

t al. (

Wei et

al., 2011a)

b P

AH

s dat

a fr

om

(Z

hou

and

Zhu,

2005

a)

200


8.3.1 Solubilization of PAHs in the presence of Gemini surfactant

The solubilization capability is evaluated by measuring the apparent solubility of

naphthalene, acenaphthene and phenanthrene in various concentrations of Gemini

surfactant, as shown in Figure 8.1. The objective is to find that there is little or no

solubility enhancement for PAHs below the CMC of Gemini surfactant, indicating a

weak interaction between the solutes and the surfactant monomers. The equilibrium

solubility of PAHs starts to increase significantly and linearly to a different extent when

the surfactant concentrations exceeded the CMC, which indicates that solubilization is

related to micellization. Moreover, the starting point of micellization for Gemini

surfactant changed slightly in the presence of different PAHs, as evidenced from Figure

8.1. It coincides with the Rosen’s research (Rosen, 1989b) that the CMC of surfactant in

the aqueous phase in equilibrium with solubilizates should change because the activity of

the surfactant is changed due to the introduction of solubilizates.

The molar solubilization (MSR) is characterized as the number of moles of

compound solubilized by one mole of micellized surfactant and denotes the effectiveness

of a particular surfactant in solubilizing a given solute and can be expressed as follows

(Edwards et al., 1991):

s

s

-

-

cmcS SMSR

C CMC (8.1)

201

where Ss is the total apparent solubility of PAH in a given surfactant solution, at a

specified surfactant concentration Cs (the surfactant concentration above CMC at which

Sm is evaluated), and Scmc is the apparent solubility of PAH at CMC.

In the presence of excess PAH, MSR values could be obtained from the slope of the

linear fitted line in which the concentration of PAH is plotted against a surfactant

concentration above the CMC (surfactant concentration in mM vs. PAH concentration in

mM) given in Figure 8.1. The solubilization capacities of the selected C12-3-12 surfactant

for PAHs, quantified by MSR, follow the order of naphthalene > phenanthrene >

acenaphthene. The higher solubilization power of C12-3-12 surfactant, in the present case,

may be attributed to the two head group with a positive charge, which has a widespread

interaction with π-electrons of PAHs and consists of a more hydrophobic content,,

assisting in excessive micellar core solubilization.

202

0.0 0.5 1.0 1.5 2.0 2.5

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Naphthalene

Phenanthrene

Acenaphthene

[PA

Hs]

/m

g L

-1

[Surfactant] /mM

Figure 8.1 Water solubility enhancement of PAHs as a function of the concentration of

C12-3-12 at 25 ◦C

203

To further characterize the effectiveness of solubilization, the micelle-water partition

coefficient (Kmc L M-1) (Jafvert et al., 1994), which is defined as distribution of organic

solutes between the surfactant micelles and the aqueous phase, may be calculated from

experimental measurement, using the following formula:

mmc

m w w

S MSRK

C S S (8.2)

where Sm and Sw are the concentration of solubilizate in the micelles and water phase

(mM), respectively. Cm is the concentration of surfactant in micellar form (mM). The

calculated Kmc of three PAHs by C12-3-12 surfactant are presented in Table 8.2. The values

of Kmc and Kow follow a straight line relationship with the slope being 0.5038, as shown

in Figure 8.2, thus indicating that solubilizate with larger Kow has a greater tendency to

participate in the micellar phase. The results are in accord with the results of Zhou et al.

(Zhou and Zhu, 2005a) who investigated the solubilization of several PAHs by TX-100.

204

Tab

le 8

.2 P

ara

mete

rs o

f th

e d

istr

ibutio

n o

f P

AH

s in

a s

oil-w

ater

-Gem

ini su

rfac

tant sy

stem

Com

po

und

K

ow

Km

c (L

M-1

) K

d (m

l g-1

) * d

K (m

l g-1

) K

oc (m

l g-1

) K

ss (m

l g-1

)

Nap

htha

lene

2291

852

28

551

2169

5634

Ace

nap

hthe

ne

8318

3149

50

1283

3817

35776

Phe

nanth

rene

37154

16722

129

2225

9847

215864

aT

he a

vera

ge e

rrors

in

the

Km

c, K

d,

* dK

, K

oc

and K

ss a

re ±

2.7

%,

±7.9

%,

±8.3

%,

±3.5

% a

nd ±

6.6

%,

resp

ect

ively

.

205

0 10000 20000 30000 40000

0

5000

10000

15000

20000

Km

c \

L M

-1

Kow

Figure 8.2 Correlation of the micelle-water partition coefficient (Kmc) with the octanol–

water distribution coefficient (Kow) for PAHs

206

8.3.2 Gemini surfactant sorption onto soil

The surfactant sorption onto soil and the subsequent sorption of HOCs to sorbed

surfactants will potentially influence the SER strategies for contaminated soils. Thus,

before gaining insight into the partition of PAHs in the water-surfactant-soil system, the

sorption of cationic Gemini surfactant is investigated first.

The sorption isotherm of Gemini surfactant onto natural soil is presented in Figure

8.3. It shows a steep initial slope at low concentrations and eventually the slope reaches a

plateau at an elevated equilibrium concentration, indicating the saturation of the binding

surface. All the standard errors are less than 10% of the average. Surfactant sorption

exceeds 90% of the added Gemini surfactant at concentrations below effective CMC and

is observed to be relatively weak up to equilibrium, when the added Gemini surfactant

concentration goes beyond CMC.

The sorption isotherm can usually be mathematically described by either a linear,

Freundlich or Langmuir sorption model (Schwarzenbach, 1993). In this study, the

sorption isotherm follows a typical Langmuir-type behavior. According to the isotherm

fitting, the maximum sorption amount of Gemini surfactant, on soil, is approximately

133.31 mg g-1, which in terms of organic carbon, is 77.31 mg g-1 (soil 13.1 mg g-1), at an

equilibrium surfactant concentration of 623.01 mg L-1, slightly higher than C12-3-12’s

CMC (596.87 mg L-1).

It is obvious that the surface coverage is superior to the monolayer, indicating that

the sorption of C12-3-12 to soils could not only be affected appreciably by the adsorption

onto to soil particles but also by the partition into SOM. As more organic carbon can be

207

bound to soil with cationic Gemini surfactant, more hydrophobic organic sites are

created, leading to more HOCs partitioning into the Gemini surfactant-treated soil

particles. It is believed that the cationic surfactant molecules are retained by a cation

exchange mechanism, i.e., displacing the soil exchangeable inorganic cations (Liao and

Xie, 2007). The isotherm plateau studied herein, corresponds to 106 meq kg-1, below the

CEC of the soil (158 meq kg-1), demonstrating that about 67% of the total CEC sites

were occupied by Gemini monomers, which is in tune with the result of Li et al. (Li and

Bowman, 1998) and Pen Wang et al. (Wang and Keller, 2008) that only a fraction of

CEC is generally available for cationic surfactant sorption.

Evidently, the C12-3-12 surfactant saturation sorption occurs near the CMC as the

micelles begin to form in the solution, indicating that the surfactant monomers are the

main sorbing species because only surfactant monomers exist below the CMC. However,

the formation of Gemini surfactant monolayer, bilayers, or vesicle-like structures on

solid surfaces is not well understood and is still being researched (Neupane and Park,

2000). In this study, since the saturation sorption occurred above the CMC, it is

reasonable to presume that bilayers may be formed by Gemini monomers. The highly

non- linear nature of the sorption isotherm signifies that an in-situ immobile zone within

an aquifer can be formed by Gemini cationic surfactant. Through the powerful

interaction of Gemini surfactant with aquifer material, the movement of the surfactant-

modified zone becomes progressively slower as the plateau of the isotherm is

approached, until finally an enhanced sorption zone is emplaced in situ. Thus, the

desorption of contaminants can be restricted, undoubtedly.

208

0 1000 2000 3000 4000 5000 6000

40

60

80

100

120

140

[A

dso

rbed

Am

ou

nt]

/m

g g

-1

[Equilibrium Concentration of C12-3-12] /mg L-1

Figure 8.3 The adsorption isotherm of C12-3-12 onto natural soil

209

8.3.3 Distribution of PAHs in a soil-water-surfactant system

The solubilization of HOCs into aqueous phase surfactant micelles, the partition of

HOCs into the soil organic matter (SOM) phase and the sorption of HOCs by the soil-

sorbed surfactant, are the three main mechanisms governing the sorption of HOCs

occurring in a soil-water system containing surfactant (Chiou et al., 1983). Thus, the

intrinsic distribution coefficient, without surfactant (Kd) and the apparent HOCs soil-

water distribution coefficient with surfactant (*

dK ), can be well employed to describe the

balance of these effects.

The soil-water distribution coefficient, without surfactant (Kd), is expressed as

(Chiou et al., 1979):

/d s wK C C (8.3)

where Cs (mg g-1) and Cw (mg L-1) are the solid-phase and aqueous-phase equilibrium

solute concentrations, respectively. Kd is a fundamental parameter to characterize the

mobility and fate of HOCs in soil-water systems. In this study, the observed Kd values

for three PAHs in the order of naphthalene < acenaphthene < phenanthrene correlate

directly with Kow and inversely with Sw of the solutes. The Kd also can be expressed as Kd

=Kocfoc (Chiou et al., 1979), where Koc is the organic-carbon normalized distribution

coefficient, and foc is the fractional organic carbon content of the soil. The Koc and Kow

are also found to have a good linear relationship (0.2596) as shown in Figure 8.4,

demonstrating that the larger the Kow of PAHs, the greater tendency of PAHs to

participate into the solid phase.

210

0 5000 10000 15000 20000 25000 30000 35000

2000

4000

6000

8000

10000

Ko

c /

ml

g-1

Kow

Figure 8.4 Correlation of the organic-carbon normalized distribution coefficient (Koc)

with the octanol-water distribution coefficient (Kow) for PAHs

211

Although Kd, for a given HOC, may vary by orders of magnitude in different soils,

Koc usually differs only by a factor of 2 to 3 (Brown and Burris, 1996). Thus, the degree

of HOC sorption is principally controlled by soil properties such as foc. Introducing a

surfactant and its subsequent sorption onto the surface of a natural solid phase should

increase foc and enhance HOC sorption on the treated soil particles. The parameter *

dK is

introduced to describe the solute mass balance between solid and solution phases in the

presence of surfactant and can be expressed as (Chiou et al., 1979):

* * */d s wK C C (8.4)

where *

sC and *

wC are the compound concentration in the solid phase (mg g-1) and the

aqueous phase (mg L-1), respectively. This equation is an embodiment of two

competitive effects inhibiting the soil-surfactant-contaminant systems, i.e., the promoted

sorption of organic pollutants by the soil organic matter and surfactant-derived organic

matter and the weakened sorption by surfactant micelles in the aqueous phase. The

mobilization or immobilization abilities of soil, with the help of surfactant, depend upon

the final competition of these two effects. Obviously, in order to improve the retention

capability of soil, effort should be directed into decreasing the undesirable surfactant

micelles in the aqueous phase and increasing the amount of soil-sorbed surfactant and

their interaction with solid phase in a surfactant-enhanced sorption zone by employing

new surfactant systems or exploring suitable conditions for effective and safe soil

remediation.

212

0 200 400 600 800 1000 1200 1400

0

1000

2000

3000

4000

5000

6000

Kd

* /

ml

g-1

Equilibrium Concentration of C12-3-12 /mg L-1

Naphthalene

Acenaphthene

Phenanthrene

Figure 8.5 The measured apparent sorption coefficients (*

dK ) of PAHs versus the

equilibrium concentration of C12-3-12

213

Figure 8.5 illustrates the dependence of *

dK values of three PAHs on the Gemini

surfactant equilibrium concentration, describing the immobilization of the PAHs within

the water-soil-Gemini surfactant systems. As shown, the sorption capacities of

naphthalene, phenanthrene and acenaphthene on C12-3-12-sorbed soils are enhanced, with

the maximal *

dK values of 19, 20 and 22 times that of the corresponding Kd, respectively,

which indicates that the levels of Gemini surfactant applied decreases the contaminant

mobility dramatically. The *

dK values increase sharply with increasing surfactant

concentration until they reach a maximum. It is attributed to the strong affinity of PAHs

to the head-on sorption of surfactant in the solid phase, which creates many hydrophobic

organic sites, making the sorbed C12-3-12 a more effective and sorptive phase for the

solutes than natural organic matter.

As a matter of fact, the surfactants are present as monomers within this surfactant

concentration range (i.e., before CMC), which prefer absorbing on the surface of the

hydrophobic solid phase to present in the water. Thus, the increased added surfactant

concentration brings about higher sorbed surfactant concentration until the sorption

equilibrium, which consequently resulted in more sorbed PAHs, suggesting that the

chosen Gemini surfactant is more suitable for in-situ immobilization of PAHs.

Furthermore, Karpanagioti et al. (Karapanagioti et al., 2005) selected one cationic and

three anionic surfactant modified sorbents in the remediation of HOC contaminated

water, and found that cationic surfactant modified zeolite was the most suitable

surfactant modified sorbent in the retardation of HOCs. Sun et al. (Sun et al., 2009) also

reported that cationic surfactant DDTMA modified sediment can reduce the mobility of

214

PAHs efficiently. As a consequence to further increases of surfactant concentration,

surfactant sorption becomes negligible. Micelles begin forming in the aqueous solution,

competing with the sorbed surfactant for PAHs molecules, thereby diminishing the

partitioning of PAHs from soil-sorbed surfactant. This result is consistent with previous

research. However, it is worth noting that the same tendencies are presented for three

PAHs wherein the sorbed PAHs reach maximum concentrations with increasing sorbed

C12-3-12 concentrations before CMC, which is expected with regard to an in situ sorption

zone forming with less usage of surfactant.

As illustrated schematically (Figure 8.6 A), cation exchange sites on the soil surface

are ordinarily occupied by inorganic cations, dominantly Ca2+ (Dance and Reardon,

1983). C12-3-12+, organic cations studied herein, are more preferred than the available

cation exchange sites, and thereby, easily displace the inorganic cations (Figure 8.6 B).

With the increase of surfactant concentrations, the Gemini monomers adsorbed on solid

surfaces begin to aggregate and form micelle- like structures called “hemimicelles”, as

defined by Yeskie and Harwell (Yeskie and Harwell, 1988) (Figure 8.6 B). Thus, the

sorbed hemimicelles may create many hydrophobic organic sites, which are more

homogeneous and more effective for the sorption of PAHs than NOM, bringing about

the highest concentration of sorbed PAHs. Whilst, the total sorption of PAHs by C12-3-12

modified soils may be considered as a combination of a partitioning fraction of PAHs

into adsorbed C12-3-12 and an adsorption fraction of PAHs on unoccupied soil surfaces.

215

Figure 8.6 Schematic representations of PAHs participation in a soil-water-Gemini

surfactant system

216

The C12-3-12+ cations in the hemimicelle occupy the CEC and generate a hydrophobic

surface, which not only adsorb organic contaminants but also subsequently uptake

additional monomers from solution by a tail-to-tail interaction mechanism leading to a

more dense coverage than the monolayer forming admicelles (Figure 8.6 C). Worth

noting are surfactant monomers, carried in hemimicelle by hydrophobic bonding, are

powerfully bound in comparison with those in the admicelles (Brown and Burris, 1996).

Therefore, it is speculated that the formation of admicelles before CMC, leads to the

reduced sorption of PAHs. With the increased Gemini surfactant concentration, the non-

retained surfactant monomers pass to the solution leading to the formation of Gemini

micelles in the aqueous phase, which are capable of desorbing the retained PAHs from

the soil phase because they offer a good hydrophobic environment to which the organic

solutes can partition and therefore, bring about the decreased sorbed amount of PAHs

(Figure 8.6 D).

Additionally, both the horizontal and vertical differences among three PAHs can be

explained by their different water-octanol partitioning coefficient, i.e., a) both the

increase and decrease percent in *

dK and the maximum *

dK values (Figure 8.5) appeared

to follow the Kow patterns (Figure 8.7): phenanthrene > acenaphthene > naphthalene, in

accordance with PAHs hydrophobicity; b) the occurred intersection under various sorbed

surfactant concentrations appeared to be positively related to the Kow of PAHs. It is

indicated that the original and obtained soil organic matter content affects more the

hydrophobic than the hydrophilic substances while the aqueous surfactant has the

opposite effect, i.e., the PAHs, which is highly soluble in water is poorly retained by

Gemini surfactant modified soils.

217

0 10000 20000 30000 40000

500

1000

1500

2000

2500

3000

K

d* /

ml

g-1

Kow

Figure 8.7 Correlation of the apparent HOCs soil-water distribution coefficient with

surfactant (*

dK ) with the octanol–water distribution coefficient (Kow) for PAHs

218

The result is out of accord with the research (Zhou and Zhu, 2005a) on the

distribution of PAHs in the soil-water in the presence of nonionic surfactant and differs

with the study of Michael J. Brown (Brown and Burris, 1996), but is supported by the

previous report (Hernández-Soriano et al., 2007a) on the enhanced sorption of pesticide

with different Kow in the presence of the conventional cationic surfactant. M. Carmen

Hernández-Soriano et al. (Hernández-Soriano et al., 2007b) also confirmed that the

cationic surfactant HDTMA had the highest effect for the more hydrophobic compounds

in terms of immobilization. Hence, general conclusions ought to be avoided because

each system including soil, water, surfactant and organic compound may behave in a

non-predicted way, requiring a deeper evaluation, respectively.

In order to precisely study the role of sorbed surfactant on the partitioning of PAHs,

an important parameter, the carbon-normalized solute distribution coefficient with the

soil-sorbed surfactant, Kss (ml g-1) needs to be studied which is derived from the

following equation (Lee et al., 2000):

*

1

d soc ssd

sm mc

K f KK

C K

(8.5)

where Csm , the equilibrium concentration of surfactant micelles, equals the difference of

surfactant concentrators in solution and CMC (mM), fsoc is the surfactant organic-carbon

fraction in the soil calculated by the following formula:

%soc ssf Q C (8.6)

where Qss is the amount of surfactant sorbed (mg g-1), and C% is the percentage carbon

within each surfactant molecule. Then, Equation (8.6) can be re-arranged to obtain an

expression for Kss:

219

* (1 ) -

%

d sm mc dss

ss

K C K KK

Q C

(8.7)

From our previously determined parameters, Kd, *

dK and Kmc, the only remaining

term, Kss can be calculated for subsequent analyses. It is found that the Kss values studied

herein, are much larger than the corresponding micellar Kmc and solid Koc values which

may be because the geometric difference between sorbed and dissolved surfactants (Ko

et al., 1998). The variation of Kss for naphthalene, acenaphthene and phenanthrene with

the sorbed C12-3-12 concentration is presented in Figure 8.8. All studied PAHs behave in a

similar manner, although they partition to a different extent. Compared with Figure 8.3

(C12-3-12 sorption), it can be observed that for each PAH, the respective Kss was a non-

linear function of the amount of surfactant sorbed, with Kss increasing initially as the

amount of sorbed surfactant increased, reaching a maximum Kss at a certain amount of

sorbed surfactant before the surfactant saturation sorption is reached.

Furthermore, investigating Figure 8.8 immediately reveals two distinct features. First,

the Kss of the sorbed C12-3-12 for a PAH passed a maximum well before the C12-3-12

saturation sorption is observed, which presents the same tendency with *

dK values.

Second, the greater the Kow values of PAHs, the earlier the maximal points are reached

and thus, higher affinity to the sorbed C12-3-12 phase. Another way to look at Figure 8.8 is

that, the greatest Kss values of PAHs also follow the same order of Kow, as indicated by

Figure 8.9. Moreover, the discrepancy of Kss is expected to vary with fsoc, due to

presumably a structure change of the sorbed surfactant with the loaded mass. Because

the adsorbed Gemini surfactant molecules may undergo a structural alteration from

220

submonolayer to bilayer or multilayer on the solid phase, the state of which having a

substantial consequence on the ability of the adsorbed Gemini surfactant molecules to

uptake organic compounds.

The Kss values are greater for these PAHs than the corresponding Koc, indicating that

the soil-sorbed Gemini is more effective per unit mass as a partitioning medium than

native soil organic matter. As the sorption of C12-3-12 increases, maximal Kss takes place

when the monolayer formation is completed by sorbed C12-3-12. Then a bilayer starts

forming. Within this step, the cationic Gemini surfactant continues to sorb onto the

monolayered surfactant molecules and present as an “end-up” configuration leading to a

gradually increasing amount of sorbed surfactant, and ultimately resulting in a

decreasing Kss (Wang and Keller, 2008) (Figure 8.8 region beyond maximal Kss and

before surfactant sorption saturation). Thus, for an in situ practical application, a sorption

zone can be formed with relatively small amounts of Gemini surfactant, which is lower

than the fully sorbed surfactant, since maximum retardation efficiency occurs before

surfactant saturation, as indicated by Kss.

221

0 20 40 60 80 100 120 140

0

50000

100000

150000

200000

250000

Naphthalene

Acenaphthene

Phenanthrene

Kss

/m

l g

-1

[Sorbed Gemini surfactant] /mg g-1

Figure 8.8 The variation of Kss for PAHs with the sorbed C12-3-12 concentration

222

0 10000 20000 30000 40000

0

50000

100000

150000

200000

250000

Kss

/m

l g

-1

Kow

Figure 8.9 Correlation of the carbon-normalized solute distribution coefficient (Kss) with

the octanol–water distribution coefficient (Kow) for PAHs

223

8.4 Summary

In this study, a new Gemini surfactant was used to evaluate its effect upon the

partition of PAHs in a soil-water-surfactant system. The results indicated that the

selected Gemini surfactant can be used in SER technology, as an amendment to the

formation of a surfactant enhanced sorption zone. Even a relatively small mass of

adsorbed Gemini surfactant can substantially enhance sorption of organic contaminants.

Results of the laboratory experiments, presented here, show that:

1) Solubilization and sorption ability of Gemini surfactant determines its effect upon

the mobilization of an immobilization application.

2) The partitioning behavior of PAHs in the soil-water-Gemini surfactant has a

strong relationship with their Kow.

3) The sorbed C12-3-12, studied herein, is a highly effective partitioning media for

PAHs to adsorb onto the soil phase from the aqueous phase and thus, can be considered a

good adjuvant for an enhanced sorption zone.

4) Kss is determined by several factors such as the availability of CEC sites for

sorption of the Gemini cationic surfactant, the amount of surfactant sorbed, and the

characteristic of organic compounds.

224

CHAPTER 9

CONCLUSIONS

9.1 Summary

In this dissertation, a set of Gemini surfactant enhanced solubilization, adsorption

and desorption methodologies towards PAHs, have been developed to 1) correlate and

evaluate the effect of the structure, interfacial and micellar properties of Gemini

surfactant, and the characteristics of contaminants on the solubilization power; 2) design

and develop multi-component Gemini surfactant systems to enhance the solubility of

PAHs; 3) investigate the partition behavior of PAHs in the soil-water-Gemini surfactant

and provide scientific proof for SER remediation technology.

(1) The enhanced solubilization of Gemini surfactant C12-2-12 for phenanthrene and its

comparison with conventional surfactant with same and different hydrophobic chain

lengths were investigated through the micellization and solubilization approach.

Although the studied Gemini surfactant has a little higher CMC value, the excellent

solubilization towards phenanthrene was exhibited due to its more micellar core

solubilization and intense attraction with phenanthrene at the micelle-water interface.

The solubilization capability of Gemini/nonionic surfactant for phenanthrene is superior

to Gemini/cationic surfactant, which may be on account of the joint effect of micellar

core solubilization and micelle-water interface attraction for organic compounds. In

addition, in the four equimolar mixed surfactant systems, the solubilizing capacities

225

towards phenanthrene by Gemini/nonionic surfactants are higher than their

corresponding conventional surfactant.

(2) The effect of composition and the category of selected multi-component Gemini

surfactant systems, including binary Gemini/cationic conventional surfactants, binary

Gemini/nonionic conventional surfactants, binary Gemini surfactants with different alkyl

chain lengths and different spacer lengths, ternary Gemini/nonionic conventional

surfactants, Gemini surfactants with different spacer lengths, in the solubilization of

PAHs, were systematic studied. The solubilities of PAHs increase linearly over the range

of mixed surfactant concentrations above the CMC, which illustrate the potential

capacities of the mixed Gemini surfactants to facilitate the solubilization of PAHs in

water. However, these chosen multi-component Gemini surfactant systems have

relatively selective solubilization capacities and are not all higher than those

corresponding individual surfactant solutions at comparable surfactant concentrations

and are not simply related to molar capacity.

(3) Binding of symmetric and dissymmetric Gemini surfactants with different

hydrophobic chain lengths onto soil particles, their distribution between solid and

aqueous phase and their effect on the partition of PAHs in the soil-water-surfactant

system, were investigated. The results indicated that the adsorption of Gemini surfactants

onto soil particles through both cation exchange and hydrophobic interaction contribute

to the bi or multilayer formation. The sorbed Gemini surfactant can be used in SER

technology as an amendment for the formation of the surfactant enhanced sorption zone.

Even a relatively small mass of adsorbed Gemini surfactant can substantially enhance

sorption of organic contaminants. The partitioning behavior of PAHs in the soil-water-

226

Gemini surfactant also has a strong relationship with their Kow. The sorbed C12-2-12 and

C12-3-12, studied herein, are a highly effective partitioning media for PAHs to adsorb onto

the soil phase from the aqueous phase and thus, can be considered as a good adjuvant for

an enhanced sorption zone, while C12-2-16 is no doubt a perfect amendment for soil

washing remediation technology.

9.2 Achievements

(1) A comprehensive systematic investigation into solubilization aspects of single or

multi-component Gemini surfactants were conducted through interfacial, micellar and

solubilizing technology. A variety of parameters associated with the solubilization and

partition process were correlated through theories to find synergism in solubilization.

The experimental results of this study will be useful in figuring out and predicting the

solubilization properties of single and mixed Gemini surfactant systems and meaningful

in providing statistical proof with which to explore new surfactant systems for practical

soil and water remediation.

(2) A set of multi-component mixed Gemini surfactant systems were developed and

proposed to improve the water solubility of PAHs, which has not been reported to the

best of our knowledge. The interaction of surfactant and contaminant in the monomer

and micelle phase was explained in order to find their synergism and antagonism. The

effect of the unique structure of surfactant and chemical nature of solute, on the change

of interface and solubilization, were revealed. A set of evaluation systems was developed

to assess the solubilization capability of a single or mixed surfactant system. The

227

analysis studied, herein, has provided valuable information for the selection of mixed

Gemini surfactants in order to solubilize water- insoluble compounds.

(3) This dissertation studied the behavior of PAHs in the absence and presence of

selected cationic Gemini surfactant. The results from this research elucidated the

relationship between structure and surface adsorbate morphology. The outputs of this

study provided some fundamental information leading to a better understanding of the

use of Gemini surfactant existing in the aqueous or adsorbed onto the solid substrate as

the desorptive or sorptive phase in order to partition the HOCs. The studies also propose

a better strategy in applying Gemini surfactants in order to optimize the treatment

efficiency of surfactant-enhanced remediation.

9.3 Recommendations for Future Work

(1) In order to investigate the implementation of Gemini surfactant in a large scale, a

pilot-scale adsorption and desorption experiment is required to scale-up experimental

results for field applications. The type and characteristic of soil and other real site

conditions, including remediation time, liquid soil ratio (L/S), temperature, etc. should be

considered, simultaneously.

(2) Studies regarding mixed Gemini surfactant systems for SER technology are

necessary, since in scientific and industrial applications, the surfactants are almost

always prepared commercially and used as mixtures rather than pure forms. Their

efficient solubilization capabilities have been demonstrated in this study but whether

they are able to facilitate the SER technology still needs investigated.

228

(3) Soil contamination is an on-going problem, worldwide, where a mixture of

different contaminants is present including organic compounds and heavy metals. The

development of a Gemini surfactant related approach to facilitate the remediation of

water and soil contaminated by mixed pollutants are highly recommended and beneficial.

229

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