advantageous supramolecular system through self ... polymer, xanthan gum self-associates through...

Research ArticleVolume 2 Issue 4 - March 2019DOI: 10.19080/AJOP.2019.02.555592

Academ J Polym SciCopyright © All rights are reserved by Laura Romero Zeron

Advantageous Supramolecular System Through Self-Association of Xanthan Gum/Cationic

Surfactant Via -Cyclodextrin Host-Guest Complexations

Laura Romero Zeron1* and Claudia Espinosa2

1Department of Chemical Engineering, University of New Brunswick, Canada2Department of chemical Engineering, University of La Salle-México, Mexico

Submission: February 20, 2019; Published: March 13, 2019

*Corresponding author: Laura Romero Zeron, Departmentof Chemical Engineering, University of New Brunswick, Canada

IntroductionXanthan gum is an anionic polysaccharide that is produced by

fermentation of a carbohydrate with Xanthomonas campestris. The primary structure of xanthan is composed of a Penta sac-charide repeating unit including two β-D-glucose residues, two D-mannose residues, and one β-D-glucuronic acid residue. The molecular weight distribution of xanthan gum ranges from 2x106 to 20x106Da [1]. Xanthan gum shows two conformations in aque-ous solutions. An ordered and rigid double helical strand structure at low temperature and a disordered and flexible coil structure at high temperature [2,3]; consequently, it undergoes thermally-in-duced order-disorder conformational transitions [1,4]. The mid-point transition temperature (Tm) is about 40 - 50°C depending on the ionic strength [2]. The ordered double-helical conformation

forms three dimensional networks [2] and this conformation is stabilized in the presence of ions in the aqueous solution [5]. For instance, divalent cations such as calcium (Ca2+) strongly binds to xanthan and stabilizes the helical conformation producing hydrogels with superior mechanical performance [1,6]; because electrolytes reduce the intra-molecular electrostatic repulsion in the xanthan gum backbone [1]. Additionally, the ordered helical structure provides thermal stability to the xanthan gum making it more resistant to hydrolysis (de-polymerization) than other poly-saccharides or synthetic polymers (i.e. HPAM) [1]. Xanthan gum is non-toxic (i.e. excellent biocompatibility), water-soluble, and ex-hibits distinctive rheological properties such as high viscosity at lower concentrations, highly shear thinning, stability under shear

Academ J Polym Sci 2(4): AJOP.MS.ID.555592 (2019) 0091

Abstract

This work takes advantage of the versatility of xanthan gum to evaluate its self-assembly potential to further improve its functionality at elevated temperatures and in aqueous solutions containing high salinity and hardness concentration. Therefore, the effect of type of surfactant (i.e. anionic and cationic) and brine concentration (2.10 & 8.41wt%) on the formation of xanthan gum supramolecular systems through non-covalent interactions was established and compared to the behavior of a parallel self-assembled system based on a partially hydrolyzed polyacrylamide (HPAM).

Experimental observations reveal self-assembly of the xanthan-gum/cationic surfactant/-CD (xanthan gum-SAP) mixture and self-association of the HPAM/anionic surfactant/-CD (HPAM-SAP) blend. Oscillatory rheology demonstrates superior viscoelasticity for both SAP systems at high brine concentration (8.41wt%) relative to their corresponding baseline polymers. Dynamic-Mechanical Analysis (DMA) using oscillatory rheometry indicates that the xanthan gum-SAP shows thixotropic behavior; while the HPAM-SAP system displays only partial regeneration of the original structural strength. Furthermore, both SAP systems exhibit superior thermal stability than their corresponding reference systems.

Static adsorption tests show that the adsorption of the encapsulated cationic surfactant onto sand and kaolin decreases by 27% compared to the adsorption behavior of the cationic surfactant in free-state. The long-term bio-stability evaluation indicates that high brine concentration (8.41wt%) seems to offer the main antimicrobial action to the polymeric systems. Overall, the xanthan gum/cationic surfactant/-CD self-assembly offers superior viscoelastic rheological behavior, thermal stability, and tolerance to high salinity and hardness concentration relative to the HPAM/anionic surfactant/-CD system. These observations are relevant for enhanced oil recovery (EOR) applications.

Keywords: Xanthan gum; Self-assembly; Surfactant; -cyclodextrin; HPAM; Inclusion complex; Non-covalent interactions; Self-association; Supramolecular

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How to cite this article: Laura Romero Zeron and Claudia Espinosa. Advantageous Supramolecular System Through Self-Association of Xanthan Gum/Cationic Surfactant Via b-Cyclodextrin Host-Guest Complexations. Academ J Polym Sci. 2019; 2(4): 555592. DOI: 10.19080/AJOP.2019.02.555592

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(i.e. thixotropy), maintains high viscosities in the presence of elec-trolytes, high temperature, and wide pH ranges [1,6,7]. Addition-ally, the large number of hydroxyl groups (-OH) and free carboxyl groups (-COO-) in the structure of the xanthan makes it a multi-purpose biopolymer for chemical modification and/or function-alization to further optimize its physicochemical properties [1,8].

Xanthan gum shows the ability to form physical networks (i.e. self-association) through hydrogen bonding, hydrophobic associ-ation, and cation bridging that results in three-dimensional net-works that contains solvent in the interstices [1,7]. For instance, xanthan gum molecules in water-ethanol solution self-aggregates in bundles of nanofibers. Generally, the self-association of xanthan gum is affected by the concentration of the polysaccharide, pH of the medium, molar mass/degree of polymerization, temperature, ionic composition, and solvent quality [7]. Furthermore, as an an-ionic polymer, xanthan gum self-associates through electrostatic interactions with functional materials [6]. Xanthan gum shows re-markable synergistic self-association with other polysaccharides (i.e. galactomannans, guar gum, konjac glucomannan, chitosan, chondroitin sulfate, rice starch, etc.) and many other components, such as proteins (i.e. sodium caseinate) that leads to the formation of supramolecular structures showing increase elasticity and vis-cosity [7-10]. For example, the self-association of xanthan gum/locust bean gum system forms thermoreversible gels with higher viscosities than the viscosities of the individual polysaccharide constituents [6,8]. Side-by-side self-aggregation of xanthan gum/protein (i.e. sodium caseinate) through hydrophobic interactions forms networks composed of rod-like fibers that are pH depen-dent and useful as building blocks for fabricating structures at the Nano scale [6]. Another example is the formation of capsular structures through xanthan gum-peptide self-assembling [11]. Numerous examples of self-aggregation of xanthan gum with several components are reported elsewhere [1,2,9,11]. The char-acterization of self-associating systems is commonly conducted through dynamic oscillatory rheometry to measure the storage modulus (G’), loss modulus (G”), and the loss factor or damping factor tan δ = G”/G’ as a function of angular frequency. The loss factor is a very important parameter, because it provides informa-tion on the strength of the self-assembling system, for instance, a G’ slightly higher than G” (G’>G”) indicates the formation of a weak self-association; while a G’>>>G” suggests the formation of a strong self-aggregating system of superior structural strength [7]. Currently, xanthan-gum is produced at low cost, hence, it is widely used in several areas including the biomedical (i.e. drug delivery and tissue engineering), pharmaceutical, food, food-packaging, water-based paint, oil recovery, water treatment industries, and others [1,6].

Partially hydrolyzed polyacrylamide (HPAM) is a copolymer of acrylamide and sodium acrylate. The degree of hydrolysis de-pends on the number of carboxyl (COO-) functional groups replac-ing the amide groups (CONH2), which increases the overall nega-tive charge of the polymer structure [12]. HPAM is a water-soluble

polymer that resists biodegradation [13]. HPAMs have been wide-ly used in many industrial applications ranging from waste water treatment, mineral processing, to enhanced oil recovery, among many other applications. The main downsides of HPAM are its susceptibility to the presence of electrolytes in the aqueous me-dia that results in significant loss of viscosity and its tendency to auto hydrolyze at elevated temperatures; which is accelerated in brines containing high salinity and hardness concentration, spe-cifically high concentration of divalent cations (i.e. Ca2+), because the hydrolyzed moieties react with divalent cations forming solid species that precipitate out of the solution [14,15]. Thus, the ap-plication of HPAM polymers in some fields such as enhanced oil recovery is reserved to reservoirs having low to moderate brine salinities and hardness, as well as low temperatures. The HPAM vulnerability has prompted research aiming to increase its per-formance in harsh conditions; therefore, several approaches have been taken, such as the copolymerization of HPAM with thermally stable and salt-tolerant monomers [14-16]. Another approach has been the evaluation of the self-assembly potential of HPAM blends with other components taking advantage of non-covalent interac-tions among the constituents of the mixture to improve its viscos-ity and thermal stability in harsh environments. For instance, the formation of HPAM pH-responsive self-assembly in aqueous solu-tions have been evaluated. In which, the primary driving force for the self-assembly of the random copolymers was hydrogen-bond-ing [17]. Formulations of self-assembling blends of HPAM/anionic surfactant/β-CD have been previously evaluated in our research group and the readers are referred to references [18-19]. Largely, self-assembly has revealed improvement of the viscosity and ther-mal stability of the HPAM-blends.

The motivation of this research was to make use of the adapt-ability of the xanthan gum biopolymer to evaluate its potential for self-assembly in order to further improve its functionality under harsh conditions. The specific objectives of this research were to establish the effect of the type of surfactant and brine concentra-tion on the formation of xanthan gum supramolecular systems through non-covalent interactions (i.e. β-CD host-guest interac-tions) and to compare the rheological behavior of the xanthan gum self-assembled system to the behavior of a parallel self-as-sembled HPAM blend.

Experimental SectionMaterials

Xanthan gum, food grade, was acquired from Groupe Mai-son Cannelle Inc. (Richmond, QC, Canada). The molecular weight of this xanthan gum is 15 x 106g/mol. Alcoflood 935 is a partly hydrolyzed polyacrylamide (HPAM), which was provided by Gel Technologies Corporation (Midland, TX, USA). The degree of hy-drolysis of this HPAM ranges from 5to10mole% and the molecular weight is 5x106g/mol. β-Cyclodextrin (β-CD) Trappsol® Technical grade (98% assay and molecular weight of 1135g/mole) was pur-chased from Cyclodextrins Technology Development Inc. (CDT,


How to cite this article: Laura Romero Zeron and Claudia Espinosa. Advantageous Supramolecular System Through Self-Association of Xanthan Gum/Cationic Surfactant Via b-Cyclodextrin Host-Guest Complexations. Academ J Polym Sci. 2019; 2(4): 555592.DOI: 10.19080/AJOP.2019.02.555592

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Inc. Alachua, Florida, USA). The cationic surfactant Dodecyl trime-thylammonium chloride (DTAC) was purchased from Sigma-Al-drich (assay≥98% and molecular weight: 263.89g/mole). The critical micelle concentration (CMC) of DTAC in distilled water at 25˚C is 22.2mmol/L [20]. The anionic surfactant, Alfoterra 167-4s, which is a primary alcohol alkoxy sulfate 30% active (Molecular weight: 580g/mol), was supplied by Sasol North America (Hous-ton, Texas). The CMC of Alfoterra 167-4s in distilled water at 25˚C is 0.66mmol/L, which was determined by conductometry using a conductivity meter manufactured by OMEGA Engineering Inc. Model PHH-80BMS. All the salts employed for the preparation of the synthetic brines were acquired from Sigma-Aldrich with the following assays: NaCl (≥99.5%), MgCl2(≥99.5%), CaCl2(≥93.0%), and Na2SO4(≥99.5%). Table 1 shows the composition of the syn-thetic brines. All chemicals were used as received.

Table 1: Composition of Synthetic Brines.

Components 2.10wt% Brine (TDS) 8.41wt% Brine (TDS)

NaCl 1.72 6.9

MgCl2 0.04 0.18

CaCl2 0.33 1.3

Na2SO4 0.01 0.04

Distilled Water 97.9 91.59

Formulation of self-assembling systemsTable 2: Viscosities of Xanthan Gum and Partly Hydrolyzed Polyacryl-amide at 25C.

Dynamic Viscosity [cP]

Instrument Xanthan Gum @ 0.5 wt%

HPAM @ 1.0 wt%

CANNON Glass Capillary Viscom-eter 1204 1020

Gemini HR 150Nano (Bohlin Rhe-ometer) Oscillatory Rheology at an angular frequency of 7.0rad/s

499.4 425.9

The preparation of the self-assembling systems was based on a formulation previously developed in our research group [21]. In which, the optimum mole ratio between the anionic surfactant and β-cyclodextrin (β-CD) was 2 moles of surfactant to 1 mole of β-CD; for a concentration of polyacrylamide of 1wt% [21]. In this exploratory work, the motivation was to determine the effect of the type of surfactant (e.g. cationic and anionic) on the self-assem-bling behavior of xanthan gum and HPAM at two different brine concentrations (2.10wt% and 8.41wt%). The polymers concen-trations were adjusted to 0.5wt% for xanthan gum and 1.0wt% for the partly hydrolyzed polyacrylamide (HPAM), to ensure that the viscosities of both polymers in distilled water were in the same order of magnitude. The viscosities of the polymer solu-tions prepared at the corresponding concentrations in distilled water were measured at 25°C using two different approaches: a

glass capillary viscometer (Capillary No. B188) (Canno Instrumen Company, USA) equipped with a viscometer bath and a Bohlin rheometer, model Gemini HR 150 Nano manufactured by Malvern Instruments (Worcestershire, UK). Table 2 shows the respective polymer concentrations and viscosities.

Rheological characterization of the supramolecular polymer-surfactant (SAP) system

Oscillatory tests were applied to evaluate the viscoelasticity of the SAP systems. Amplitude sweeps were first carried out to establish the limits of the Linear Viscoelastic (LVE) range. Sub-sequently, frequency sweeps were performed, keeping the am-plitude at a constant value within the LVE range as established from the previous amplitude sweep. The frequency sweeps allow determining the time-dependent deformation behavior of the SAP systems. These oscillatory tests rendered information on the stor-age or elastic modulus, G’, the loss or viscous modulus, G”, and the loss or damping factor, tan δ. These tests were conducted using the Bohlin rheometer, model Gemini HR 150 Nano manufactured by Malvern Instruments (Worcestershire, UK) at 25°C.

Host-guest interaction between the cationic surfactant (DTAC) and β-Cyclodextrin

H-NMR spectroscopic analysis was applied to verify the host-guest interaction between the cationic surfactant (DTAC) and β-CD. A Varian UNITY INOVA 300MHz spectrometer equipped with an Automation Triple Broadband probe was used for this analysis at 25°C. DTAC (50ppm), β-CD (107.5ppm), and DTAC (50ppm)/ β-CD (107.5ppm) solutions were prepared in heavy water (D2O) and the molar ratio DTAC to β-CD was 2moles of DTAC to 1mole of β-CD.

Adsorption of expanded vs. encapsulated cationic surfactant on solid surfaces

The adsorption of expanded and encapsulated cationic sur-factant as “inclusion complex” within the β-CD cavity were car-ried out using play sand (QUIKRETE® Premium Play Sand® No. 1113) and kaolin (Kaolin finest powder, Sigma Aldrich, Cas No. 1332-58-7) as the substrates with surfaces areas of 0.164m2/g and 8.284m2/g respectively, determined by the BET method. The corresponding masses of sand and kaolin used during each of the experimental runs were 50.51g (sand) and 1g (kaolin), so that the surface available for adsorption in both cases were the same at 8.284m2; which allow comparing the adsorption performance for both systems on the same reference.

Homogeneous stock solutions of DTAC at 0.0176mol/L, β-CD at 0.0088mol/L, and SAP (DTAC/β-CD) at 0.0176 mol/L - 0.0088mol/L (at a molar ratio of DTAC: β-CD of 2:1) were pre-pared in 8.41wt% brine. Table 3 displays the experimental matrix of the adsorption tests conducted. Each experiment was duplicat-ed. A volume of 80ml of DTAC, β-CD, or SAP was added to a beaker containing the corresponding absorbent substrate as indicated in Table 3. The beaker was covered with paraffin paper and placed




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in a shaker (IKA® 130 Basic) at 240rpm for one (1) hour at room temperature. Afterward, the beaker was left motionless for a pe-riod of 24 hours. Subsequently, the samples were filtered using filter paper (Q8, Fisher Scientific, UK) to remove suspended solids from the solution. The next step consisted in the determination of the surfactant concentration in the aqueous solution after con-tacting the solid adsorbents (i.e. sand and kaolin). The Standard Method 5540B Surfactant Separation by Sublation was applied to determine the concentration of surfactant in the diluted aqueous solutions [22].

Table 3: Experimental Matrix of the Cationic Surfactant Adsorption Evaluation in Free- and Encapsulated state.

Cationic Surfactant / Free-State

Test Name and Number Solid substrate

Baseline # 1 No solid substrate- DTACf

Baseline # 2 No solid substrate- DTACf

Play Sand # 1 DTACf

Play Sand # 2 DTACf

Kaolin # 1 DTACf

Kaolin # 2 DTACf

Cationic Surfactant / Encapsulated State

Test Name and Number Solid substrate

Baseline # 1 No solid substrate- DTACen

Baseline # 2 No solid substrate- DTACen

Play Sand # 1 DTACen

Play Sand # 2 DTACen

Kaolin # 1 DTACen

Kaolin # 2 DTACCen

Structural decomposition and regenerationThe structural decomposition and regeneration of the opti-

mum SAP systems was evaluated through dynamic-mechanical analysis (DMA) using oscillatory test based on three test inter-vals: the reference interval or low-shear, the high-shear interval, and the regeneration interval or low-shear; each interval was performed at constant dynamic-mechanical conditions as recom-mended in reference [23]. Testing was carried out at 25°C. The structural strength of the SAP systems was described in terms of the G’(t) values, which are commonly used to describe the struc-tural strength of this type of materials [23]. The pre-set for each interval was as follows:

1st Interval (200 measuring points, t=960s): γ=2% and γ=6.28rad/s in the LVE range

2nd Interval (100 measuring points, t=474s): γ=1000% and γ=6.28rad/s outside the LVE range

3rd Interval (200 measuring points, t=960s): γ=2% and γ= 6.28rad/s in the LVE range

Long-term thermal stability evaluationSample preparation for the long-term thermal stability analy-

sis of the optimum SAP systems and the corresponding reference solutions was conducted by first removing any free O2 from the solutions by bubbling the solutions with N2 in an O2-free cham-ber for 30minutes. Afterward, samples of 20ml volume of the SAP and baseline solutions were poured into flat bottom vials equipped with Teflon headspace septa and pressure release seals (Thermo Scientific, TN, USA). The vials were sealed using a man-ual crimper (Thermo Scientific, TN, USA). A total number of 32 samples were prepared (duplicated samples for each SAP systems and each baseline solutions). The samples were placed in an oven (Precision, Model 6530, Thermo Scientific, OH, USA) at 90°C for a period of 4 weeks. Each week, four (8) vials (4 vials containing the SAP solution and 4 vials containing the polymer baseline solu-tion) were retrieved from the oven. The vials were left still until the temperature of the solutions reached room temperature. Sub-sequently, the vials were then opened using a manual decrimper (Thermo Scientific, TN, USA) and all the solutions were subjected to rheological analysis through oscillatory testing) at 25°C to ob-tain the G’- & G”-curves.

Long-term bio-stability evaluationTable 4: Experimental Design/Long-Term Bio-Stability Evaluation (*DW means Distilled Water).

Test #

Brine wt%

Xanthan Gum (0.5 wt %) Formulation

HPAM (1.0 wt%) Formulation

1 0 (DW)* Baseline (Bulk + Glass beads)

Baseline (Bulk + Glass beads)

2 2.1 Baseline (Bulk + Glass beads)


3 2.1Xanthan gum + 20ppm DTAC (Bulk and Glass

beads)-


beads)-


beads)

HPAM + 50ppm DTAC (Bulk and Glass beads)

6 2.1Xanthan gum + 50ppm β-CD (Bulk and Glass

beads)

HPAM + 50 ppm β-CD (Bulk and Glass beads)

7 2.1 SAP-Alfoterra 167-4s (Bulk and Glass beads)

SAP-Alfoterra 167-4s (Bulk and Glass beads)

8 2.1 SAP-DTAC (Bulk and Glass beads)

SAP-DTAC (Bulk and Glass beads)




beads)-


beads)-



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beads)

HPAM + 50 ppm DTAC (Bulk and Glass beads)

13 8.41Xanthan gum + 50ppm β-CD (Bulk and Glass

beads)

HPAM + 50 ppm β-CD (Bulk and Glass beads)


SAP-Alfoterra 167-4s (Bulk and Glass beads)


SAP-DTAC (Bulk and Glass beads)

The reason of this evaluation was to determine if the cationic surfactant, DTAC, which “is a quaternary ammonium compound, commonly used as an antimicrobial sanitizer due to its surface activity and germicidal efficiency” [24] was effective in prevent-ing the biodegradation of the xanthan gum. Hence, this evaluation was conducted to establish the effect of different concentrations of surfactant, types of surfactant, SAP formulations, and brine

concentrations on the bio-stability of xanthan gum and HPAM. This bio-stability analysis was conducted for a period of 2 years and 6 months at room temperature (25°C). In this experimental phase, two series of samples were prepared. In the first series of samples, 20ml of baseline polymer solutions and SAP solutions were placed in flat bottom vials and covered using Teflon head-space septa and pressure release seals (Bulk samples). In the sec-ond series of samples, 15ml of the solutions were placed in flat bottom vials containing 5grams of glass beads having an average size ranging from 80 to 200 mesh. These vials were also covered using Teflon headspace septa and pressure release seals (Glass bead samples). All the vials were left still on a laboratory bench at room temperature. Every 6 months the samples were visually checked to determine the presence of mould, which would indi-cate the biodegradation of the sample. Table 4 shows the experi-mental design of this long-term bio-stability evaluation.

Results and DiscussionFormulation and rheological characterization of the self-assembling systems

Figure 1: G’, G”, and tan vs and Brine Concentration. (a) Xanthan Gum and (b) HPAM.

Figure 1a & 1b display the G’, G”, and tanδ= G”/G’ -curves for xanthan gum and HPAM as a function of angular frequency (ω) and brine concentration. These curves show important and con-trasting behaviors for these polymers. Increasing brine concen-tration from 2.10wt% to 8.41wt% significantly affects the elastic (G’) and viscous (G”) moduli of HPAM compared to the behavior of xanthan gum at the same conditions. While, the loss factor curves (G”/G’) demonstrate that for the case of xanthan gum the elastic behavior dominates (G’>G”) in the entire range of angular

frequency evaluated, which is consistent with the behavior of a three-dimensional network structure built up by non-covalent intermolecular interactions [2,7,23]. On the contrary, in the case of HPAM, the viscous behavior dominates (G”>G’) showing a flu-id-like behavior [23]. Overall, the baseline xanthan gum solution shows a more stable viscoelastic behavior as function of salinity concentration and a significantly higher elasticity and viscosity than the baseline HPAM solution at the same conditions.

Figure 2: G’, G”, and tan vs and Brine Concentration. (a) Xanthan Gum and (b) HPAM.




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Figure 2 displays the frequency sweeps in terms of G’ and G” -curves for both baseline polymers and the corresponding blends of polymer-DTAC at different brine concentrations. Figure 2a & 2b indicate that the addition of 50ppm DTAC to the xanthan gum

solutions shows unimportant interactions demonstrated by over-lapping G’&G”-curves before and after DTAC addition at different brine concentrations. Likewise, Figure 2c & 2d demonstrate negli-gible interactions between HPAM and the cationic surfactant.

Figure 3: Frequency Sweeps of Polymer/-CD Blends and Corresponding Baselines. (a) and (b) G’&G”-curves for Xanthan gum Solutions and (c) and (d) G’& G”-curves for HPAM Solutions.

Figure 3 demonstrates that the addition of 107.5ppm of β-CD does not influence the viscoelastic behavior of the polymer solu-tions. All the G’ and G” -curves of the polymeric systems before and after addition of β-CD show overlapping, which provides ev-idence of lack of intermolecular interactions between these poly-mers and β-CD.

The next experimental phase consisted in evaluating the be-havior of different formulations in which the xanthan gum and the HPAM polymer solutions were blended with an anionic or a

cationic surfactant in the presence of β-CD at different brine con-centrations (e.g. 2.10wt% and 8.41wt%) to establish the potential self-association among these components; which would produce supramolecular structures based on non-covalent interactions. In this work, the surfactants (i.e. anionic and/or cationic surfactant) and β-CD were blended at a molar ratio of 2 moles of surfactant to 1 mole of β-CD with the polymer solutions. The formation of three-dimensional network structures via self-assembling was verified through oscillatory rheology [23].

Figure 4: Rheological Behavior of Xanthan Gum/-CD/Surfactant Blends as a function of Type of Surfactant and Brine Concentration. (a) G’-curve Xanthan Gum/-CD/DTAC, (b)G’-curve Xanthan Gum/-CD/Alfoterra 167-4s, (c) G”-curve Xanthan Gum/-CD/DTAC, and (d) G”-curve Xanthan Gum/-CD/Alfoterra 167-4s.



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Figure 4 displays the effect of the charge of the surfactant on the self-assembly behavior of xanthan gum/β-CD/surfactant sys-tem. Figure 4a & 4b show the effect of adding a cationic surfactant (CS) and an anionic surfactant (AS) at different brine concentra-tions on the formation of xanthan gum self-assembly systems that was demonstrated by their elastic behavior, G’-curve, and viscous behavior, G”-curve, (Figure 4c & 4d). Oscillatory rheology con-firms that a stable self-assembly xanthan gum/β-CD (107.5ppm)/

DTAC (50ppm) system is formed at high brine concentration (i.e. 8.41wt% brine) with substantial increase of the G’- and G”- values in the entire range of angular frequency evaluated. At the lower brine concentration of 2.10wt% spontaneous association of the components in the system does not take place. Similarly, Figure 4c & 4d, demonstrate that the addition of the anionic surfactant (Alfoterra 167-4s) does not trigger self-association of the compo-nents in this blend.

Figure 5: Rheological Behavior of HPAM/-CD/Surfactant Blends as a function of Type of Surfactant and Brine Concentration. (a) G’-curve HPAM/-CD/DTAC, (b)G’-curve HPAM/-CD/Alfoterra 167-4s, (c) G”-curve HPAM/-CD/DTAC, and (d) G”-curve HPAM/-CD/Alfoterra 167-4s.

Figure 6: Frequency Sweep of the Xanthan Gum SAP and the HPAM SAP in terms of G’-, G”-, & G”/G’-curves.

In the case of HPAM, Figure 5, indicates that the addition of anionic surfactant in the blend renders self-association of the blend HPAM/β-CD/AS (Alfoterra 167-4s) at low and high brine concentrations; while, the addition of the cationic surfactant, does not promotes self-association. Furthermore, Figure 5b & 5d reveal that stronger self-assembling structures are formed at high brine concentration (i.e. 8.41wt% brine) as verified by the superior viscoelastic behavior within the low to medium range of angular frequencies. Based on these experimental observations, the op-

timum self-assembling systems selected in this work for further evaluation were the xanthan gum/β-CD/DTAC and the HPAM/β-CD/Alfoterra 167-4s both prepared in 8.41wt% brine as the aqueous phase. Figure 6 displays the frequency sweep in terms of G’-, G”-, and G”/G’- curves for the xanthan gum and HPAM super-structures. As expected, both self-assembling systems show G’>G” therefore the elastic behavior dominates the viscous behavior. This rheological behavior is characteristic of physical networks built up through non-covalent intermolecular interaction forces




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[2]. Furthermore, Figure 6 exhibits G’- & G”-curves that are “al-most parallel straight lines throughout the entire frequency range showing a slight slope only” [23] behavior that is characteristic of physical 3-D network structures “exhibiting a relatively constant structural strength in the whole frequency range” [23]. The supe-

rior structural strength of both SAP systems it is also demonstrat-ed by the consistent increase of the G’-values at the upper range of angular frequencies. Additionally, Figure 6 indicates that the xan-than gum-SAP shows higher elasticity and viscosity in the entire range of angular frequency relative to the HPAM SAP system.

Host-guest interactions between the cationic surfactant (DTAC) and β-cyclodextrinTable 5: 1H-NMR Chemical Shifts (β, ppm) for C-H protons of β-CD alone, β-CD/DTAC complexation, and their complexation induced shifts, CIS, in D2O at 25°C.

C-H Protons C-1-H C-2-H C-3-H C-4-H C-5-H C-6-H

β-CD Alone 4.934 3.552 3.83 3.49 3.72 3.74

β-CD/DTAC com-plex 4.929 3.545 3.747 3.489 3.688 3.744

CIS -0.005 -0.007 -0.083 -0.001 -0.032 0.004

The encapsulation of the cationic surfactant (DTAC) into the cavity of the β-CD was verified by 1H-NMR spectroscopy. The en-capsulation of a guest molecule into the hydrophobic cavity of the β-CD causes significant chemical upfield shifts of protons C-3-H and C-5-H that are situated in the interior of the cavity; while the hydrogen protons located at the outer surface (C-2-H, C-4-H, and C-6-H) of the β-CD structure remain unaffected or showing mar-ginal upfield or downfield chemical shifts [25]. Table 5 lists the 1H-NMR chemical shifts (δ, ppm) obtained for β-CD alone, β-CD/DTAC inclusion complex, and the complexation induced shifts or

complex freeCIS δ δ= − In Table 5, a positive sign of CIS (∆δ, ppm) indi-cates a downfield chemical shift and a negative sign shows an up-field chemical shift.

The CIS of protons C-3-H and C-5-H were -0.083ppm and -0.032 ppm, respectively. While, the CIS of the protons C-1-H, C-2-H, C-4-H, and C-6-H were very small (see Table 5), which corroborates that the hydrophobic tail of the cationic surfactant

(DTAC) only interacts with the protons inside the β-CD cavity; these experimental observations are in agreement with previous research [25,26]. Table 6 summarizes the 1H-NMR chemical shifts obtained for DTAC alone, encapsulated DTAC, and the CIS. The prominent C-H protons for the DTAC structure were named from right to left as A, B, C, D, E, and F.

The data in Table 6 reveals that all the protons in the encapsu-lated DTAC were shifted upfield. According to [27] upfield chem-ical shifts might result from the shielding effect due to van der Waals forces indicative of interactions with hydrogen atoms inside the β-CD cavity; while the magnitude of the complexation induced shift provides information on the relative strength of those inter-actions [27]. Thus, the complexation induced shifts observed for all the DTAC C-H protons further verifies the inclusion of the DTAC hydrophobic tail into the cavity of the β-CD. The complexation of the anionic surfactant (e.g. Alfoterra 167-4s) with β-CD has also been established through 1H-NMR spectroscopy in our preceding work [18], therefore, it will not be discussed any further here.

Table 6: 1H-NMR Chemical Shifts (β, ppm) for the C-H protons of DTAC alone, encapsulated DTAC, and the complexation induced shifts, CIS, in D2O at 25°C.

C-H Protons C-A-H C-B-H C-C-H C-D-H C-E-H C-F-H

DTAC Alone 3.012 3.189 3.211 3.232 4.64 4.641

Encapsulated DTAC 2.977 3.168 3.189 3.21 3.595 4.545

CIS -0.035 -0.021 -0.022 -0.022 -1.045 -0.096

Adsorption of expanded vs. encapsulated cationic surfactant (DTAC) on solid surfacesTable 7: Experimental Results of the Cationic Surfactant Adsorption Tests in Free- and Encapsulated.

Cationic Surfactant / Free-State

Baseline Play Sand Kaolin

Surfactant concentration in aqueous solution (mg/L) 1151 273 372

% of Surfactant adsorption onto solids substrates 76.30% 67.70%

Cationic Surfactant / Encapsulated-State

Baseline Play Sand Kaolin

Surfactant concentration in aqueous solution (mg/L) 1151 589 684

% of Surfactant adsorption onto solids substrates 48.80% 40.60%



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Table 7 summarizes the results of the cationic surfactant ad-sorption evaluation. The adsorption results of the free-DTAC are presented first, followed by the adsorption results of the encap-sulated DTAC in sand and kaolin. Table 7 shows the average val-ues of duplicated samples for each experiment, with the excep-tion of the baseline data, which corresponds to the average of 4 samples. Table 7 provides important insights on the DTAC/β-CD complexation adsorption behavior. First, it is clear that the com-plexation (i.e. encapsulation) of DTAC into the βCD cavity reduces its absorption onto sand and kaolin by 27%. This observation is significant because it confirms the advantage of surfactant encap-sulation for several applications such as enhanced oil recovery. Secondly, the data in Table 7 also indicates that there is a larger adsorption of DTAC onto play sand relative to kaolin. Play sand is 99.0% composed of crystalline silica (quartz) [28], which exhibits a negatively charged surface [29], therefore a maximum adsorp-tion of the cationic surfactant (DTAC) onto the grain surface of the play sand is expected.

Kaolin is mainly composed of the clay mineral kaolinite, which is a hydrous aluminosilicate [30], and exhibits an amphoteric mineral surface [31], with the silica tetrahedral face negatively charged at pH>4, while the alumina octahedral phase is positively charged at pH<6 and negatively charged at pH>8. Consequently,

the surface charge of these two faces are pH-dependent [31,32]. The pH of the 8.41wt% brine used in this work is 5.8, so at this pH, the surface of kaolinite mineral simultaneously exhibits neg-ative and positive charges. Hence, for the same surface area, the kaolin mineral exposes a less negatively charged surface relative to the negatively charged surface offered by sand grains; which explains the consistently lower adsorption of the cationic surfac-tant (DTAC) onto the kaolin surface at the pH of the brine used in this work.

Structural decomposition and regenerationTable 8 presents the experimental results of the DMA test

showing the behavior for the HPAM SAP and for the xanthan gum SAP systems in the regeneration step in terms of G’(t) and in %. The results of the DMA test (Table 8) indicate that the xanthan gum-SAP shows practically complete regeneration of the initial structural strength within 960 s, while the HPAM-SAP displays a slower structural regeneration, achieving only 78.6% of structural regeneration in the same time frame. The xanthan gum-SAP sys-tem exhibits a thixotropic behavior. This observation agrees with previous research on self-assembling of xanthan gum/methylcel-lulose [2]. This thixotropic recovery property is essential for prac-tical applications as is the case of polymer flooding in enhanced oil recovery [33-36].

Table 8: Structural Regeneration of the HPAM SAP System and the Xanthan Gum System in terms of G’(t) and in %.

HPAM SAP System Xanthan Gum System

Test Intervals and Conditions G’ [Pa] % Regeneration G’ [Pa] % Regeneration

At the end of the first interval, at low-shear conditions; the reference value of G’-at-rest (t =960s) 8.554 100 7.59 100

At the end of the second interval, at high-shear conditions (t = 474s) 0.615 7.2 0.205 2.7

Regeneration in the third interval After t = 300s 6.695 78.3 7.351 96.9

After t = 600s 6.713 78.5 7.395 97.4

After t = 960s 6.72 78.6 7.537 99.3

Long-term thermal stability evaluation





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Figure 7 displays the G’-& G”- curves for the baseline poly-mers (i.e. xanthan gum and HPAM) and for the corresponding SAP systems (i.e. xanthan gum-SAP and HPAM-SAP) as a function of angular frequency and time. This long-term thermal stability eval-uation consisted in subjecting samples of the different polymer systems at 90°C for a period of 4 weeks, thus figure 7 illustrates the rheological behavior of the polymeric systems at day 1, after week #3, and after week #4. Overall, figure 7 demonstrates that the viscoelastic behavior of the baseline polymer solutions and of both SAP systems were thermally degraded as a function of time. However, Figure 7a indicates that the xanthan gum SAP system exhibits superior structural strength (e.g. less thermal degrada-tion) than the baseline xanthan gum solution as a function of time; while in terms of the viscous modulus, Figure 7b suggests that both the xanthan gum SAP system and the corresponding base-line solution underwent similar thermal degradation with over-lapping or very close G”-curves.

Comparable thermal degradation behavior was observed for the HPAM-SAP system and its matching baseline. In terms of the G’-curves, Figure 7c reveals that by week #4, the structural strength of the HPAM-SAP is also significantly higher than the structural strength of its baseline. Likewise, Figure 7d indicates that after week #4, the HPAM-SAP system is more viscous (i.e. up-

per G”-curve) than the baseline. This suggests that the HPAM-SAP system underwent less thermal degradation than its correspond-ing reference HPAM solution.

Figure 8 shows pictures of the different polymeric systems after week #3 and week #4 of the thermal stability tests. Fig-ure 8a & 8d show the vials containing the xanthan gum baseline solution and the xanthan gum/SAP after week #3 and week #4, respectively. The main variation observed as a function of time from these solutions was color change, which turned from a transparent-clear yellowish color to a transparent-darker yellow more noticeable for the xanthan gum/SAP system. Similar obser-vations were obtained for the HPAM baseline and for the HPAM/SAP system (Figure 8c & 8d), however, a darker brownish color was observed for the HPAM/SAP system. The appearance of sol-id particles precipitated out of the solutions was not observed. In some of the samples, only a very small amount of very fine par-ticles was observed. At large, the samples showed macroscopic stability. Overall, this long-term thermal evaluation demonstrates that the structural strengths of both SAP systems are more ther-mally stable than the corresponding reference polymers. In other words, the self-assembling systems built up through non-covalent interactions generate three-dimensional networks that are more stable to thermal degradation.


Long-term bio-stability evaluationTable 9: Long-Term Bio-Stability Evaluation: Experimental Results (*DW means Distilled Water).

Test # Brine wt% Xanthan Gum (0.5 wt%) Formulation

11 8.41 Xanthan gum + 35ppm DTAC (Bulk and Glass beads)

Test # Brine wt% HPAM (1.0 wt%) Formulation

1 0 (DW)* Baseline (Bulk + Glass beads)




12 8.41 HPAM + 50ppm DTAC (Bulk and Glass beads)

13 8.41 HPAM + 50ppm β-CD (Bulk and Glass beads)





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Table 9 shows only the polymeric-surfactant systems that remained undegraded (i.e. stable) during the evaluation period of 2 and ½years. The experimental results presented in Table 9 clearly confirms the fact that xanthan gum is particularly vulner-able to biodegradation [1]. Only one of the xanthan formulations remained stable during the testing period, which was the formu-lation containing xanthan gum (0.5wt%) and 35ppm of DTAC prepared in 8.41% brine. On the contrary, the HPAM formulations demonstrated to be more resistance to biodegradation, as expect-ed [13]. Furthermore, it seems that the concentration of the brine played an important role in maintaining the integrity of the HPAM formulations, due to the fact that all HPAM formulations prepared in 8.41wt% brine remained undegraded. Only one the HPAM formulations and the baseline (e.g. without additives) solution prepared in the 2.10% brine remained stable during the testing period. In addition, to the baseline HPAM solution prepared in dis-tilled water. Therefore, it seems that the antimicrobial properties of DTAC are not effective at the concentrations used in this work. These experimental observations also suggest that high brine concentration (i.e. 8.41wt%) offers the main antimicrobial action.

ConclusionOverall, the baseline xanthan gum solution shows a more sta-

ble viscoelastic behavior as function of salinity concentration and a significantly higher elasticity and viscosity than the reference HPAM solution at the same conditions. Blends of xanthan gum/DTAC and HPAM/DTAC do not show meaningful interactions. However, oscillatory rheology demonstrated that self-assembly occurred when the xanthan-gum solution (0.5wt%) was mixed with the cationic surfactant (50ppm) and β-CD (107.5ppm) at a molar ratio of 2moles of cationic surfactant to 1 mole of β-CD. The formation of β-CD/DTAC host-guest interaction was verified via 1H-NMR spectroscopy. Self-association was also observed for blends of HPAM solution (1wt%)/anionic surfactant (50 ppm)/ β-CD (50ppm) at a molar ratio of 2moles of anionic surfactant per 2moles of β-CD. Superior structural strength of both self-assem-bling systems was observed at high brine concentration (8.41wt% brine). Hence, self-assembling provides stability to high salinity concentrations relative to the stability of the matching baseline polymers. The static adsorption tests of the cationic surfactant in free-state and complex-state demonstrated that the encapsulation of the cationic surfactant (DTAC) into the β-CD cavity decreases the adsorption of the cationic surfactant onto the sand and kaolin surface by 27% compared to the adsorption behavior of the cat-ionic surfactant in free-state.

In terms of structural strength of the supramolecular systems, the DMA test indicates that the xanthan gum-SAP shows a thixo-tropic behavior, while the HPAM-SAP system shows only partial regeneration (i.e. 78.6%) of the original structural strength after the application of a high-shear rate. Furthermore, the long-term thermal stability analysis of the self-assembling systems formu-lated for the xanthan gum and HPAM polymers showed superi-or thermal stability than the matching baseline systems. Finally,

the long-term bio stability evaluation indicates that high brine concentration (i.e. 8.41wt%) seems to offer the main antimicro-bial action to the baseline polymers and self-assembling systems. Overall, this research established that molecular self-assembly of xanthan gum/cationic surfactant/β-CD blend offers the advantag-es of superior viscoelastic rheological behavior, thermal stability, and tolerance to high salinity and hardness concentration relative to the HPAM/anionic surfactant/β-CD system. These observations are relevant for enhanced oil recovery (EOR) applications.

AcknowledgmentThe authors are grateful to Mr. Otto Morales, Chemical

Engineering Department, University of New Brunswick, now with the ADI Group Inc. for his contributions in determining the adsorption behavior of the cationic surfactant in free- and encapsulated- state. The authors would also like to acknowledge the research funding from the MITACS Globalink program.

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