surf act ant and colloidal cb

8/2/2019 Surf Act Ant and Colloidal CB

1/105

THE ADSORPTION OF MIXED SURFACTANT SYSTEMS ONCOLLOIDAL CARBONBLACK

Scott Michael Richardson

A Thesis Submitted to the D epartment of Chemistry inConformity with the Requirements for the Degree of Masterof Science.

Queen's UniversityKingston, Ontario, CanadaJuly 1997

Copyright O Scott Michael Richardson, 1997


2/105

National Library(*I f Canada Bibliothque nationaledu CanadaAcquisitions and Acquisitions etBibliographie Services services bibliographiques395 WellingtonStreet 395. rueWellingtonOttawaON K t A O N 4 OttawaON K t A O N 4canada Canada

Your fi& vcrrs relwmcuOur N m eMIIMQ)

The author has granted a non- L'auteur a accord une licence nonexclusive licence allowing the exclusive permettant laNational Library of Canada to Bibliothque nationale du Canada dereproduce, loan, distribute or seil reproduire, prter, distribuer oucopies of this thesis in microfonn, vendre des copies de cette thse souspaper or electronic formats. la forme de microfiche/flm, de

reproduction surpapier ou su r foxmatlectronique.

The author retains ownership of the L'auteur conserve la proprit ducopyright in this thesis.Neither the droit d'auteur qui protge cette thse.thesis nor substantial extracts fkom it Ni la thse ni des extraits substantielsmay be printed or othenvise de celle-ci ne doivent tre imprimsreproducedwithout the author's ou autrement reproduits sans sonpermission. autorisation.


3/105

This thesis investigates the behaviour of carbon black as a mode1 hydrophobiecolloid in mixed surfactant systems. Adsorption isotherms were prepared for a series ofnonylphenol polyethylene oxide surfactants of vg chah length. Additionalisotherms were prepared for sodium dodecyl sulfate and tetradecylaimethylammoniumbromide. Subsequent work was done in order to determine the individual surfactantconcentrations in mixed surfactant systems.

Electrokinetic and acoustophoretic measurements were used to measure thecharge on the carbon black particle surface in the presence and absence of surfactants.Measurernents were carried out in both single and mixed surfactant systems.Experimental design was directed at understanding the behaviour of the surfactantadsorption under changing conditions of pH and temperature.

A preliminary study of the particle size distribution in aggregated carbon blacksystems was aIso conducted.


4/105

TABLE OF CONTENTSPage

........................................................................................................................BSTRACT i.TABLE OF CONTENTS .................................................................................................... riLIST OF FIGURES .............................................................................................................. v.ABBREVIATIONS ........................................................................................................... vri...SYMEIOLS....................................................................................................................... vu1LIST OF TAB LES ............................................................................................................... x

.......................................................................................HAPTER 1.NTRODUCTION 1CHAPTER 2.BA CKGROUND AND THEORY .............................................................. 5

2.1 .1 Electrostatic Repulsion........................................................................ 52.1.1 Gouy-Chapman Mode1 of the Electncal interface.............................122.1 -3 van der Waals Forces B etween C olloida1 Particles...........................13

..............................................................................1.4 Harnaker Equation 15.............................................................................1.5 Steric Stab ilization 17...............................................................1.6 interaction Potential Curves 21

2.1.7 Effect o f Added Stabilizers ............................................................... 5

2.2 Surfactants.............................................................................................................. 262.2.1 Micelle Definition and Energy Description ...................................... 7.........................................................................2. 2 Nonionic Surfactants -2 92.2.3 Cloud Point Temperature .................................................................. 02.2.3 HLB Classification ............................................................................1


5/105

2.2.5 Applications .....................................................................................3 2

.................................................3 Adsorption lsotherms for Nonionic Surfactants .... 34...........................3.1 Methods for Evaluating Surfactant Concentrations 35

2.3.2 Shape o f Isotherms ............................................................................ 62.3.3 Steric Layer Thickness ......................................................................39

2.4 Acoustophoresis.................................................................................................... 422.1.1 Principles o f Operation ..................................................................... 4 22.4.2Electroacoustics .............. .........................................................442.4.3 Advantages o f Electroacoustics .......................................................-462.4.4 Electrical Nature o f the Solid Liquid [nterface .................................48

CHAPTER 3 .EXPERlMENTAL ......................................................................................493.1 Materials.................................................................................................................49

..........................................................................1 -1 Nonion ic Surfactants 49.................................................................................12 Ionic Surfactants 50

......................................................................................1 -3 Carbon Black 5032 Adsorption Isotherm Preparation .......................................................................... -52

3 .3.1 Nonionic Surfactant Analysis............................................................ 332 . 2 Anionic Surfactant Analysis..............................................................5 33 -3 3 Cationic Surfactant Anaiysis............................................................-5 6

3.4 Pen Kem 7000 Acoustophoretic Titrator ..............................................................-5 7........................................................................................5 Pen Kem 50 1 Zeta Meter 58


6/105

. .Conductivity Experiments...................................................................................... 59..............................................................................................ggregation Studies -59

3.7.1 Zeta Potential Measurements of Ionic Surfactants............................ 603.7.2 Image AnaIysis Work ....................................................................... 13.7.3 Analysis of SD S as a Function of Tirne ............................................ 61

. .-..................*.......*.-.............*....................HAPTER 4 RESULTS AND DISCUSSION 623.1 Adsorption Isotherms............................................................................................. 62

...........................................................2 Electrokinetic Zeta Potential Measurements 714.3 Acoustophoresis Experirneots................................................................................ 801.4 Spike Addition of TTAB to SD S Stabilized System s ............................................82

.4.5 Conductivlty Experiments.................................................................................... 85CHAPTER 5. CONCLUSIONS AND FURTHER WORK ..............................................86

REFERENCES ..................................................................................................................91

VITAE................................................................................................................................94


7/105

LIST OF FIGURES

Figure 2.1 Schematic Drawing of th e Electrical Double Layer.(after israelachvili 1992)................................................................................................. 1 1Figure 2.2 (a) Ste nc Stabilization of Colloidal Particles w ith Nonionic Surfactants................b) Electrosteric Stabilization with Ionic Surfactants. .....................................,.. 20Figure 2.3 (a) Electrostatic Repulsion Energy as a Function of Surface Potentialfor a PS Latex System r = 0.1 pun'= 25 O C : (1) 40 mV, 2) 60 mV, (3) 80 mV.(b) van der Waals A ttraction Energy as a Function o f Particle Radius,

................................i j i= 1.95 x 1 0 - l ~, T = 25 O C : (1 ) 50 nm, (2) 100 nm, (3 ) 150M 23Figure 2.4 (a) Total interaction Potential of a PS Latex System, r = 0.05p,A i i = 1.95 x 1 0 - l ~ . T = 15 O C : 1) 80 mV, (2) 60 mV, (3 ) 40 mV.( b ) Effect of S teric Layer Thckness, r = 0.05p, l l = 1.95 x 10'19 J, T = 25 O C :..............................................................................1) 2.25 m ~ ,2 ) 2.90 nm, 3 ) 3.45 m. 24Figure 2.5 Idealized L4 Type Isothenn for Adsorption o f Nonionic Surfactants....................t the Solid/So lution interface (after G.D. Parfitt and C.H. Rochester 1983) 37Figure 2.6 Schematic of Acoustophoretic Mechanism(afier B.J. Marlow and D. Fairhurst 1988).........................................................................43Figure 3.1 V Spectrum of SDS:M ethylene Blue Cornplex. ............................................55Figure 4.1 Adsorption Isotherm for CO-850 on ST1120 C arbon Black(+)- pmol g-'. ( p n o l m-'........................................................................................... 65Figure 4.2 Adsorption Isotherm for SDS on ST1120 Carbon Black.............................................................................................+ ) - p o l g'i, ( pmol rn" 66Figure 4.3 Mixed A dsorption Isotherm of SDS and CO-720 on ST1110Carbon Black (*)otal Adsorption, (a)CO-720 ( 0 ) DS .............................................9Figure 4.4 Adsorption Isotherm for SDS, TTAB an d CO-720 on ST1120Carbon Black. (m ) SDS, (+) CO-720 ............................................................................... 70Figure 4.5 Equilibrium Process in Mixed AnionicK ationic Surfactant System(after Scamehom et al. 1988)............................................................................................. 75


8/105

Figure 1.6 Zeta Potential as a Function of Temperature for CO-710/SDS Systems(+) 6 OC. i ) 24 O C . (m ) 32 O C ........................................................................................ 80Figure 4.7 PIot of Zeta Potential as a Function ofAdded T'T'AB to STI 120

.............................................................................................tablized with SDS/CO.720 84Figure 1 .8 Determination of CMC fT'LU fiom Conductivity

.................easurements in DDW . .......................................................................... 87Figure 4.9 Titration ofT ' A B with SD S in DDW ............................................................. 88


9/105

RAMSDSTTABCMCDDWRPM

w

PEO

EOMBASCVP

ST1120DLVO

PB

C.C.C.

PS

Relative acoustophoretic mobilitySodium dodecyl sulfateTetradecyItrimethylammonium bromideCritical micelle concentrationDeionized distilled waterRevolutions per minuteUltravioletPoiyethylene oxideEthylene oxide unitMethylene blue active substanceCotloid vibrational potentialSterling carbon blackColloidal Stability theones of B. De jaguin. L.D. Landau, J.W. Venveyand J. Th. OverbeekPoisson-BoltzmannCritical coagulation concentrationPo1ystyrene


10/105

Electrical force. NElectrical charges, CPermittivity of fiee space, kg-'m-' 4A'Dielectric constant of the mediumSeparation distance between the centers of the charges. mWork to b ~ gwo charges together &om an infite distance, JConcentration of positive ions in the bulk medium. moles L-'Charge on the ion, CElernentary electric charge, CSurface potential, VBoltzmann constant, J K"Temperature, KCharge density, C m-'Valency nurnberConcentration of n type ions in the bulk medium. moles L"The reciprocal of the thickness of the double layer, m"Electrostatic potential on the particle, VCounter ion charge numberParticle radius, mHamaker constant, ISurface to surface distance, m

Ha , + a 2Particle radius, mVolume of a molecule of the dispersion medium, rn3Polymer solvent interaction parameterConcentration of the polymer in the steric layer. mol m"

.V l l l


11/105

Stenc layer thickness, mNurnber of moles of surfactant adsorbed on a unit mass of soiid,moles kg-'Total nurnber of moles o f solution before adsorption, molesChange in mole fraction o f surfactant resulting from adsorptionMass of insoluble adsorbent, kgAmount adsorbed, mol rnS2Number of moles of surfactant adsorbed. molesSurface area of substrate, m'Zeta potential, VFluid density, kg m"Particle density, kg m-'Fluid viscosity.N rn"Volume fiaction of the particleDielectric constantParticle weight BactionWavelength of light. nm


12/105

LIST OF TABLES

9...................................................................able 1.1 Exarnples of ColIoidal Dispersions...............................................able 2.1 Selected HLB Values of Nonionic Surfactants 32

...............................................able 3.1 Nonionic Surfactants and Physical Data ,.. 39...................................................able 3.2 Absorptivity Data for Nonionic Surfactants 50

3 .....................................................able 3. 3 Physical Properties of Sterling 1120 .. 51..................................able 3.4 Surface Area Measurements on ST1 120 Carbon Black 51

Table 4.1 Absorption Isothenn Data for Surfactantson STI 130 ...................................64..........able 4.2 Zeta Potential of Carbon Black Solutions as a function of added SDS 72

........................able 4.3 Zeta Potential Measurements of Mixed TTAB/SDS Solution 73....................able 4.3 Zeta Potential of ST l 120 Stabilizedwith Nonionic Surfactants 77

Table 4.5 Effect of Temperature on the Zeta Potential of SDSKO-720 Systems..........79.........................able 4.6 Detemination of the Critical Micelle Concentration of SDS 85

Table 3.7 Determination of the Critical Micelle Concentrationof TTAB......................85


13/105

CHAPTER 1. INTRODUCTION

Colloids are an intricate part of our world. One accepted definition of a colloid isany system that has one or more of its componentswith at least one dimension in thenanometer-micrometer size range. Exarnples include: aerosols, foams, inks, andpharmaceuticals. These are al1 systems that contain srna11 particles or large molecules.

Th e ability to control and predict the stability of a colloidal system is of vitalinterest in a broad range of industries including: pharmaceuticals, detergency, xerography,and cosmetics. EEorts to understand this stability stem fiom increased product life,efficient dmg delivery. and consistent product performance. The ability to manipulate acolloid is based upon an understanding of the factors that impart stability to the system.Equally important is a thorough understanding of the factors which can destabilize asystem.

One of the more frequently encountered types of colloidal systems is dispersions.Dispersions consist of one phase of matenal homogeneously rnixed in a second. n i ephases can be liquid, gas or solid. Examples of some of these systems are listed below inTable 1.1 .' As will be discussed in Chapter 2. there has been an intensive effort duringthe past several decades to develop a unimg theory to understand and predict colloidalstability. One of the primary means of generating stable dispersions is through theintelligent application of stabilizers or surfactants.


14/105

Table 1.1 Examples of Colloidal Dispersions

Dispersed Phase Dispersion Medium Name ExarnplesLiquidSolidGasLiquidSolidGasLiquidSolid

GasGasLiquidLiquidLiquidSolidSolidSolid

Liquid aerosolSolid aerosolFoarnEmulsionColloidal SuspensionSoIid FoamSolid EmulsionSolid suspension

Fog, liquid spraysSmoke, dustSoap solutionsMilk. mayonnaiseAu sol, Agi solExpanded PolystyreneOpal, pearlPigmented plastics

Surfactants are a class of molecules that have a unique chernical structure. Thereare three general classes of surfactants: anionic, cationic and nonionic. Ln general theirmolecular makeup consists of both a hydrophobieand a hydrophilic portion. in this thesisthe following terrns will be used to descnbe surfactant structure. The hydrophobicportion (usually a hydrocarbon chah) will be referred to as the tail. The hydrophilic(charge bearing or containhg polar groups) will be referred to as the head. The dualnature of these types of molecules allows thern to preferentially position themselves at theinterface between non miscible components. Surfactants find widespread use in colloidalsystems. They are often the only means of generating stable dispersions in some systems.

More recently, mixtures of surfactants have been employed in the area of colloidalstability. Mixtures oflen exhibit spergist ic behaviour which is unavailable in single


15/105

surfactant systems. However, the properties of these mixtures are complex and are atpresent not well understood.

One of the driving forces for understanding surfactant behaviour in these rnixedsystems is the potential to optirnize their use and performance. Emulsion polyrnerizationis an example of a colloidal system that relies heavily on the properties of surfactants.The synthesis of th e polymer and the final particie size distribution in the product areintncately linked to surfactant behavior in the colloidal system. Of fundamentalimportance is a thorough understanding of surfactant behaviour during the seeding orgrowing process of the primary particles. During this process seed particles are firstgenerated, these primary particles cm be aggregated to form the secondary particles.Surfactants play a prime role in both the growth of the pnmary particles and thesubsequent stabilization of the secondary particles. Control over the extent anduniformity of the secondary particle size is a result of a number of processes. Currentlythere exist some opposing ideas regarding the role of the surfactants during the particlegrowth and stabilization phases.

Emulsion polyrnerization typically involves a number of di fferent chemicals andfactors that exert influence on the final particle size distribution. Some of the moreimportant factors include shear rate. temperature, pH1and type of surfactants.

Recent research into narrowly dispersed aggregates at the Xerox Research Centreof Canada has provided a starting point to begin this investigation into mixed surfactantsystems. From the procedure developed at Xerox, narrowly dispersed aggregates areproduced fiom a pnmary particle size of 0.15 p ith a final aggregafe size anywherebetween 5 and 10Fm. The primas. particles are stablized with a mixture of anionic and


16/105

nonionic surfactants. Cationic surfactant is added to the systems to destabilize it and tobegin the aggregation process. There are several hypotheses directed at explaining thebehavior/roie of the individual surfactant components in the mixture.

The surfactants in this system initially serve to keep the latex particles stable insoiution. Aggregation is induced by the addition of cationic surfactant, which causes theformation of a thick viscous gel. This gel is subsequently broken up at higher temperatureto form aggregateswith a remarkablynarrow size distribution. The restabilization isthought to be primarily a function of the aggregate size and of the surfactant concentrationand conformation on the particle surface. in order to M e r nderstand and explain theprocess, a method of determining the concentration and effect of individual surfactants int h i s complex mixture was required.

This thesis describes a first attempt at obtaining analytical techniques fordetermining individual surfactant concentrations is such complex systems. A secondaryobjective of this research proj ect was to investigate if narrow aggregate distributionscould be obtained by aggregating dispersions of colloidal carbon black.


17/105

CHAPTER 2. BACKGROUND AND THEORY

2.1 DLVO TheoryThe ability to control and manipulate colloidal stability has been the focus of a

large and concentrated effort for the past several decades. Depending on the applicationof a colloidal system, fme-control over the stability of the system can have a ciramaticinfluence on the end-use properties. In the area of colfoid science the theones ofDejaguin, Landau, Verwey and Overbeek (DLVO theory) are often used to defme andpredict the stability of a given system.

The DLVO theory unified the theones goveming attraction and repulsion incolloidal particle systems. The theory deals with the potential energy of interactionbetween colloidal particles as a function of distance. It combines the attractive (van derWaals) and repulsive (electrostatic) energies between particles to predict the totalinteraction energy. The major contributing factors to the repulsive and attractive forcesacting between colloidal particles will be outlined next.

2.1.1 Electrostatic RepulsionElectrostatic repulsion arises between colloidal particles when two similarly

charged surfaces approach each other at a small distance of separation. The repulsion is adirect consequence of the interaction between the sirnilady charged surfaces.'

There has been a vast amount of theoretical and experimental work directed atexplaining the nature of the electrical interface. The interface exists between the solid and


18/105

its surrounding solution environment. Repulsive electncal forces acting between the

surfaces of similarly charged colloids are often the primarymeans of stabilizationwithin asystem.'

Colloidal particles may acquire surface charge through one or a combination of thefollowing mechanisms: (i) preferential dissolution of surface ions. (ii) direct ionization ofsurface groups, (iii) substitution of surface ions, (iv) specific ion adsorption." The forcesacting between "charged surfaces" have their ongin in Coulomb's Law. This lawdescribes the interaction between point charges separated by a distance r in a vacuum.Coulomb's law m u t be modified in order to extend its applicability to colloicial systems.'

The interaction of two charges in a vacuum, separated by distance r, can bedescribed by Equations 2.1 and 7.2

where

Fe1 = elecicaI force, Nq ,qz = electncal charges, C&O = permittivity of fiee space, kg" m*' s4A'E = dielectric constant of the medium


19/105

r = separation distance between the centers of the charges, mw = the work necessary to bring two charges together fiom an infinite

distance, J.For charges of the same sign the work will be positive and the interaction will be

repulsive. The quantity of work is defined as the "elec-ical potential" at r due to th echarge q,, and is given the symbol Y.

in order to describe systems of more practical interest Coulomb's law must bemodified by the Boltzmann distribution to account for al1 ions present in the system.which is descnbed by Equatior?2.3. If he surface has a negative electrical potential(negative surface charge), the concentration of positive charges in the region surroundingthe surface cm be calcuiated by the following expression:

wherec, = concentration of positive ions surrounding the surface, moles L*'Co = concentrationof positive ions in the bulk medium, moles L"Z, = charge on the ion, Ce = elementary electnc charge, CY = surface potential, Vk = BoItmiann constant, J K-'T = temperature. K.


20/105

One o f the approxim ations often invoked to describe the electrical interface is toexpress the charge density of the surface as a fnction o f potential upon rnoving awayfiom the charged surface. Charge density is related to the surro und hg ion concentrationprofile as s h o w by Equation 2.4j

whereP'

21

e

Ykn1o

T

= charge density. C rn'j= vaiency nurnber= eieme ntary electric charge. C= surface potential, V= Boltzmann constant, J K-'= concentration of n type ions in the b u k medium, moles L-'= temperature, K.

Reiating E quation 2.4 to the po tential results in the Poisson-Boltzmann (PB) equation

The Poisson eq uation implies that the potentiais associated with the variouscharges combine in an additive manner, whereas the Boltzma nn distribution irnplies an


21/105

exponential relationship between the charges and the potential. The PB equation has noexplicit solution and must be soived for limiting cases. The solution to the abovedifferential equation under conditions of low surface potential and the condition ofelectroneutrality results in the Debye-Huckel approximation which is expressed asEquation 2.6,

Wherey 0 = the potential at the particle surface, VK = th e reciprocai of the thickness of the double layer, m-'.

The thickness of the double layer for surface potentials less than 25 mV is calculatedaccording to Equation 2.7

The thickness of the double layer varies inversely with the concentration ofsolution electrolytes, and the square of the valency of the counter ion. By changing eitherthe concentration or the identity of the surroundhg electrolyte, the thickness of the doublelayer cm be manipulated. in systems whose stability is entirely dependent on electricdouble layer interactions this has important implications. This strong dependence on ionvalency and concentration is the basis for the Schulze-Hardy rule! The Schulze-Hardymle predicts the amount of inert electrolyte necessary to destabilize a colloidal system.


22/105

this arnount is most often referred to as the critical coagulation concentration (c.c.c). Atthe C.C.C. the surface charge of the particles will have been screened by th e addedelectrolyte, this causes the attractive forces to dominate and the dispersion will becomeunstable.

The solution taken as a whole will be electrically neutral. However in the vicinityof the charged surface, at the particle-solvent interface. there will exist an irnbalance ofelectrical charges. This charge irnbalance in the system depends heavily on the net chargeof the surface. The region of excess charge of opposite sign around a charged surface iscommonly referred to as the "ionic atmosphere" or "charge cloud" associated with thatpotential. This ionic atmosphere consists of a higher concentration of counter ions overCO-ions s predicted fiom Equation 7.3. In colloidal systems the "charge cloud" inconjunction with the charged interface is commonly referred to as the "electncal doubielayer" associated with the particle.

As a consequence of the dynarnic nature of a solution, the ions present in thedouble layer exist in a dif i se state. This results in a surrounding ionic environment thatis rapidly undergoing change. Taking account of this added complexity, Gouy andChapman developed a mode1 to describe the electrical double layer that relates thepotential of the surface to the diffise portion of the double layer.' It does not involve thessurnption of low potentials invoked in the Debye-Huckle approximation. A schematic ofthe interface is given in Figure 2.1.


23/105

FFUSE LAYER (MOBILE IONS)

I \ BOUND (IMMOBILE) COUNTER-IONS

PARTICLE SURFACE IONS

Figure 2.1 Schematic Drawing of the Electncal Double Layer(after Israelachvili 1992).


24/105

2.1.2 Gouy-Chapman Mode1 of the Electrical Interface

The "Stem Layer" is the small space separating the ionic atmosphere around asurface rom the acnial diffuse double layer. and consists of tightly bound counter ionsthat are not ''fiee" to move with the thermal motion of the aqueous system. The StemLayer has a thickness on the order of a few angstroms, and its width accounts for the finitesize of charged groups and ions specifically associated with the surface.'

It is assumed that the electrical potential in the solution surrounding the surfacedecreases exponentiallywith distance. This approximation is not valid at points close tothe surface, where the potential decreases much more rapidly due to the presence of boundcounter ions in the Stem Layer which are more effective at screening the surface charge.Several simplifjmg assumptions are employed to theoretically treat the nature of theinterface. These include: (i) ions fiom both the solution and the surface are treated aspoint charges, (ii) the surface is treated as a Mform charge, (iii) charges of opposite signc m approach infinitely closely. and (iv) the dielectric constant of the solvent is assurnedto remain constant throughout the double layer.

The actual surface potential represented by y~ is replaced in the Gouy-Chapmanmodel with y,. This is the potential of the surface at th e interface between the Stem planeand the solution. The Gouy-Chapman model accounts for the mobility of ions in aqueoussolution. The diffuse model of the double layer reflects the change in potential associatedwith the surface on moving away fiom the interface.

No exact analytical expressions exist for solving the equations associated with thenature of the interface, recourse to nurnencal solutions or to various approximations areofien invoked. According to Overbeek. the rate of double layer overlap in typical


25/105

Brownian motion between particles is too fast for adsorption equilibrium to bemaintained.' Often models of the interface assume that the potential remains constantduring particle collision or that the surface charge remains constant; the true situation liesbetween these two assurnptions.

Reerink and Overbeek developed an expression to calculate the interactionpotential caused by the overlap of the d i h e portion of two double layers. Thisinteraction is referred to as the electrostatic repdsion term (TlR). The main assumption intheir derivation is that the interparticle separation is large compared to the thickness of thedouble layers. For equal sphencal particles this denvation is represented by Equations2.8-2.10.


26/105

where'+'dz

E

a

= electrostatic potential on the particle, V= counter ion charge number= permittivity of the medium (water), kg*'m-' 4 A'= particle radius, m.

Viscosity effects in solution dictate that only a portion of the double layer willmove up to approximately the Stem Layer. The dividing line is referred to as the shearplane position where viscosity effects in the solution change drarnatically. The potentialat the shear plane is termed the "electrokinetic" or "zeta potential". It is normallyassumed that the zeta potential and the Stem potential are the same in magnitude. Theslight difference arises as a consequence of a structured aqueous layer which displaces theshear plane of the zeta potential slightly outward from that of the Stem plane.

2.1.3 van de r WaaIs Forces BetweenColloidal ParticlesThe prirnary attractive force between two molecular bodies is the van der Waals

attractive force. There are many subdivisions of these forces, some of the more importantones include: ( i ) permanent dipole-induced dipole, (ii) permanent dipole-permanentdipole, and (iii) induced dipole-induced dipole interaction.' The "induced dipole -inducecidipole" interaction is generally referred to as the London dispersion force. London forcesact between non-polar molecules through polarization of one molecule by fluctuation inthe charge distribution in a second molecule and vice versa. This short-range attractionvaries inverselywith d6where d is the intemolecular distance.


27/105

The attractive force that is operative between molecules can also be applied in thearea of colloids with the appropriate modifications. Hamaker derived an expression toscale-up the van der Waals attraction between molecules to descnbe colloidai systems.Th e derivation assumes that the attraction between particles are additive, and is calculatedby summing the interactions between al1 interparticle molecular pairs. The summationspredict that London interactions decay much Iess rapidly than those of individualmolecules, and the law generally obeys an nverse relationship with distance.

The van der Waals attractive force (VA) is overestimated at large distances sincethe derivation neglects the finite time required for propagation of electromagneticradiation between particles. This weakens V, because the particles will oscillate furtherout of phase the greater their separation. For rnost practical applications in colloidalscience this "retardation" effect is not important.

2.1.4 Hamaker Equation

For non-polar particles the predominating attractive forces are London dispersionforces. Harnaker derived the following expression to describe the attractive dispersionforces acting between two spherical particles of colloidal dimensions (Equation 2.1 1):

whereA = Hamaker constant. J


28/105

H = surface to surface distance, m

ai , a2 = particle radii, m.

One of the difficulties in calculating the van der Waals attraction betweencolloidal particles is the evaluation of the Harnaker constant. This constant c m becalculated fiom either a microscopie or a macroscopic approach. Both approaches havetheir own merit depending upon the separation distances between the particles. Theoriginal equations describing the interparticle interactions were deveioped under vacuumconditions. To account for the influence of the dispersion medium, the Hamaker constantis replaced with an effective Harnaker constant (Equation 2.11) that evaluates theinteraction between particles 1 and 2 interacting through an intervening mediurn 3. '

Equation 2.12 simply states that as two particles approach on e another the particle-dispersion medium interactions are replace with particle-particle and dispersion medium-dispersion medium interactions. The interaction of particles of the same material isalways attractive. The strength of the interaction increases the greater the chemicaldifference between the particles and the dispersion medium. That is, the attractionbetween non-polar particles in a polar medium is stronger than that of polar particles in apolar medium. The main assumptions in the Hamaker rnolecular theory include the


29/105

following: (i) the interactions can be considered pair-wise, (ii) bodies are assurned to have

uniforni density (iii) the interactions of the molecular clouds are instantaneous, (iv) al1dispersion force attractions are due to one dominant frequency, and (v) the bodies are notdistorted by the attractive forces.

2.1.5 Steric StabiiizationAn altemate method of hnparting stability to a colioidal system involves the

adsorption of polymers or surfactant molecules on the surface of the colloidal particles.The term "steric stabilization" is fiequently used to describe this method of stabilization.Adsorbed polymer c h a h acting as stabilizers offer several advantages over electrostaticstabilization. They are generally insensitive to electrolytes. have utility in both aqueousand nonaqueous systems, and exhibit good fieeze-thaw stability7

An effective stabilizer will be strongly anchored to the particle surface and exhibit

hi@ enough surface coverage (to avoid lateral movement during stress) to maintainstabilityon close approach of the particles. Spontaneous re-dispersion of dried particles isa characteristic feature of stencally stabilized systems. There have been severaiexpressions developed to describe the interaction between colloidal particles withadsorbed stabilizers.' One of the more fiequently encountered is the Fischer equation forthe enthalpy of mixing (Equation 2.13):

where5 = volume of a molecule of the dispersion medium. rn j

17


30/105

4 = surface to surface distance,mAli = polyrner-solvent interaction parameter

7ci = concentration of the polyrner in the stenc layer, mol rn';-3

Pi = tabilizer density in th e steric layer, kg rn"6 = steric layer thickness, rna = particle radius, m.

The Fischer equation relates the change in fiee energy of the steric layer as afunction of the restricted volume upon approach of the particles. It relates the excesschernical potential to the excess osrnotic pressure. The fast two terms of the equationreflect the overlap volume occupied by the approach of two spheres. The first and secondterm take into account the interactionbetween the adsorbed layer and the solvent, and theinteraction between the two adsorbed layers.

Consideration must be given to the interaction between the stabilizer and thesolvent in order to achieve the desired level of stability. The dispersion medium must bea "goo d solvent for the stabilizer, in order to prevent the mutual attraction betweenadsorbed layers.

ORen stabilizers are added to systems which already posses some electrostaticstability fiom surface charges on the particle. Some systems derive their stability fiomboth stenc and electrostatic contributions. The stabilizers themselves may containcharged groups. as is the case with ionic surfactants. The term "electrostenc" is oftenemployed to descnbe such systems. These methods of stabilization are illustrated in


31/105

Figure 2.2. in nonaqueous systems where electrostatic stabilization is rarely possible.steric stabilization is oflen the only m eans o f stabilizing a ~ ~ s t e r n . ~

There are several different methods available for directly measuring the interactionbetween adsorbed steric layers. These include: (i ) crossed mica cylinders, (ii) crossedquartz filaments, and (iii) small hemi-spherical caps. Other methods are based onparticdate dispersions.


32/105

Overlap of Steric(a)

Overiap of Electrosteric

Figure 2.2 (a) Steric Stabilization o f Colloidal Particles with Nonionic Surfactants.(b ) Electrostenc Stabilization with Ionic Surfactants.


33/105

2.1.6 Interaction PotentialCurvesTo summarize, the stability of a given colloidal system can be predicted by

cornblliing the attractive and repulsive terms outlined in equations2.1 - 2.13. Thecalcuiations Uivolved areofien presented in the form of potential energy curves whichrelate the interaction potential between two colloidal particles a s a function of distance.Figure 2.3 (a) is a graphical representation of equation 1.8, the effect of increasing surfacepotential c m be easily seen. When the distance between the particle centres approaches 4nm the repulsive energy becomes strong, the energy barrier becomes steeper uponincreasing the surface charge. The van der Waals attractive energy between twopolystyrene particles of equal size is shown in the c u v e below (Figure 2.3 (b)). Thesteeper curves reflect the increase in attractive force that accompanies an increase inparticle size. The effect of surface charge on the stability of the system is s h o w in Figure3.1 (a) which combines the repulsive and attractive terms fiom equations 2.8 and 2.1 1 .The system represented by the top c uve exhibits stability even at small separationdistances, whereas the system represented by the bottom curve is not stable. Figure 2.4(b) dernonstrates the effect of adding a stenc stabilizer to the system. The drarnatic effectof the added stabilizer, can been seen at close separation distances between the particles.The energy barrier becomes large and the system is stable. At these close distances ofseparation in the absence of added stabilizer attractive forces would dominate and theparticles would undergo flocculation.

These theoretical calculations provide the fiamework to measure the stability of adispersion of particles. They can be used to explain and predict the stability of adispersion if an understanding of the properties and particle size are known. These curves


34/105

can be used to test experirnental data and provide the fiamework fiom which to

investigate the aggregation behavior of dispersions. First the system consists of prirnaryparticles that are stabilizedwith a mixture of anionic/nonionic urfactant, destabilizationis induced by addition of cationic surfactant. The caiculated curves provide a means ofdeterminhg how the system should respond if the parameters involved in the calculationsare known or can be determined fkom expenment. Further consideration of stabilizerbehaviour and selection will be outlined in the next section.


35/105

Interparticleseparation, nm

O 4 8 12 16interparticle separation, nm

Figure 2.3 (a) Electrostatic Repulsion Energy as a Function of Surface Potential for a PSLatex System r = 0.1 prn, T = 25 OC: 1 ) 40 mV, 2) 60 mV, 3) 80 m V b) van derWaals Attraction Energy as a Function of Particle Radius, Al 1 = 1.95 x 1 J,T = 25 OC: (1) 50 m , (2) 100 MI, (3) 150 nm.


36/105

O 4 8 12 16lnterparticleseparation,nm

lnterparticleseparation, nm

Figure 2.4 (a ) Total Interaction Potential of a PS Latex System, r = 0.05p,A t j l = 1.95 x ~ o ' ' ~ J .= 2 5 OC: (1 ) 80mV,(2 ) 60 mV, 3)40 m V ( b ) EffectofStencLayer Thickn ess, r = 0.05 pn,Al j I = 1.95 x 10-l9. T = 25 O C : 1) 2.25 nm. 2 ) 2.90 nm.(3) 3.45m.


37/105

2.1.7 Effect of Added Stabilizers

The adsorptior. of nonionic surfactants on a particle surface force the shear planeaway fom the surface reducing the zeta potential relative to the Stem potential.Adsorbed nonionic surfactant gives rise to a steric barrier on the particle. The stenc layeris composed of both surfactant molecules and, in the case of polyethylene oxidenonionics. a highly ordered water layer. The plane of shear is then effectively movedM e r iom the surface of the particles.

If the potential energy barrier is large compared to the thermal energy (kT) f theparticles, the system is kinetically stable. For larger particles, flocculation into thesecondary minimum may have observable effects. The stability of a colloidal system canbe manipulated by changing the solvent or temperature. Changing these variables canmodi@ the interaction potential from virtually hard sphere repulsive to moderatelyattractive. For a particle size of 100nm, he change fiom strong repulsion to attraction inthe dispersion occurs only within a srnaIl temperature or pressure range.


38/105

2.2 SurfactantsSurfactants are a class of rnolecules that have a unique chemical structure. These

molecules consist of a hydrophobic non-polar portion bonded to a hydrophilic polarportion. There are three major types of surfactants: anionic, cationic, and nonionic. Thisclassification scheme is based on the nature of the hydrophilic portion of the molecule.

Surfactants have been used in product formulations for centuries &om primitiveinks and paints to modem detergents and medicines. Several fields rely heavily on the useof surfactants including: detergency, enhanced oil recovery, and pharmaceuticalindustries. Surfactants are most widely employed for their ability to lower the interfacialenergy. A cornmon example of a surfactant is soap. Natural soaps are th e sodium salts offatty acids. One of the disadvantages of these materials is hat diey are converted intoinsoluble magnesium and calcium salts in hard water.' This lirnits their effectiveness inthe area of detergency. This phenornena lead to the development of synthetic soaps orsurfactants.

One of the defming features of surfactant solutions is the critical micelleconcentration or CMC. This is the concentration of surfactant in solution above whichaddition of more surfactant results in the formation of rnice~les.~he structure of amicelle is described below. At the CMC several properties of the bulk solution alsochange, including: density, solubility, osmotic pressure, conductivity and light scatteringability. At the CMC the addition of M e r urfactant to the solution does not increase thefiee surfactant in solution but gives rise to additional micelles. The consequence of thisequilibriurn is that the surface properties of the solution remain relativeiy constant.


39/105

2.2.1 Micelle Definition and Energy Descriptionin a micelle, hydrocarbon chahs are shielded fiom water; the entire structure as

seen by water is hydrophilic and compatible. However, detailed consideration of micelleeeometry indicates that the rnolecular chahs are more "randomly" arr-mgedthroughoutCthe micelle interior, Le. they are not totally shielded by the head groups in typical systems.

For micelles to fom, the Gibbs fiee energy of their formation has to be negative(Equation 2.14). Individual surfactants have a characteristic concentration range in whichmicelles form, this indicates that there are both positive and negative contributions to theGibbs fkee energy of formation:

whereAH = enthalpy contribution, .iAS = entropy contribution, J K".

Upon rnicellization, the hydrophobie parts are shielded (AH egative) but polarhead groups are brought close together. This gives rise to a positive interaction energybetween the head groups of ionic surfactants. The entropy of the surfactant uponmicellization will be negative, consequently there is a need for a sufficient change inenthalpy for the overall reduction in fiee energy.

There is a contribution fiom the solvent on the micellization process. The effect isgreater in water than in less polar solvents. The nature of th e volume available to the


40/105

solvent molecules changes in such a way as to increase the entropy of the solvent, sinceprior to micelle formation, these molecules were tightly associated to the polar or ionichead in the surfactant.

However, the inability of srna11 solvent molecules to penetrate the relatively largesolute-molecule domain reduces the entropy of the solvent molecule near these domains.he decrease in total entropy of the solvent is approximately proportional to the overallarea of the solute domains. Micellization reduces this area, therefore entropic effects of

this sort favour micekat ion. It is clear entropy changes for both surfactant and watermust be considered in calculating the fiee energy of micelli~ation.~

Water molecules form structured clusters around hydrophobic molecules ( i.e. theybecome ordered ). The entropy of these stnictured water layers are lower than they are inthe bulk solution. Upon the formation of micelles, these water molecules are released intothe bulk solution with a subsequent increase in entropy. At higher temperatures, energyeffects usually dorninate and enthalpy is negative.

The CMC decreases and the aggregation nurnber (the nurnber of individualsurfactant molecules within a micelle) increases with increasing hydrophobic chah lengthand increasing salinity. Both of these effects cause the surfactant to become lesshydrophilic. Increasing the salinity increases the screening of the charges associated withthe ionic head groups; this lowers the CMC considerably. The presence of branches,chahs, or double bonds has the opposite effect and hinden micelle formation increasingth e CMC .


41/105

2.2.2 Nonionic SurfactantsThe most fiequently used nonionic surfactants are prepared by adding ethylene

oxide to long chah hydrocarbonswith temiinal polar groups (OH, COOH). Ethyleneoxide adductswere first patented in 1930. These surfactants are produced by the reactionof ethylene oxide with a reactive hydrogen atom on the hydrophobic moiety as illustratedby the reaction mechanism belowl':

'O'

X is one of the following species:NH,O, or S.R is an alkyl or allcylphenyl group

The ethoxylation is carried out in the presence of catalysts. The quality of theproduct depends on the purity of the ethylene oxide and starting materials. *f iemajorimpurity in commercially produced products are polyglycols. These are forrned by thereaction of water with ethylene oxide. Commercial products produced in this fashiongenerally do not exist in pure fom.

The CMC's are much lower for nonionic surfactants than ionic surfactants withcomparable chain lengths, as illustrated by the following values at room temperature: 6.8x 10" M for C,?EO, (EO represents one ethylene oxide unit) versus 8.0 x IO-' M foranionic sodium dodecyl sulfate (SDS). ' The electrical repulsion between head groupsstrongly opposes micelle formation for ionic surfactants. The differences in CMC have a


42/105

tremendous impact on the properties of a system depending on the particular requirementsof the surfactant.

2.2.3 Cloud Point TemperatureSensitivity to temperature is a distinctive feature of nonionic micellar solutions.

These solutions become turbid beyond the so-called cloud point temperature. Thistemperature marks the condition where a surfactant rich liquid begins to f o m inequilibrium with the rnicellar solution. This phenornenon is a consequence of thebreakdown of hydrogen bonding between the ethoxy groups of the surfactant and thesurrounding water. This was confirmed by experiments that compared the molecularvolume occupied per surfactant molecule at different temperatures. h e cloud point canbe manipulated by changing the length of the polar portion of the molecule. Ln contrast,the CMC s far more sensitive to the number of carbon atoms in the tail than to thenumber of ethoxy groups. Understanding Cloud Point behaviour is an importantconsideration when selectinga particular surfactant for an application.

The -'head group" is the polar or ionic portion of a surfactant molecule. Thisgroup is easily solvated and can have specific interactions with the solvent including:solvation, dipole-dipole, ion-dipole, and in polar solvents hydrogen b ~ n d i n ~ . ~

The Kraft Temperatureor Kraft Point is the temperature at which the solubility ofa surfactant becomes equal to the CMC. h ther words the solubility of individualsurfactant molecules in the solution is equal to the CMC of the s~rf acta nt.~


43/105

The lifetime of a swfactant molecule in a micelle is of the order of 1 x IO-'seconds. This reflects the dynarnic nature of micelles; there exists a constant interchangeof individuai surfactant molecules between micelles and solution.

2.2.4 HLB ClassificationOne of the primary means of c l as s i mg nonionic surfactants is through the

hydrophilic-lipophilic balance or HLB classification scheme. This is an ernpiricalclassification fint developed by ~r iffen. ' A number assignment is calculated based onthe weight percent of hydrophilic portion of the molecule. Features such as solubility andmicelle forrning behaviour can be derived fiom this classification. For the nonionic alkyl-aryl polyethylene oxides the following simple formula (Equation 2.16) can be used tocalculate this parameter:

weight% POE glycolHLB = 5

The HLB classification of some representative commercial nonionic nonylphenolpolyethylene oxides are shown in Table 2.1.


44/105

Table 2.1 Selected HLB Values of Nonionic Surfactants

IGEPAL CO-720IGEPAL CO-850IGEPAL CO-880IGEPAL CO-890

Nonionic surfactants may interact with the surface through one or a combinationof the following mec hanisms: hyd rogen bonding, adsorption by polarization o f rrelectrons, van der W aals dispersion forces, and altemating hydrophob ic bonding.Alternating hydrophobic bo nding may re sult when the surfactant chahs orient thernselvesso that their respective head groups are joined. This then allows tw o separate tail sectionsto interact with the surface. Surfactants which contain an aromatic ring in thehydrophobic region may exe rt specific interaction with nonpolar solids.

2.2.5 ApplicationsMain fields of domestic and industrial applications that emp loy the use o f

nonionic surfactants include: deterge nts, cleaners, cosmetics, textile, leather, fur, oit, andplastics. As a general guide, the application of a pmicular surfactant can be generallyclassified according to the nurnber of ethoxy groups in the polar portion o f the mo lecule.The HLB classification outlined above can be considered a general guide for applications:5-6 are suitable as emulsifiers for hydrocarbons. 8- 12 for wetting agents and detergents,


45/105

12-15 in dispersing agents. For a constant c h a h length o f hydrophobie groups theproducts range fiom waxy, pasty to iiquid which is a function of the length of thehydrophilic portion of the molecule.


46/105

2.3 Adsorption Isotherms forNonionic SurfactantsFor an understanding of surfactant behaviour and design it is equally important to

characterize the material with which the surfactant will interact. Throughout the literaturethe terni adsorbate is used to designate the matenal which the surfactant adsorbs on. Theterm adsorbent is reserved for the species that is adsorbed. Adsorbates can be classifiedinto two general categones depending on the nature of the surface. Hydrophilic or polaradsorbates are matenals with surfaces containhg ionogenic sites or dipolar moleculargroups including hydroxyl or carbonyl. These matenals ofien have a hi& a f i t y forwater; examples include silicates. inorganic oxides, hydroxides, and natural fibres. Theother broad category of adsorbates are "low energy" or "non-specific", since they interactwith adsorbate through van der Waals dispersion forces rather than the more specific andgenerally stronger dipolar or electrostatic forces. Examples of non-polar adsorbatesinclude: carbon blacks, organic pigments, and some polymers.

The surface properties of the solid substrate depend on the treatment and additivesused during the manufacturing process. For this reason it is highly desirable tocharacterize the adsorbents as thoroughly as possible. Methods of characterizationinclude particle sizing techniques and surface analysis. Surface analysis may inctudeconductometric titrations to assess the number and concentration of ionizable groups onthe surface.

Adsorption isotherms can be used to study the adsorption process of surfactants asa function of surfactant concentration. The isotherms are generally computed bydeterminhg the amount of surfactant that is depleted fiom solution due to the adsorption


47/105

process. This change in the nurnber of moles of surfactant adsorbed on a unit mass ofsolid is given to a good approximation by Equation 2.16."

where

4 = number of moles of sufactant adsorbed on a unit mass of solid. molrio = total number of moles of solution before adsorption, molAx2 = change in mole fiaction of surfactant resulting fiom adsorptionm = mass of insoluble adsorbent, g.

2.3.1 Methods for Evaluating Surfactant Concentrat ionsThere have been nurnerous different approaches to determinhg the change of

surfactant concentration in solution upon adsorption. Most of them ivolve a prioricalibration of th e surfactant at the appropriate experimental concentrations involved.Some of the more fiequently used methods include: changes in refractive index, UVabsorbante, surface tension, innared spectroscopy,and radio-tracer techniques.

As previously rnentioned, cornmercially produced surfactants contain a mixture ofisomers. The extent of polydispersity mu t be taken into consideration when calculatingan isotherm. Depending on both the substrate and the solution conditions this may result


48/105

in preferential adsorption of on e of th e isomeric species. tsotherms that exhibit this typeof behaviour tend to deviate fiom their expected concentration dependenc e.

An alternative expression for describing the amount of surfactant adsorbed for lesswell defined surfaces is th e surface excess which can be calculated fiom Equation 2.17:

whereTI= = arnount adsorbed, mol m-'4 = number of moles of surfactant adsorbed, molAs = surface area of substrate, m2.

The surface area of the rnaterial c m be determined by a variety of techniqu es; oneof the most comrnon is the BET adsorption isotherm named after Brunauer, Emmett andTeller. The limitation of this technique is that the surface area is calculated on the basisof the area occupied by inert gas molecules (usually nitrogen). This area is not equivalentto that accessible to the surfactant due to the difference in molecular geometry.Consequ ently there exists so me un certainty on the calculation of the surface excess.

2.3.2 Shape of IsothermsAdsorp tion isotherms for nonionic surfactants are generally Langmuir-type

isotherms and have the general structure illustrated in Figure 2.5.


49/105

Figure 2.5 Idealized L4 Type Isotherm for Adsorption of Nonionic Surfactants at theSoIid/SoIution Interface (after Parfin and Rochester 1983).


50/105

The plateau found in A is only relevant to a few systems. The dif icultyencountered in this region of the isotherrn is to accurately assess the small concentrationsof surfactant involved. The idec ti on and sharp increase in adsorption at B occurs at bulksolution concentrations that are close to the CMC of the surfactant. This is followed by asecond plateau in region C. The molecular structure of the surfactant significantlyinfluences the shape of the isotherm in many ways. Increasing the length of thehydrocarbon chah generally increases the magnitude of the maximum adsorption for agiven hydrophobic substrate. This is usually due to the combination of increased solute-surfactant interactions coupled with increased tail-tail interactions afier the adsorption hastaken place. The opposite effect is observed on increasing the size of the polar headgroup. This c m be explained by an increased steric demand of th e hydrophilic portion ofthe molecule coupled with their increased affinity for th e solvent (Le. as the proportion ofth e rnolecule that is soluble is increased the greater the tendency for the surfactant toremain in solution). The relationship may not hold for substrateswith polar surfaceswhere the adsorption of nonionics on the substrate occurs with the hydrophilic or "head-group" of the surfactant oriented towards the surface to th e substrate.

Comrnercially produced surfactants exhibit similar isotherm shapes at high surfacecoverage in al1 cases. Adsorption reflects the same hydrophilic/hydrophobic forces thatdrive self-assembly of the micelles in s~lut ion. '~sotherms for the adsorption ofnonionics at the solid Iiquid interface are observed to plateau at concentrations in excessof the CMC. Generally it is believed that the mixture behaves in a similar manner to thecomponent that has the average rnolecular weight of the mixture.


51/105

The strength of the interaction between the adsorbent-adsorbate can be inferredfkom the shape of the isotherm. S '-shaped" isotherms are typical, in the limit of lowsurface coverage, this represents a weak adsorbent-adsorbate interaction. A sharpincrease in the isotherm is consistent with the onset of a CO-operative dsorption arnongthe surfactant molecules. It has been experirnentally observed that CO-operativeadsorption occurs at lower concentrations in commercially produced surfactants than theequivalent homogeneous material. Plateau adsorption is reached at one to three times theCMC of the surfactant.

2.3.3 Steric Layer ThicknessIn addition to understanding of the properties of an adsorption isotherm, it is

important to consider the factors that influence the conformation of these materials afieradsorption is cornplete. The adsorption of surfactant results in an outward movement ofthe plane of shear between the solid and solution interface. This can be detected by adecrease in the zeta potential in the system afier adsorption has taken p l a ~ e . ' ~hanges inthe thickness of the adsorbed layer are reflected in the plane of shear which can bedetected by measuring zeta potential values within a system.

At low ionic strength, the thickness of the adsorbed layer approaches a limit equalto the hydrodynamic thickness. '' The electrokinetic thickness at low ionic strengthshould approximately equal the steric thickness. Only at low ionic strengths can theeffective slipping plane be identified with the electrokhetic thickness. At high saltconcentrations the electrokinetic thickness is always smaller due to compression of thedouble layer resulting fiom a screening of surface charges.


52/105

Large repulsive energies occur at slight overlap of the adsorbed layers if thehydrophilic portion of the surfactant is in a good solvent. This repulsion was discussedearlier as the stenc stabilization mechanism. Water is a good soivent for polyethyleneoxide head groups, this re sdts in strong repulsion and negligible interpenetration ofadsorbed head groups and imparts good stability in the system. The effect ofpolydispersity of surfactant samples results in a preferential adsorption of the shorterchahs. The phenornena was investigated by Kronberg et al. who concluded "'the final

distribution of adsorbed species fiom polydisperse systems may take considerable time.due to the preferential adsorption of short-chah homologues at equilibrium".'5 Due tothe manufacturing conditions of nonionics there ofien exists a wide EO ch ah lengthdistribution that may vaxy considerably behveen dif5erent batches. in order to assesstheoretical trends it is necessary to work with pure materials. With commercial samplesthe adsorption isotherms rnay not be reproducible arnong different batches. Thus it isimportant to have adequate knowledge of the purity of the materials being used in order toassess the size and behaviour of the steric layer.

The dependence of adsorption strength on EO chah length, provides anexplanation of the variation in adsorption characteristics between different batches ofnonionic surfactant, the content of fiee polyethylene oxide typically varies between3 %-8 % by weight in commercial samples. For nonionic surfactantswith 20 and 50 EOunits al1 isotherms reach a plateau value at solution concentrations correlating with theCMC's of the surfactants. 5

Isotherm plateau coverage reflects the affmity of the hydrophobic moiety of thesurfactant to a non-polar environment. This plateau coverage usually occurs when the


53/105

concentration of th e surfactant in the solution has reached a value of 85-90 96 of the CMCof the surfactant under consideration. With technical p d e surfactants it is difficult toestablish theoretical trends. The affinityof the ethylene oxide c h a h to the particle may beinfluenced by the presence of carboxylic groups which hydrogen-bond with ether oxygenatorns. The standard fiee energy of adsorption becomes less negative as the ethyleneoxide chah length is increased. Anionic SDS has a much lower affinity to non polarsufaces than nonionics. Nonionics adsorb more strongly and can displace anionics fiom asurface.I6This is anticipated by considering their respective CMC values which reflectthe a&ity of the surfactantto dissolve in solution. Surfactants have the ability to modiQthe interface and prornote dispe:sion between unlike phases. Ionic surfactants also havethe ability to impart a charge to a neutral surface. in the next section the importance ofmeasuring and understanding the effect of surface charge will be discussed.


54/105

2.4 AcoustophoresisOneof th e primary rneans of measuring charges on colloidal particles is to

evaluate the zeta potential. There are severalmethods available to perform thesemeasurements; one of themore versatile is the technique of acoustophoresis. Thetechnique of acoustophoresis c m be used to characterize colloidal dispersions of Ioadingsup to 50% by volume. Acoustophoresis utlilizes the interactionof sound waves with theelectrically charged colloidalparticles to detemine the relative acoustic mobility (RAM).This value can subsequentlybe converted to a zeta potential. The technique is based onthe phenornena that particles in solution are surrounded by diffuse ciouds of ions knownas the "double layer". When this layer is subject to an altemating acoustic field itbecomes polarized and the particles movewith respect to the field (see Figure 2.6).

2.4.1 Principles o f OperationSeparation is induced between a colloidal particle and its surrounding diffuse

double layer through the interaction with sound waves. This results in an altematingpotential termed the colloid vibration potential (CVP). TheCVP is several orders ofmagnitudegreater than the related ion vibration potential (IVP)which was predicted anddiscussed by Debye in the 1930's.


55/105

PROPAGATIONDIRECTION

Figure 2.6 Schemaiic of Acoustophorectic Mechanism (after Marlow and Fairhurst, 1988).


56/105

The CVP is dependent upon the following characteristics of the colloid: (i) zetapotentiai (ii) particle concentration, (iii) fiequency of the acoustic wave, and (iv) thenature of the supporting electrolyte.

Acoustophoretic measurements are given in units of Relative Acoustic Mobility(RAM). in order to perform a measurement. the machine is calibrated using a zirconiumoxide colloid at pH 4. The charge on the particles is unarnbiguousiy positive at this pH.Al1 subsequent measurernents are based on this reference including phase detection and

absolute magnitude of the signal. The zeta potential is rclated to the RAM ccording toEquation 2.18.

where4 = zeta potential, VP = fluid density, kg m"Pz = particle density, kg m"E = pemittivity of fiee space, kg-' m" s4 A'rt = fluid viscosity, Ns rn"0 = volume fraction of the particleD = Dielectric constant.


57/105

The volume fraction of the particles cm be easily calculated fkom their weightfiaction according to Equation 2.19:

whereP1 = fluid density, kg rn"P2 = particle density, kg m-'x = particle weight fraction4) = volume fhction of the particle.

2.4.2 ElectroacousticsAs rnentioned previously, electroacoustics involves passing a hi& fkequency

sound wave through a colIoidal sus~onsion.The wave fiequencies are usually on theorder of several hundred kHz. The sound wave causes the particles to oscillate with thesame frequency.

The particle motion can be detected because it gives rise to an altematingelectncal signal. The signal arises because in addition to the very low amplitude motionof the particle, there is a larger rnovement of the ions associated with the double layer.Since the fluid can respond to the pressure wave more quickly than the particle (if pz > pl)a small dipole is generated. The presence of many such dipotes in the suspension, al1


58/105

pointing in the same direction, creates a macroscopic electnc field. This field can bedetected by placing two electrodes in the suspension positioned at the peak and trough ofthe sound ~ a v e . ~

Cornpressional sound waves give nse to a penodic polarkation of the ionicatmosphere surrounding the particles, causing each particle to act as a vibrating dipolewhich results in an altemating voltage. This altemating voltage is the CVP mentionedabove. During this phenomena double layer relaxation is the dominant process. A morecornplete description of the phenomena is outlined below.17

2.4.3 Advantages of ElectroacousticsThe Pen Kem systern is highly versatile and offers several advantages over

traditional electrokinetic techniques. Some of its more salient features Uiclude: ability tooperate within a wide range of particle sizes(nm o pm), sample concentration rangesfiom as low as ppm to volume f i l h g systems, and adaptability to on-line measurements.Measurements can be applied to a wide range of materials including opaque,photosensitive or mobile livingorganisms.

The interaction of sound waves with particles has been studied for decades. Thewell known Debye Eflect was first proposed in the 1930's.'' The general prernise is asfollows: dynarnic reactions of ions in an ultrasonic field will be difTerent for ions ofdifferent nasses. The relative displacement of anions and cations produces a separationof charge accompanying the sound wave, resulting in a potential difference.


59/105

Further study in this field by Hemans, Rutgers and Enderby indicate the CVP isdependent upon the following characteristics of the system: zeta potential, concentrationand nature of the particles, fiequency of the acoustic wave, and supporthg electrolytenature and concentration. '

The sound source m u t be able to generate a monochromatic, plane progressivesound wave, whose wavelength mut be much greater than the radius of the particles inorder to promote the harmonies necessary for signal generation. The fluid will respond tothe sound wave by varying harmonically and it will transfer its momenturn to the particlethrough '&cous coupling" resulting in the same harmonica1 motion for the particles.Due to the difference in density between the tw o mediurns the particles will experience"relative" harmonic motion with respect to their surrounding aqueous environment. Inorder for this process to be effective, the double layer relaxation time must be less thanthe period of oscillation of the sound wave. Th~sliows the particles to reach a steady-state prior to disturbance fiom the next wave.

The polarization of the electric double layer is a dynamic process. Le. the electricfield is measured as a function of time. in an acoustic field the double layer ions lead theparticle, and the theory assumes that the penod of the acoustic wave is much greater thanthe dynamic relaxation time of the particle as well as the double layer relaxation time.That is to Say, the particles and their surrounding double layer have relaxed prior to beingdisturbed by the next portion of the sound wave.

The CVP is the signal generated in the receiver by th e peak amplitude of thealtemating potentiai o f the particles, when the electrodes are separated by U2. The


60/105

moment of each dipole is proportional to the zeta potential and the relative velocitybetween particle and fluid as described by Equation 2.18.

Retardation is a phenornena that stems fiom the fact that polarization of the doublelayer ions result in an induced electric field. The induced electric field opposes particlemotion. This is important for small particles that are highiy charged in a media of Iowconductivity where the double layers of the particles would be extremely thick ( 10


61/105

CHAPTER 3. EXPERIMENTAL3.1Materials

3.1.1 Nonionic SurfactantsThe nonion ic surfactants used in the study were supplied by R hne-Pou lenc's

Surfactants and Specialities Group of Cranbury, New Jersey. They are comm erciallyproduced surfactants with a polydisperse size distribution. The surfactants are listed inTable 3.1 along with some of their p hysical characteristics. These surfactants werechosen primarily because of their widespread use as stabilizers in commercial colloidaisystems.

Table 3.1 Nonionic Surfactants and Pbysical Data.Trade Narne Nurnber of EO units Appearance@ 25 O C Molecular WeightIgepal CO-720 12 hazy viscous liquid 748Igepal CO-850 20 soft wax I l 0 0Igepal CO-880 30 waxy solid 1540Igepal CO-890 40 waxy solid 1980Igepal CO-970 50 hard waxy solid 2420


62/105

Table 3.2 Absorptivity Data for Nonionic Surfactant, h = 174 nm.

Trade Name Molar Absorptivity.L mol-' c dIgepai CO-720 1400Igepal CO-850 2700Igepal CO-880 1300lgepal CO-970 1600

3.1.2 Ionic SurfactantsTwo types of ionic surfactants were employed in this study. A cationic

teaadecyltnmethylammonium bromide (TTAB) [C ,,H2,N(CH3)$3r was purchased fiomsigma" Chernical Company of St. Louis, Missouri. This matenal is a white powder andhas a reported 99.9 % pur@, it was used as received. The anionic surfactant was Sodiumdodecylsulfate (SDS), Electro Pure, [C,2H2,0S03]Na nd was purchased fiomPolysciences of Warrington, Pennsylvania. This matenal is also a white powder and wasused as received.

3.1.3 Carbon BlackThe carbon black used in a11 the adsorption studies was sterling@l 20 (ST1120)

produced by Cabot Corporation. The powdered sample has physical characteristics asListed in Table 3.3.


63/105

Table 3.3 Physical Properties of ~terling'@l120.

Property ValueIodine Absorption Nurnber (g kg-') --7 + 4Dibutyl Phthalate Absorption Nurnber (cm3/100g) 32 fSolvent Discolouration@ 124nrn 70% minimumS eve Residue US 35 mesh 10ppm maximum

US 325 mesh 200 ppm maximumOxygen Content not availablePH 7.0Heating Loss (as packaged) 1.O%maxirnun

Values taken fiom technical data sheet provided by Cabot Corporation.

n i e surface area of the matenal was determined experimentally on a~icromeritics@lowsorb II 2300 with a Nz urface area determination. The results arelisted below in Table 3.4.

Table 3.4 Surface Area Measurements on STI 120 Carbon Black.Sarnple Mass, Nitrogen Adsorption, SurfaceArea, Average,

g rn' ' - 1m-2.1637 49.49 22.87 22.8 O.1 )


64/105

Carbon black is classified as an arnorphous carbon and s characterized by animperfect or degenerate graphite structure. The se m aterials have physical properties inthe fo llowing ranges: specific gravity 1.86-2.04, carbon-carbon bond distance 0.142 nm,and interlayer spacing distance of 0.365 m .'' This material was chosen for adsorptionstudies because of its uniform hydrophobie surface.

3.2 Adsorption Isotherm PreparationA controlled amount of adsorbent was d ispersed in aqueo us solution, using

distilled water or deionized distilled water (DDW). This was accomplished by adding 30g of powdered carbon black to 1.5 L of water. This solution was then stirred at 200 rpmusing a Janke & Kunkel mechanical stirrer to ensure a homogeneous mixture.Subsequently 30 mL of this continuously stirred solution was pipetted to a screw capcentrifuge tube (capacity 50 mL).

Controlled incremental arnounts of surfacta nt were added to the tubes. Distilledwater was added to each tube to make the final volum e up to 40 mL. Each adsorptioncurve was composed of 8-10 points whose concen trations were selected to encompass thesurface saturation of the solid adsorbent.

The tubes were then rolled on a collo id ro ll mi11 for 12-15 hours to ensure that theadsorption equilibrium had been reached between the p article surface and the solution.To sepa rate the particles fiom the supernatant, the sarnples were centnfuge d for one hourat 10,000 rpm ushg a Milton-Roy MR 18.22 centrifuge. Any remaining particdates wereremoved during a subsequent filtration step through a c ellulose nitrate filter with anaverage pore size of 0.05 Fm.


65/105

3.3 Surfactant Analysis3.3.1 Nonionic Analysis

The UV-visible analysis of the supernatant was performed using a HewlettPackard 8452A diode array spectrophotom eter. Th e absorbance of the solution wasmeasured at 274 nm, the wav elength at which the aromatic portion of the nonionicsurfactant exhibits a strong absorban ce signal. The measu red absorbance values wereconverted into concentration values by means of a calibration curve. The calibrationc u v e was prepared to encom pass al1 concentration ranges encountered during the courseof the adsorption studies. An individual curve was prepared for each nonionic surfactantto account for changes in molar ab sorptivity that accornp any changes in molecular weight.

3.3.2 Anionic SurfactantAnalysisSD S is among a c lass of compo unds know n as rnethylene blue active substances

(MBAS). The anionic surfactant combines with the rnethylene blue (a cationic dye) in a1 : com plex formation. Methy lene blue exhibits a strong [IV spectnim, and theconcentration of th e SD S species in solution can be determined fiom the stoichiometry ofthe reaction.

The anion ic surfactant was analyzed by first coupling it with methylene blue dyethen extracting this complex into chloroforrn. This reac tion occurs through a 1: ion pairformation by the surfactant anion and the m ethylene blue cation. The intensity of theresulting blue colour (at 652 nrn) n the chloroform phase is a measure of anionicsurfactant concentration. The amount of surfactant adsorbed in p o l g'l was calculatedby determining the change in th e nurnber of mo les between the initial an d equilibriurn


66/105

surfactant concentration. This technique was adopted fkom a method used in the analysisof surfactants in wastewater.19The method relies on the strong interaction between theionic head groups of the dye and surfactant. The ion-pair forms a large organic moleculewhich is easily removed h m he aqueous solution by extraction into an organic solvent.The interpretationof the absorbance values is accomplished by means of a calibrationcurve. The calibration curve is obtained by preparing a series of standards and measuringthe absorbance of these standardsat 652 nm. Experirnental values are then calculatedbased on the regression line fiom the calibration curve. A V spectrum of theSDS:Methylene Blue complex is illustrated in Figure 3.1.


67/105

MethyleneBlue:SDS ComplexSignal\

100 200 300 400 500 600 700 800 900Wavelength, nm

Figure 3.1 UV-visible Spectrum of SDS:Methylene Blue Complex.


68/105

3.3.3 Cationic Surfactant AnalysisThe anaIysis of the cationic surfactant was carried out using the analytical

technique developed by Tsubouchi et al." The method involves the determination ofcationic surfactmts by a two-phase titratiun. The cationic surfactant solution is treatedwith an indicator under controlied pH conditions. The indicator and the cationicsurfactant form an ion pair that is blue in colour. This mixture is then subsequentlytitrated with the sodium salt of tetraphenyl borate. Under these conditions the dye-

surfactant complex is replaced by a surfactant-surfactant complex, with subsequentprotonation of the indicator. The protonated indicator has a yellow colour which signalsthe end point of the reaction. Knowing the concentration of the anionic surfactant in thetitrant solution the amount of cationic surfactant cm be determined. The reaction can berepresented accordhg to Equation 3.1.

blue yellow colourless

whereCAS'InO, w

- cationic surfactant- indicator- organic or aqueous phase respectively.


69/105

3.4 Pen Kem 7000 AcoustophoreticTitratorAcoustoph oresis experimen ts were performed using the Pen Kern 7000

Acoustophoretic ~ it ra to ? As described in the introduction, this technique exploits theinteraction of sound w aves with the el ec tic al interface. The device measures the relativeacoustophoretic rnobility(wf the sample. These values can be converted to zetapotentiai values if the density and volume percent of the colloid are known.

Th e sampleswere prepared at 2-5 % solids by weight. This was accomplished byweighmg the ST1120 particles into a 1 L Nalgene screw top bottle and add ing acontrolled amoun t of surfactant. The sample was then made up to volume with distilledwater. Washed zirconia grinding media were added to the bottle which was then placedto roll on a colloid mi11 for a minimum of 15hours. After equilibration of the particlesand surfactant, the sample could be measured. A minimum of 250 rnL of solution isrequired to perform a single experiment. If the experimen t does not involve caIculationsof surfactant arnounts, accurate volumes are not necessary provided al1 probes on themeasuring unit are sufficiently submerged fo r data collection.

In addition to acoustophoretic rneasurements the unit is equipped w ithconductivity, pH and temperature probes. These probes allow for m e r haracterizationof the sample during the course of an experiment. An automated burette can also beemployed to add reagents such as acid, base o r surfactant solutions to the sample.

Experim ents were conducted in "titra tion " or "time mode". in time mode heinstrument simply monitors the properties of the colloid as a function of tirne. Themeasurem ent intervals are operator controlled. In titration mode. measurem ents are


70/105

generated as a function of added reagent. Both the volume and mixing time during thetitration can be set by the operator allowhg for flexibility in experimental design and datacollection. Experiments on carbon black solutions were made in both titration and timemode.

3.5 Pen Kern 501Zeta MeterElectrokinetic measurements were performed ushg the Pen Kern 501 Zeta

~ e t e r ?This instrument measures the electrokinetic potential of the sarnple. The sarnpleis placed in a quartz chamber and a voltage is applied. The particles migrate in responseto th e applied potential, and are illurninated with a laser bearn and monitored with anoptical microscope. Adjustments are made to keep the particles motionless in the appliedtield, the zeta potential of the particles can then be read fkom the digital display. Thesarnple volume of the chamber is approxirnately 25 mL. The concentration of the colloidshould be in the following range; 1o6 to 1o9 particles cm", shce this allows for optimalpassage of the laser light while permitting suscient particles to establish a consistentsignal. The colloidal dispersions had to be diluted pnor to measurement. Where possiblethis was accomplished by dilutingwith the mother solution from which the dispersion hadbeen prepared.

This instrument was used in conjunction with the Pen Kern 7000 system as acheck on the stability of the systems. The advantage of this instrument was that it wasable to provide unambiguous assignment of positive or negative charge to the particlesunder study. One of the limitations of this device was the diEculty in recreating thebackground supernatant solution fiom which the particleswere taken. Values were


71/105

recorded by takmg five individual measurements. the mean of the these values was takento represent the zeta potential. The systems studied included nonionics with variablehydrophilic chah lengths, anionic and cationic systems at dif'ferent concentrations an dmixed surfactant systems adsorbed on STI 120.

3.6 Conductivity ExperimentsThe CMC's of TTAB an d SD S were determined experimentally through th e

measurement of conductivity in aqueou s solution. The conductivity w as recorded duringthe titration of DD W with a concentrated surfactant solution. The CMC is determined bya change in the dop e o f the conductivity curve that occurs at the onset o f micellization.The solutions were titrated in a the mo stat ed ce11 and repeated in triplicate. Additionalexperiments were performed to investigate the change in conductivity that occurs duringthe rnixing of ionic surfactants. These experiments were u s e h l for providing informationregarding the strength of the formation of the "pseudo-nonionic" complex fonned fromthe interaction of the two ionic surfactants.

3.7 Aggregation StudiesSarnples for aggregation studies were prepared by rolling ST1 120 at 3 wt% solids

in grinding media and excess anionic surfactant. After equilibration the solution wasplaced in a ta11 graduated cylinder to allow the larger size aggregates to settle out ofsolution. Incremental arnounts of cationic surfactant were added to this solution toencompass the CMC o f the cationic surfactant. Any change in the particle sizedistribution was monitored using an optical microscope.


72/105

The ST 120 particles mixed with SDS appear unifom and stabilized. Variable-sized aggregates do not appear visible under this magnification. Microfiltration of thisdispersion with cellulose nitrate filter pore size O. 1pn does not remove the smallestparticles fiom solution. M e n he pore size of the filter is changed to 0.05p l1 of theparticles are removed nom solution. This was confirmed by measuring the W-spectra ofthe filtered solution which did not reveal any scattering caused by particles remaining insolution.

3.7.1 Zeta Potential Measurements of Ionic Surfactants

A solution of anionically stabilized particles was prepared (50 mi,of SDSmothersolution concentration = 2 x IO- ' M, 3 g of ST1120 carbon black, and 950 mL of distilledwater) and controlled arnounts of cationic surfactant solution were added. Observationsto the final state of aggregationwere made.

Controlled addition of SDS to the particles was accomplished by adding thesurfactant solution to 30 mL of 0.03 wtiwt solution of ST1120. The volume was made upto 40 rnL with distilled water. Solutions were equilibrated for 24 hours prior tomeasurement. Initial measuring attempts revealed the samples were too concentrated tobe used in the PenKemSO 1 system. Samples were diluted by pipetting 1mL of thesurfactant mixture and adding 39 mL of distilled water. The resulting solutions

surf act ant and colloidal cb

Documents