cationic surfactant via β-cyclodextrin host-guest complexations for Enhanced Oil Recovery Applications

cationic surfactant via β-cyclodextrin host-guest complexations for Enhanced Oil Recovery Applications

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Journal of Petroleum Science and Engineering xxx (xxxx) xxx

Contents lists available at ScienceDirect

Journal of Petroleum Science and Engineering journal homepage: http://www.elsevier.com/locate/petrol

Advantageous supramolecular system through self-association of xanthan gum/cationic surfactant via β-cyclodextrin host-guest complexations for Enhanced Oil Recovery Applications �n a, *, Claudia Espinosa b, 1 Laura Romero-Zero a b

Chemical Engineering Department, University of New Brunswick, PO Box 4400, Head Hall, 15 Dineen Drive, Fredericton, NB, E3B 5A3, Canada Facultad de Ciencias Químicas, Universidad La Salle-M�exico, Av. Benjamin Franklin 45, Cuauht�emoc, Mexico City 06140, Mexico

A R T I C L E I N F O

A B S T R A C T

Keywords: Xanthan gum Cationic surfactant Polymer-surfactant self-assembly Self-assembly Self-association High salinity and hardness tolerance

The self-assembly of xanthan gum/surfactant/β-cyclodextrin to further improve its functionality at elevated temperatures and in brines with high salinity and hardness concentration was evaluated. The effect of surfactant type (i.e. anionic and/or cationic) and brine concentration (2.10 and 8.41 wt%) on self-association was estab­ lished and compared to the behavior of a self-assembled system of a partly hydrolyzed polyacrylamide (HPAM)/ surfactant/β-CD. Results demonstrate that self-assembly is surfactant dependent. Self-assembled polymeric (SAP) systems with superior viscoelasticity, tolerance to brine concentration, and improved thermal stability relative to their corresponding baseline polymers were obtained for the xanthan-gum/cationic surfactant/β-CD (XG-SAP) blend and the HPAM/anionic surfactant/β-CD (HPAM-SAP) mixture. Only the XG-SAP system shows thixotropy. Encapsulation of the cationic surfactant decreases its adsorption onto sand and kaolin by 27% relative to the adsorption of the surfactant in free state. The long-term bio-stability tests indicate that high brine concentration offers the antimicrobial action to the SAP and polymer systems. The XG-SAP system has inherent advantages for Enhanced Oil Recovery Applications.

1. Introduction Xanthan gum (XG) is an anionic polysaccharide that is produced by fermentation of a carbohydrate with Xanthomonas campestris. The primary structure of XG is composed of a Penta saccharide repeating unit including two β-D-glucose residues, two D-mannose residues, and one β-D-glucuronic acid residue. The molecular weight distribution of XG ranges from 2 � 106 to 20 � 106Da (Kumar et al., 2018). Xanthan gum shows two conformations in aqueous solutions: an ordered and rigid double helical strand structure at low temperature and a disordered and flexible coil structure at high temperature (Liu and Yao, 2015; Pastuszka and MacKay, 2016). Consequently, it undergoes thermally-induced order-disorder conformational transitions (Kumar et al., 2018; Liu and Yao, 2015; Pastuszka and MacKay, 2016; Pham et al., 2016). The midpoint transition temperature (Tm) is about 40–50 � C depending on the ionic strength of the aqueous solution. The ordered double-helical conformation forms three dimensional networks (Liu and Yao, 2015) and this conformation is stabilized in the presence of ions in the aqueous

solution (Roy et al., 2014). For instance, divalent cations such as calcium (Ca2þ) strongly binds to XG and stabilizes the helical conformation producing hydrogels with superior mechanical performance (Kumar et al., 2018; Tiwari, 2010). The stabilizing effect occurs because elec­ trolytes reduce the intra-molecular electrostatic repulsion in the XG backbone. Additionally, the ordered helical structure provides thermal stability to XG making it more resistant to hydrolysis (de-polymeriza­ tion) than other polysaccharides or synthetic polymers (i.e. HPAM) (Kumar et al., 2018). XG is non-toxic (i.e. excellent biocompatibility), water-soluble, and exhibits distinctive rheological properties such as high viscosity at lower concentrations, highly shear thinning, stability under shear (i.e. thixotropy), maintains high viscosities in the presence of electrolytes, high temperature, and wide pH ranges (Kumar et al., 2018; Tiwari, 2010; Saha and Bhattacharya, 2010). Additionally, the large number of hydroxyl groups (-OH) and free carboxyl groups (–COO–) in the structure of XG makes it a multipurpose biopolymer for chemical modification and/or functionalization to further optimize its physicochemical properties (Kumar et al., 2018; Alhaique et al., 2015).

* Corresponding author. E-mail addresses: [email protected] (L. Romero-Zer� on), [email protected] (C. Espinosa). 1 Now with BASF Mexicana Care Chemicals. https://doi.org/10.1016/j.petrol.2019.106644 Received 18 June 2019; Received in revised form 22 September 2019; Accepted 1 November 2019 Available online 6 November 2019 0920-4105/© 2019 Elsevier B.V. All rights reserved.

Please cite this article as: Laura Romero-Zerón, https://doi.org/10.1016/j.petrol.2019.106644

Claudia

Espinosa,

Journal

of

Petroleum

Science

and

Engineering,

L. Romero-Zer� on and C. Espinosa

Journal of Petroleum Science and Engineering xxx (xxxx) xxx

XG shows the ability to form physical networks (i.e. self-association) through hydrogen bonding, hydrophobic association, and cation bridging that results in three-dimensional networks that contains sol­ vent in the interstices (Kumar et al., 2018; Saha and Bhattacharya, 2010). For instance, XG molecules in water-ethanol solution self-aggregates in bundles of nanofibers. As explained by Saha and Bhattacharya (2010), the self-association of XG is affected by the “con­ centration of the polysaccharide, pH of the medium, molar mass/degree of polymerization, temperature, ionic composition, and solvent quality”. Furthermore, as an anionic polymer, XG self-associates through elec­ trostatic interactions with functional materials (Tiwari, 2010). XG shows remarkable synergistic self-association with other poly­ saccharides (i.e. galactomannans, guar gum, konjac glucomannan, chi­ tosan, 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 increased elasticity and viscosity (Saha and Bhattacharya, 2010; Alhaique et al., 2015; Chen et al., 2016; Kim and Yoo, 2006). For example, the self-association of XG/locust bean gum system forms thermoreversible gels with higher viscosities than the viscosities of the individual polysaccharide constituents (Tiwari, 2010; Alhaique et al., 2015). Side-by-side self-aggregation of XG/protein (i.e. sodium caseinate) through hydrophobic interactions forms networks composed of rod-like fibers that are pH dependent and useful as building blocks for fabricating structures at the nano scale (Tiwari, 2010). Another example is the formation of capsular structures through XG-peptide self-assembling (Mendes et al., 2012). Numerous examples of self-aggregation of XG with several components are reported else­ where (Kumar et al., 2018; Liu and Yao, 2015; Chen et al., 2016; Mendes et al., 2012). The characterization of self-associating systems is commonly conducted using dynamic oscillatory rheometry through the response of the storage modulus (G0 ), loss modulus (G00 ), and the loss factor or damping factor (tan δ ¼ G00 /G0 ) as a function of angular fre­ quency. The loss factor is an important parameter, because it provides information on the strength of the self-assembled system, for instance, a G0 slightly higher than G00 (G0 >G00 ) indicates the formation of a weak self-association; while a G0 ≫>G00 suggests the formation of a strong self-aggregating system of superior structural strength (Saha and Bhat­ tacharya, 2010). Currently, XG 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 (Kumar et al., 2018; Tiwari, 2010; Imeson, 2012). Partially hydrolyzed polyacrylamide (HPAM) is a copolymer of acrylamide and sodium acrylate (Lewandowska, 2007). The degree of hydrolysis depends on the number of carboxyl (COO-) functional groups replacing the amide groups (CONH2), which increases the overall negative charge of the polymer structure (Lewandowska, 2007). HPAM is a water-soluble polymer that resists biodegradation (Song et al., 2011). HPAMs have been widely used in many industrial applications ranging from wastewater 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 media 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. Specifically at high concentrations of divalent cations (i.e. Ca2þ), because the hydrolyzed moieties react with divalent cations forming solid species that precipi­ tate out of the solution (Liu et al., 2012; Sarsenbekuly et al., 2017). Thus, the application of HPAM polymers in some fields such as enhanced oil recovery is limited 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 performance in harsh reservoir conditions; therefore, several approaches have been taken, such as the copolymerization of HPAM with thermally stable and salt-tolerant monomers (Liu et al., 2012; Sarsenbekuly et al., 2017; Co et al., 2015). In addition, the self-assembly of HPAM with other components

has been explored. Self-assembly of HPAM with other components via non-covalent interactions improves the viscosity and thermal stability of HPAM in harsh reservoir environments. For instance, the formation of HPAM pH-responsive self-assembly in aqueous solutions have been evaluated. In which, the primary driving force for the self-assembly of the random copolymers was hydrogen-bonding (Fang et al., 2011). Formulations of self-assembling blends of HPAM/anionic surfac­ tant/β-CD have been previously evaluated in our research group and the readers are referred to citations (Wei et al., 2014a, 2014b). Largely, self-assembly has revealed improvement of the viscosity and thermal stability of the HPAM-blends. The motivation of this research was to improve the functionality of XG in terms of viscoelasticity, tolerance to high salinity and hardness concentration, and thermal stability through self-assembly. Therefore, this work evaluated the self-association of XG with anionic and cationic surfactants through non-covalent β-CD host-guest interactions. The specific objectives of this research were to establish the effect of the type of surfactant, salinity and hardness concentration on the properties of the XG supramolecular system relative to the performance of an analo­ gous self-assembled HPAM blend. 2. Materials and methods 2.1. Materials XG food grade was acquired from the Groupe Maison Cannelle Inc. (Richmond, QC, Canada). The molecular weight of this XG is 15 � 106 g/ mol. Alcoflood 935 is a partly hydrolyzed polyacrylamide (HPAM), which was provided by Gel Technologies Corporation (Midland, TX, USA). The degree of hydrolysis of this HPAM ranges from 5 to 10 mol% and the molecular weight is 5 � 106 g/mol. β-Cyclodextrin (β-CD) Trappsol® Technical grade (98% assay and molecular weight of 1135 g/ mol) was purchased from Cyclodextrins Technology Development Inc. (CDT, Inc. Alachua, Florida, USA). The cationic surfactant dodecyl tri­ methylammonium chloride (DTAC) was purchased from Sigma-Aldrich (assay �98% and molecular weight: 263.89 g/mol). The critical micelle concentration (CMC) of DTAC in distilled water at 25 � C is 22.2 mmol/L � (Sarac and Be�ster-Roga�c, 2009). The anionic surfactant, Alfoterra 167-4s, which is a primary alcohol alkoxy sulfate 30% active (Molecular weight: 580 g/mol), was supplied by Sasol North America (Houston, Texas). The CMC of Alfoterra 167-4s in distilled water at 25 � C is 0.66 mmol/L, which was determined by conductometry using a con­ ductivity 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: NaCl (�99.5%), MgCl2 (�99.5%), CaCl2 (�93.0%), and Na2SO4 (�99.5%). All chemicals were used as received. Table 1 shows the composition of the synthetic brines. 2.2. Methods 2.2.1. Formulation of self-assembling systems The preparation of the self-assembling systems was based on a formulation previously developed in our research group (Alribi, 2016). In which, the optimum mole ratio between the anionic surfactant and β-cyclodextrin (β-CD) was 2 mol of surfactant to 1 mol of β-CD; for a concentration of polyacrylamide of 1 wt% (Alribi, 2016). In this Table 1 Composition of synthetic brines.

2

Components

2.10 wt% Brine (TDS)

8.41 wt% Brine (TDS)

NaCl MgCl2 CaCl2 Na2SO4 Distilled Water

1.72 0.04 0.33 0.01 97.90

6.90 0.18 1.30 0.04 91.59

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exploratory work, the motivation was to determine the effect of the type of surfactant (e.g. cationic and anionic) on the self-assembling behavior of XG and HPAM at two different brine concentrations (2.10 wt% and 8.41 wt%). The polymers concentrations were adjusted to 0.5 wt% for XG and 1.0 wt% for the HPAM to ensure that the viscosities of both polymers in distilled water were in the same order of magnitude. The viscosities of the polymer solutions prepared at the corresponding con­ centrations in distilled water were measured at 25 � C using two different approaches: a glass capillary viscometer using Capillary No. B188 manufactured by Cannon Instrument Company (State College, PA, USA) equipped with a viscometer bath and a Bohlin rheometer, model Gemini HR 150 Nano manufactured by Malvern Instruments (Worcestershire, UK). The Bohlin rheometer is a “precision instrument designed to pro­ vide accurate rheological measurements” (Instruments, 2004). Detail specifications of this rheometer are provided in reference (Instruments, 2004). Table 2 shows the respective polymer concentrations and dy­ namic viscosities.

kaolinite) negatively charged significantly increases the adsorption of cationic surfactants (Cui et al., 2014). Nevertheless, it has been demonstrated (Shehata and Nasr-El-Din, 2017) that in reservoir brines of elevated salinity and hardness concentration (e.g. sea water), the surface charge of sandstone tends to become positive. In this study, the adsorption of expanded and encapsulated cationic surfactant as “inclusion complex” within the β-CD cavity were carried 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 surface areas of 0.164 m2/g and 8.284 m2/g respec­ tively, determined by the BET method. The corresponding masses of sand and kaolin used during each of the experimental runs were 50.51 g (sand) and 1 g (kaolin), so that the surface available for adsorption in both cases were the same at 8.284 m2; which allows comparing the adsorption performance for both systems on the same reference. Homogeneous stock solutions of DTAC at 0.0176 mol/L, β-CD at 0.0088 mol/L, and SAP (DTAC/β-CD) at 0.0176 mol/L - 0.0088 mol/L (at a molar ratio of DTAC: β-CD of 2:1) were prepared in 8.41 wt% brine. Table 3 displays the experimental matrix of the adsorption tests con­ ducted. Each experiment was duplicated. A volume of 80 ml of DTAC, β-CD, or SAP solution was added to a beaker containing the corre­ sponding absorbent substrate as indicated in Table 3. The beaker was covered with paraffin paper and placed in a shaker (IKA® 130 Basic) at 240 rpm for one (1) hour at room temperature. Afterward, the beaker was left motionless for a period of 24 h. Subse­ quently, the samples were filtered using filter paper (Q8, Fisher Scien­ tific, UK) to remove suspended solids from the solution. The next step consisted in the determination of the surfactant concentration in the aqueous solution after contacting 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 (Franson, 1989).

2.2.2. Rheological characterization of the supramolecular polymersurfactant (SAP) systems 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. Subsequently, frequency sweeps were performed, keeping the amplitude at a constant value within the LVE range as established from the previous amplitude sweep. The frequency sweeps allow determining the time-dependent deforma­ tion behavior of the SAP systems. These oscillatory tests rendered in­ formation on the storage or elastic modulus, G0 , the loss or viscous modulus, G00 , and the loss or damping factor, tan δ. These tests were conducted using a Bohlin rheometer, model Gemini HR 150 Nano manufactured by Malvern Instruments (Worcestershire, UK) at 25 � C. 2.2.3. Host-guest interaction between the cationic surfactant (DTAC) and β-cyclodextrin 1 H-NMR spectroscopic analysis was applied to verify the host-guest interaction between the cationic surfactant (DTAC) and β-CD. A Var­ ian UNITY INOVA 400 MHz spectrometer equipped with an Automation Triple Broadband probe was used for this analysis at 25 � C. DTAC (50 ppm), β-CD (107.5 ppm), and DTAC (50 ppm)/β-CD (107.5 ppm) solutions were prepared in heavy water (D2O) and the molar ratio DTAC to β-CD was 2 mol of DTAC to 1 mol of β-CD.

2.2.5. Structural decomposition and regeneration The structural decomposition and regeneration of the optimum SAP systems was evaluated through dynamic-mechanical analysis (DMA) using an oscillatory test based on three test intervals: the reference in­ terval or low-shear, the high-shear interval, and the regeneration in­ terval or low-shear; each interval was performed at constant dynamicmechanical conditions as recommended in reference (Mezger, 2014). Testing was carried out at 25 � C. The structural strength of the SAP systems was described in terms of the G0 (t) values, which are commonly used to describe the structural strength of this type of materials. The pre-set for each interval was as follows:

2.2.4. Adsorption of the cationic surfactant in free- and encapsulated state onto solid surfaces In sandstone formations, larger adsorption of cationic surfactants takes place relative to the adsorption of anionic surfactants, due to the negative surface charges exhibit by silica sands at pH > 4 (Cui et al., 2014). In carbonate reservoirs, which contain more than half of the remaining oil reserves worldwide (Gupta and Mohanty, 2010), the most common minerals are calcite and dolomite (Shehata et al., 2014), which display positive surface charges at pH � 9 (Cui et al., 2014; Gupta and Mohanty, 2010; Khaledialidusti and Kleppe, 2018). Therefore, anionic surfactants readily adsorb onto these carbonate minerals, in contrast to cationic surfactants. Though, the presence of impurities in natural car­ bonate minerals such as silica minerals (e.g. quartz) or clays (i.e.

Table 3 Experimental matrix of the cationic surfactant adsorption evaluation in free- and encapsulated- state. Cationic Surfactant/Free-State

Table 2 Viscosities of xanthan gum and partly hydrolyzed polyacrylamide at 25 � C. Xanthan Gum @ 0.5 wt%

HPAM @ 1.0 wt %

CANNON Glass Capillary Viscometer Gemini HR 150 Nano (Bohlin Rheometer) Oscillatory Rheology at an angular frequency of 7.0 rad/s

1204.0 499.4

1020.0 425.9

Solid substrate

Baseline # 1f Baseline # 2f Play Sand # 1f Play Sand # 2f Kaolin # 1f Kaolin # 2f

No solid substrate- DTACf No solid substrate- DTACf DTACf DTACf DTACf DTACf

Cationic Surfactant/Encapsulated-State

Dynamic Viscosity [cP] Instrument

Test Name and Number

3

Test Name and Number

Solid substrate

Baseline # 1en Baseline # 2en Play Sand # 1en Play Sand # 2en Kaolin # 1en Kaolin # 2en

No solid substrate- DTACen No solid substrate- DTACen DTACen DTACen DTACen DTACen

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� First Interval (200 measuring points, t ¼ 960s): γ ¼ 2% and γ ¼ 6.28 rad/s in the LVE range � Second Interval (100 measuring points, t ¼ 474s): γ ¼ 1000% and γ ¼ 6.28 rad/s outside the LVE range � Third Interval (200 measuring points, t ¼ 960s): γ ¼ 2% and γ ¼ 6.28 rad/s in the LVE range

Table 4 Experimental Design/Long-Term Bio-Stability Evaluation (*DW means Distilled Water).

2.2.6. Long-term thermal stability evaluation Sample preparation for the long-term thermal stability analysis of the optimum SAP systems and the corresponding reference solutions was conducted by first removing free O2 from the solutions (i.e. bubbling N2 in an O2-free chamber for 30 min). Afterward, samples of 20 ml 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 manual crimper (Thermo Scientific, TN, USA). In total 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, eight (8) vials (4 vials containing the SAP solutions and 4 vials containing the polymer baseline solutions) were retrieved from the oven. The vials were left still until the temperature of the solutions reached room tem­ perature. Subsequently, 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 obtain the G0 - and G00 -curves.

Test #

Brine wt %

XG Formulation (Bulk þ Glass beads

HPAM Formulation (Bulk þ Glass beads

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

0 (DW)* 2.10 2.10 2.10 2.10 2.10 2.10 2.10 8.41 8.41 8.41 8.41 8.41 8.41 8.41

Baseline Baseline XG þ 20 ppm DTAC XG þ 35 ppm DTAC XG þ 50 ppm DTAC XG þ 50 ppm β-CD SAP-Alfoterra 167-4s SAP-DTAC Baseline XG þ 20 ppm DTAC XG þ 35 ppm DTAC XG þ 50 ppm DTAC XG þ 50 ppm β-CD SAP-Alfoterra 167-4s SAP-DTAC

Baseline Baseline – – HPAM þ 50 ppm DTAC HPAM þ 50 ppm β-CD SAP-Alfoterra 167-4s SAP-DTAC Baseline – – HPAM þ 50 ppm DTAC HPAM þ 50 ppm β-CD SAP-Alfoterra 167-4s SAP-DTAC

helical strand structure of XG at low temperatures in aqueous solutions (Pastuszka and MacKay, 2016; Liu and Yao, 2015). On the contrary, in the case of HPAM, the viscous behavior dominates (G00 >G0 ) showing a fluid-like behavior (Mezger, 2014). Overall, the baseline XG solution shows a more stable viscoelastic behavior as function of salinity con­ centration and a significantly higher elasticity and viscosity than the baseline HPAM solution at the same conditions. Fig. 2 displays the frequency sweeps in terms of G0 and G00 -curves for both baseline polymers and the corresponding blends of polymer-DTAC at different brine concentrations. Fig. 2a and b indicate that the addition of 50 ppm DTAC to the XG solution shows unimportant interactions that is demonstrated by the overlapping of the G0 and G00 -curves before and after DTAC addition at different brine concentrations. Likewise, Fig. 2c and d demonstrate negligible interactions between HPAM and the cationic surfactant. Similarly, Fig. 3 demonstrates that the addition of 107.5 ppm of β-CD does not influence the viscoelastic behavior of the polymer solutions. The G0 - and G00 -curves of the polymeric systems before and after the addition of β-CD show overlapping in both low and high salinity con­ centrations, which provides evidence of lack of intermolecular in­ teractions between these polymers and β-CD. In the next experimental phase, different formulations of XG and HPAM polymer mixed with anionic surfactant (AS), cationic surfactant (CS) and β-CD at different brine concentrations (e.g. 2.10 wt% and 8.41 wt%) were evaluated. The surfactants (i.e. anionic and/or cationic surfactant) and β-CD were blended at a molar ratio of 2 mol of surfactant to 1 mol of β-CD in the corresponding baseline polymer solutions. The formation of three-dimensional network structures via self-assembling from these formulations was verified through oscillatory rheology (Mezger, 2014). Fig. 4 displays the effect of surfactant charge on the self-assembly behavior of XG in the presence of β-CD. Fig. 4a and b show the G0 curves as a function of angular frequency and brine concentration for XG/β-CD/CS and XG/β-CD/AS respectively. Fig. 4c and d display the corresponding G00 -curves as a function of angular frequency and brine concentration for the systems XG/β-CD/CS and XG/β-CD/AS respec­ tively. The structural strength of viscoelastic materials is typically described through the behavior of the G0 -curve as a function of angular frequency. Besides, three-dimensional network structures formed via non-covalent interactions display G’ ≫ G00 that corresponds to gel-like materials (i.e. elastic behavior dominates) (Mezger, 2014). Fig. 4a and c demonstrate a substantial increase of the G0 - and G00 -values for the system XG/β-CD/CS in 8.41 wt% brine in the entire range of angular frequency. Likewise, Fig. 4a and c, show that the G0 - values ≫ G00 - values for the system XG/β-CD/CS in 8.41 wt% brine, which indicates the for­ mation of a stable self-assembled network. These observations agree

2.2.7. Long-term bio-stability evaluation The objective 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” (Sigma-Aldrich, 2019) was effective in pre­ venting the biodegradation of the XG. Hence, this evaluation was con­ ducted to establish the effect of different concentrations of surfactant, types of surfactant, SAP formulations, and brine concentrations on the bio-stability of XG 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, 20 ml of baseline polymer solutions and SAP solutions were placed in flat bottom vials and covered using Teflon headspace septa and pressure release seals (Bulk samples). In the second series of samples, 15 ml of the solutions were placed in flat bottom vials con­ taining 5 g 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 mold, which would indicate the biodegradation of the sample. Table 4 shows the experi­ mental design of this long-term bio-stability evaluation. 3. Results and discussions 3.1. Formulation and rheological characterization of the self-assembling systems Fig. 1a and b display the G0 , G00 , and tan δ ¼ G”/G’ -curves for XG and HPAM as a function of angular frequency (ω) and brine concentration. These curves show important and contrasting behaviors for these poly­ mers. Increasing brine concentration from 2.10 wt% to 8.41 wt% significantly affects the elastic (G0 ) and viscous (G00 ) moduli of HPAM compared to the behavior of XG at the same conditions. The loss factor curves (G”/G0 ) demonstrate that for the case of XG the elastic behavior dominates (G’>G00 ) in the entire range of angular frequency evaluated. This is consistent with the behavior of 3-D network structures built up by non-covalent intermolecular interactions (Liu and Yao, 2015; Saha and Bhattacharya, 2010; Mezger, 2014) due to the ordered and rigid double 4

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Fig. 1. G0 , G00 , and tan δ vs ω and Brine Concentration. (a) XG and (b) HPAM.

Fig. 2. Frequency Sweeps in terms of G0 and G00 -curves as a function of Surfactant-Polymer Blends and Brine Concentrations. (a) and (b) G0 - and G00 -curves for XG baseline and XG/DTAC blend respectively. (c) and (d) G0 - and G00 -curves for HPAM baseline and HPAM – DTAC blend respectively.

with previous research on xanthan gum/rice starch networks based on non-covalent interactions (Chen et al., 2016) and other self-assembling systems based on host-guest interactions (Yang et al., 2015; Tan et al., 2014; Adler-Abramovich and Gazit, 2014). In this formulation, the concentration of β-CD and DTAC were 107.5 ppm and 50 ppm respec­ tively. The formulation in lower brine concentration (i.e. 2.10 wt%) does not prompt the spontaneous association of the components in the sys­ tem. Similarly, Fig. 4b and d, demonstrate that the addition of the anionic surfactant (Alfoterra 167-4s) does not trigger self-association of the components in these blends. In the case of HPAM, Fig. 5b and d indicate that the addition of anionic surfactant (AS) to the blend renders self-association of the sys­ tem HPAM/β-CD/AS at low and high brine concentrations. While, the addition of cationic surfactant (CS) to the blend does not promotes selfassociation (Fig. 5a and c). Furthermore, Fig. 5b and d reveal that

stronger self-assembling structures are formed at high salinity concen­ tration (i.e. 8.41 wt% brine) as verified by the superior viscoelastic behavior within the low to medium range of angular frequencies. Based on these experimental observations, the optimum selfassembling systems selected in this work for further evaluation were the XG/β-CD/CS (XG-SAP) and the HPAM/β-CD/CS (HPAM-SAP) both prepared in 8.41 wt% brine as the aqueous phase. Fig. 6 displays the frequency sweep in terms of G’-, G00 - and G"/G0 - curves for the XG and HPAM superstructures. As expected, both self-assembling systems show G’>G00 , corroborating the dominance of the elastic behavior over the viscous one, which is characteristic of physical networks built up through non-covalent intermolecular interactions. Furthermore, Fig. 6 exhibits G0 - and G00 -curves that are “almost parallel straight lines throughout the entire frequency range showing a slight slope only” (Mezger, 2014) behavior that is distinctive of physical 3-D network 5

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Fig. 3. Frequency Sweeps of Polymer/β-CD Blends and Corresponding Baselines. (a) and (b) G0 - and G00 -curves for solutions in 2.1 wt% Brine (c) and (d) G0 - and G00 curves for solutions in 8.41 wt%.

Fig. 4. Rheological Behavior of XG/β-CD/Surfactant Blends as a function of Type of Surfactant and Brine Concentration. (a) and (b) G0 -curves for the XG/β-CD/CS and XG/β-CD/AS respectively, (c) and (d) G00 -curves for XG/β-CD/CS and XG/β-CD/AS respectively.

6

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Journal of Petroleum Science and Engineering xxx (xxxx) xxx

Fig. 5. Rheological Behavior of HPAM/β-CD/Surfactant Blends as a function of Type of Surfactant and Brine Concentration. (a) and (b) G0 -curves for the HPAM/ β-CD/CS and HPAM/β-CD/AS respectively, (c) and (d) G00 -curves for HPAM/β-CD/CS and HPAM/β-CD/AS respectively.

Fig. 6. Frequency Sweep of the XG-SAP and the HPAM-SAP in terms of G’-, G"-, and G"/G0 -curves.

structures “exhibiting a relatively constant structural strength in the whole frequency range” (Mezger, 2014). The superior structural strength of both SAP systems it is also demonstrated by the steady in­ crease of the G0 -values at the upper range of angular frequencies. Additionally, Fig. 6 indicates that the XG-SAP shows higher elasticity and viscosity in the entire range of angular frequency evaluated relative to the HPAM-SAP system.

3.2. Host-guest interactions between the cationic surfactant (DTAC) and β-cyclodextrin The encapsulation of the cationic surfactant (DTAC) into the cavity of the β-CD was verified by 1H-NMR spectroscopy. Fig. 7a to c show the chemical structures of β-CD, DTAC, and the respective 1H-NMR spectra of β-CD alone, DTAC-alone, and β-CD/DTAC inclusion complex, in 7

L. Romero-Zer� on and C. Espinosa

Journal of Petroleum Science and Engineering xxx (xxxx) xxx

Fig. 7. Chemical structures and 1H-NMR Spectra for: (a) β-CD alone, (b) DTAC alone, and (c) β-CD/DTAC inclusion complex. 8

L. Romero-Zer� on and C. Espinosa

Journal of Petroleum Science and Engineering xxx (xxxx) xxx

Fig. 7. (continued).

which the corresponding protons are labeled. The encapsulation of a guest molecule into the hydrophobic cavity of the β-CD causes significant chemical upfield shifts of protons H3 and H5 that are situated in the interior of the β-CD cavity; while the hydrogen protons located at the outer surface (H2, H4, and H6) of the β-CD structure remain unaffected or showing marginal upfield or downfield chemical shifts (Marques, 2010). Table 5 lists the 1H-NMR chemical shifts (δ, ppm) obtained for β-CD-alone, the β-CD/DTAC inclusion complex, and the complexation induced shifts or CIS ¼ δcomplex δalone . In Table 5, a positive sign of CIS (Δδ, ppm) indicates a downfield chemical shift and a negative sign shows an upfield chemical shift. The CIS of protons H3 and H5 were - 0.083 ppm and - 0.028 ppm, respectively. While, the CIS of the protons H1, H2, H4, and H6 were very small (see Table 5), which corroborates that the hydrophobic tail of the cationic surfactant (DTAC) entered into the β-CD cavity interacting with the internal protons H3 and H5; these experimental observations are in agreement with previous research (Marques, 2010; Pessine et al., 2012; Zhao et al., 2016). Table 6 summarizes the 1H-NMR chemical shifts obtained for DTACalone (free), encapsulated DTAC, and the CIS. The prominent H protons for the DTAC structure were named from right to left on the 1H-NMR spectrum as A, B, C, D, E, and F (Fig. 7).

Table 6 1 H-NMR Chemical Shifts (δ, ppm) for the C–H protons of DTAC alone, encap­ sulated DTAC, and the complexation induced shifts, CIS, in D2O at 25 � C.

H1

H2

H3

H4

H5

H6

β-CD Alone β-CD/DTAC complex CIS

4.935 4.929

3.524 3.529

3.830 3.747

3.470 3.473

3.742 3.714

3.690 3.688

0.006

0.005

0.083

0.003

0.028

HA

HB

HC

HD

HE

HF

DTAC Alone Encapsulated DTAC CIS

3.012 2.977

3.189 3.168

3.211 3.189

3.232 3.210

4.600 3.595

4.647 4.645

0.035

0.021

0.022

0.022

1.045

0.002

The data in Table 6 reveals that all the protons of the encapsulated DTAC were shifted upfield. According to Veiga et al. (2001) upfield chemical 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 interactions (Veiga et al., 2001). Thus, the complexation induced shifts observed for all the DTAC H protons further verifies the inclusion of the DTAC hy­ drophobic tail into the cavity of the β-CD. The complexation of the anionic surfactant (e.g. Alfoterra 167-4s) with β-CD has been established through 1H-NMR spectroscopy in our preceding work (Wei et al., 2014a), therefore, it will not be discussed any further here.

Table 5 1 H-NMR Chemical Shifts (δ, ppm) for H protons of β-CD alone, β-CD/DTAC complexation, and their complexation induced shifts, CIS, in D2O at 25 � C. H Protons

H Protons

3.3. Adsorption of free vs. encapsulated cationic surfactant (DTAC) onto solid surfaces Table 7 summarizes the results of the cationic surfactant adsorption study. The adsorption results of the free (alone)-DTAC are presented in the upper part of the table, followed by the adsorption results of the encapsulated DTAC onto sand and kaolin. Table 7 shows the average

0.002

9

L. Romero-Zer� on and C. Espinosa

Journal of Petroleum Science and Engineering xxx (xxxx) xxx

Table 7 Experimental results of the cationic surfactant adsorption tests in free- and encapsulated state.

Table 8 Structural Regeneration of the HPAM SAP System and the Xanthan Gum System in terms of G0 (t) and in %.

Cationic Surfactant/Free-State [DTAC] (mg/L) % of DTAC adsorption

Test Intervals and Conditions Baseline

Play Sand

Kaolin

1151

273 76.3

372 67.7

Baseline

Play Sand

Kaolin

1151

589 48.8

684 40.6

At the end of the first interval (low-shear stage). Reference value of G0 -at-rest (t ¼ 960 s) At the end of the second interval (high-shear stage) (t ¼ 474 s) Regeneration in the third interval After t ¼ 300 s After t ¼ 600 s After t ¼ 960 s

Cationic Surfactant/Encapsulated-State [DTAC] (mg/L) % of DTAC adsorption

values of duplicated samples for each experiment, except for the base­ line data, which corresponds to the average of 4 samples. Table 7 provides important insights on the DTAC/β-CD complexation adsorption behavior. First, the complexation (i.e. encapsulation) of DTAC into the β-CD cavity reduces its absorption onto sand and kaolin by 27%. This observation is significant because it confirms the advan­ tage of surfactant encapsulation for several applications such as enhanced oil recovery particularly in carbonate reservoirs. Secondly, the data in Table 7 also indicates that there is a higher adsorption of DTAC onto play sand relative to kaolin. Play sand is 99.0% composed of crystalline silica (quartz) (QUIKRETE, 2011), which exhibits a nega­ tively charged surface (Earle, 2016), therefore a maximum adsorption 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 (Churchman et al., 2016) that exhibits an amphoteric mineral surface (Zhou and Gunter, 1992). According to Churchman et al. (2016) “the silica tetrahedral face [is] 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 (Zhou and Gunter, 1992; Gupta, 2011). The pH of the 8.41 wt% brine used in this work is 5.8, so at this pH, the surface of kaolinite mineral simultaneously ex­ hibits negative 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 the surface of the sand grains; which explains the consistently lower adsorption of the cationic sur­ factant (DTAC) onto the kaolin surface at the pH of the brine used in this work. These experimental observations indicate that the XG-SAP system could be advantageous por applications in carbonate formations con­ taining low content of silica and clay minerals as suggested by Cui et al. (2014).

HPAM -SAP

XG-SAP

G’ [Pa]

%R

G’ [Pa]

%R

8.554

100

7.59

100

0.615

7.2

0.205

2.7

6.695

78.3

7.351

96.9

6.713 6.72

78.5 78.6

7.395 7.537

97.4 99.3

samples of the different polymer systems in an oven at 90 � C for a period of 4 weeks. Fig. 8a to d display the G0 - and G00 - curves for the baseline polymers (i.e. XG and HPAM) and for the corresponding SAP systems (i. e. XG-SAP and HPAM-SAP) as a function of angular frequency and time (day # 1, week # 3, and week # 4). The viscoelastic behavior (Fig. 8a to d) of the baseline polymer solutions and SAP systems indicates thermal degradation of all the samples as a function of time. However, the curve for week # 4 (W # 4) in Fig. 8a shows that the XG-SAP system exhibits superior structural strength (e.g. less thermal degradation) than the baseline XG solution. While, in terms of the viscous modulus at W # 4 (Fig. 8b), the XG-SAP system and its corresponding baseline show very close G00 -curves. Comparable thermal degradation behavior was observed for the HPAM-SAP system and its matching baseline. In terms of the G0 -curves, Fig. 8c reveals that by W # 4, the structural strength of the HPAM-SAP was also significantly higher than the structural strength of its baseline. Likewise, Fig. 8d indicates that at W # 4, the HPAM-SAP system is more viscous (i.e. upper G00 -curve) than the baseline. This suggests that the HPAM-SAP system underwent less thermal degradation than its corre­ sponding reference HPAM solution. Fig. 9a to d show pictures of the different polymeric systems at week # 1, week # 3, and week # 4 of the testing period. Fig. 9a and d show the vials containing the XG baseline solution and the XG-SAP at week # 1, week # 3, and week # 4, respectively. The main variation observed for these solutions as a function of time was color change, which turned from an initial transparent-clear in W #1 (same physical appearance as the fresh polymeric solutions at t ¼ 0), to a transparent-clear yellowish color (W # 3), to a transparent-darker yellow (W # 4), more noticeable for the XG-SAP system, probably related to the thermal aging of the CS and β-CD. Similar observations were obtained for the HPAM baseline and for the HPAM-SAP system (Fig. 9c and d), however, a darker brownish color was observed for the HPAM-SAP system. In some of the samples, only an insignificant amount of very fine particles was observed. At large, the samples showed macroscopic stability. Overall, this long-term thermal evaluation demonstrates in terms of the rheological analysis that both SAP systems are more thermally stable than the corresponding baseline polymers. Thus, the three-dimensional network structures built up through non-covalent interactions are more stable to thermal degradation.

3.4. Structural decomposition and regeneration of the SAP systems Table 8 presents the experimental results of the DMA test showing the behavior for the HPAM-SAP and for the XG- SAP systems in the regeneration step in terms of G0 (t) and percentage of regeneration (%R). The results of the DMA test (Table 8) indicate that the XG-SAP system shows practically complete regeneration of the initial structural strength within 960 s, while the HPAM-SAP displays a slower structural regen­ eration, achieving only 78.6% of structural regeneration in the same time frame. The XG-SAP system exhibits a thixotropic behavior. This observation agrees with previous research on self-assembling of XG/ methylcellulose (Liu and Yao, 2015). This thixotropic recovery property is essential for practical applications as is the case of polymer flooding in enhanced oil recovery (Sydansk et al., 2004; Wei et al., 2015).

3.6. Long-term bio-stability evaluation Table 9 shows only the polymeric-surfactant systems that remained undegraded (i.e. stable) during the evaluation period of 2 and ½ years. The experimental results presented in Table 9 clearly confirms the fact that XG is particularly vulnerable to biodegradation (Kumar et al., 2018). Only one of the XG formulations remained stable during the testing period, which was the formulation containing xanthan gum (0.5 wt%) and 35 ppm of DTAC prepared in 8.41% brine. On the

3.5. Long-term thermal stability evaluation This long-term thermal stability evaluation consisted in placing the 10

L. Romero-Zer� on and C. Espinosa

Journal of Petroleum Science and Engineering xxx (xxxx) xxx

Fig. 8. G0 - and G00 - curves as a function of angular frequency and time (week # 3, and week # 4). (a) and (b) XG system and (c) and (d) HPAM systems.

Fig. 9. Thermal Stability Test. Vials retrieved from the oven at Week # 1, Week # 3, and Week # 4. (a) XG baseline (b) XG-SAP (c) HPAM baseline, and (d) HPAM-SAP.

contrary, the HPAM formulations demonstrated to be more resistance to biodegradation, as expected (Song et al., 2011). Furthermore, it seems that the concentration of the brine played an important role in main­ taining the integrity of the HPAM formulations, since all HPAM for­ mulations prepared in 8.41 wt% brine remained undegraded. Only one of the HPAM formulations and the baseline (e.g. without additives) so­ lution prepared in the 2.10% brine remained stable during the testing period. In addition, to the baseline HPAM solution prepared in distilled 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. 8.41 wt%) offers the main antimicrobial action. 4. Conclusions Overall, the baseline XG polymer solution shows a more stable viscoelastic behavior as function of salinity concentration and a signif­ icantly higher elasticity and viscosity than the HPAM polymer solution 11

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Journal of Petroleum Science and Engineering xxx (xxxx) xxx

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.

Table 9 Long-Term Bio-Stability Evaluation: Experimental Results (*DW means Distilled Water). This table lists only the polymeric-surfactant systems that remained undegraded (i.e. stable) during the evaluation period of 2 and ½ years. Test #

Brine wt%

XG System (Bulk and Glass beads

11 Test #

8.41 Brine wt%

XG þ 35 ppm DTAC HPAM System

1 2 8 9 12 13 14 15

0 (DW)* 2.10 2.10 8.41 8.41 8.41 8.41 8.41

Baseline Baseline SAP-DTAC Baseline HPAM þ 50 ppm DTAC HPAM þ 50 ppm β-CD SAP-Alfoterra 167-4s SAP-DTAC

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at the same conditions. Blends of XG-DTAC and HPAM-DTAC do not show meaningful interactions. However, oscillatory rheology demon­ strated that self-assembly occurred when the XG solution (0.5 wt%) was mixed with the cationic surfactant (50 ppm) and β-CD (107.5 ppm) at a molar ratio of 2 mol of CS to 1 mol of β-CD. The formation of β-CD-DTAC host-guest interaction was verified via 1H-NMR spectroscopy. Selfassociation was also observed for blends of HPAM solution (1 wt %)/anionic surfactant (50 ppm)/β-CD (50 ppm) at a molar ratio of 2 mol of AS per 1 mol of β-CD. Superior structural strength of both selfassembling systems was observed at high brine concentration (8.41 wt % brine). Hence, the optimum self-assembled systems are tolerant to brine containing high salinity and hardness concentration relative to the performance of the corresponding baseline polymers in the same brine. The static adsorption tests of the CS in free-state and complex-state demonstrated that the encapsulation of the CS (DTAC) into the β-CD cavity decreases its adsorption onto the sand and kaolin surface by 27% compared to the adsorption behavior of the CS in free-state. In terms of structural strength of the supramolecular systems, the DMA test indicates that the XG-SAP shows a thixotropic 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 thermal stability of the self-assembling systems formulated using XG and HPAM polymers show superior thermal sta­ bility than the matching baseline systems. Finally, the long-term bio stability evaluation indicates that high brine concentration (i.e. 8.41 wt %) seems to offer the main antimicrobial action to the baseline polymers and self-assembling systems. Overall, this exploratory research established that molecular selfassembly of XG/CS/β-CD blend offers the advantages of superior viscoelasticity, thermal stability, and tolerance to high salinity and hardness relative to the HPAM/AS/β-CD system. These observations are relevant for practical applications as is the case of enhanced oil recovery (EOR) in carbonate formations containing low concentration of mineral impurities and/or in carbonate reservoirs in which the formation brines contain elevated salinity and hardness concentration. Forthcoming research will evaluate the performance of the XG/CD/ β-CD system in displacing heavy oil through core flooding displacement tests. Author contributions Claudia Espinosa contributed equally. Notes The authors declare no competing financial interests. Acknowledgment This research was supported by the MITACS Globalink program. The 12

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