Disrupting Admicelle Formation and Preventing Surfactant Adsorption

May 14, 2014 - molecular weight of 70 000, was procured as a 30 wt % solution in water, and was used as received. The silica used was procured from...
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Disrupting Admicelle Formation and Preventing Surfactant Adsorption on Metal Oxide Surfaces Using Sacrificial Polyelectrolytes Javen S. Weston,*,† Jeffrey H. Harwell,‡,§ Benjamin J. Shiau,†,§ and Mahfuz Kabir† †

Chemical, Biological and Materials Engineering; ‡Department of Chemical Engineering; and §Petroleum Engineering, University of Oklahoma, Norman, Oklahoma 73019, United States

ABSTRACT: The adsorption of anionic, cationic, and nonionic surfactants was measured on high-surface area silica and alumina nanoparticles when in the presence of the proposed polyelectrolyte sacrificial agents. Surfactant adsorption was characterized using two types of adsorption isotherms: one with constant polymer concentration and varying surfactant concentration, and another with a varying polymer concentration and constant surfactant concentration. Polystyrenesulfonate and Polydiallyl dimethylammonium chloride were tested as potential sacrificial agents on alumina and silica, respectively. Each surfactant/ polymer system was allowed to reach equilibrium and supernatant surfactant concentrations were measured. This information was then plotted in order to determine what, if any, effect the proposed sacrificial agent had on the equilibrium adsorption. Results indicate that both of these polymers can have a large effect on total surfactant adsorption at a variety of surfactant concentrations.

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INTRODUCTION

Consequently, any method that can reduce the amount of surfactant adsorption during a chemical flood will result in a large increase in the number of viable applications for chemical flooding. Adsorption prevention is especially important because of surfactant molecules’ ability to form admicelles on solid surfaces. When surfactant concentrations are above the critical admicelle concentration (CAC), they deviate from the linear Henry’s Law adsorption region where single surfactant molecules are adsorbing onto the surface with limited to no adsorbate−adsorbate interactions (Region “1” in Figure 1) and enter a period of cooperative adsorption where each adsorption site on a surface can adsorb multiple surfactant molecules (Region “2” in Figure 1) until the surface becomes a nearcomplete double-layer/admicelle slowing surfactant adsorption (Region “3” in Figure 1). The adsorbed double-layer is then slowly filled until a maximum adsorption plateau is reached (Region “4” in Figure 1). This nonlinear adsorption behavior is a major reason a sacrificial agent method is very attractive for preventing surfactant adsorption; it not only prevents the adsorption of the initial surfactant molecule, but could also aid

Surfactant adsorption on metal oxides is extremely important for a variety of practical applications from detergency1 to enhanced oil recovery2−4 and surfactant-aided environmental remediation.5,6 In chemical flooding applications, a specially designed surfactant mixture is pumped into either the contaminated aquifer or oil reservoir to mobilize trapped oil by lowering the interfacial tension and, occasionally, changing the wettability of the rock formation or aquifer. Many variables can be changed in order to optimize the efficiency of the chemical flood, such as surfactant type, surfactant concentration, injection volume, pH, counterion valence and concentration, surfactant ratios (in multicomponent systems), and so forth. Surfactant adsorption onto soil or reservoir rock can impact many of these variables, most notably surfactant concentration and surfactant ratio, and have a negative effect on oil recovery. Surfactants lower interfacial tension by organizing themselves along the oil−water interface, therefore, any surfactant that is adsorbed on reservoir rock or aquifer soil cannot participate in lowering the interfacial tension and is not used to mobilize any hydrocarbon. Significant surfactant adsorption can greatly affect the economic viability of a chemical flood due to lower efficiency. © 2014 American Chemical Society

Received: March 21, 2014 Revised: May 14, 2014 Published: May 14, 2014 6384

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Figure 1. Model adsorption isotherm.

significant effects on the conformation of coadsorbing polyelectrolytes and surfactants14 which can have significant effects on the total surfactant adsorption onto the mineral surface. Velegol also demonstrated that poly-L-lysine (which is cationic at certain pH’s) can prevent the adsorption of hexadecyltrimethylammonium bromide (a cationic surfactant) on silica,15 results that are directly comparable to those investigated here. Of those cited, only Esumi intensely studied the effect of order of addition on surfactant adsorption. This study hopes to prove the concept that a polymeric sacrificial agent will adsorb onto a mineral surface and prevent the adsorption of surfactant molecules and the formation of admicelles. Silica and alumina will be used to approximate negatively and positively charged reservoir/soil minerals, respectively.

in preventing the formation of admicelles on the surface,7−9 reducing the amount of surfactant lost to adsorption at a greater than one-to-one ratio, thus increasing productivity and efficiency of surfactant flooding. Currently, surfactant adsorption is most commonly mitigated by changing the type of surfactant(s) used. In reservoirs/soils with a net negative surface charge on the mineral surface, for instance, anionic surfactants are preferred because the electrostatic repulsion between the mineral surface, and the surfactant headgroup helps minimize adsorption. Similarly, cationic surfactants are preferred for use in reservoirs/soils with a positive net surface charge. However, oil reservoirs and aquifer soils are inherently complex and made up of a variety of minerals including silicates, aluminates, carbonates, and various clays. The surface charge of all of these minerals varies with both pH and the type of mineral, which creates a substrate that has regions of negative charge and regions of positive charge. In these situations, it is difficult to prevent patchy, but significant, surfactant adsorption when using either anionic or cationic surfactants. Therefore, it is hoped that by using a low molecular weight polyelectrolyte as a sacrificial agent in these reservoirs the charge at the mineral surface can be effectively neutralized or reversed and electrostatically driven surfactant adsorption prevented. Polyelectrolytes were chosen as potential sacrificial agents due to their tendency to irreversibly adsorb onto charged mineral surfaces10 due to the numerous adsorption sites on each polymer molecule; i.e., although each charged group on the polymer is adsorbing and desorbing from the surface individually, desorption of the entire molecule is energetically unfavorable from a practical standpoint if the number of adsorption sites per molecule is significantly large. Multiple investigators have studied the coadsorption of polyelectrolytes and surfactants on metal oxides. Somasundaran11 proposed a variety of mechanisms where simultaneous adsorption of polyelectrolytes and ionic surfactants onto a charged metal oxide surface can either promote or inhibit surfactant adsorption via cooperative or competitive pathways, respectively. Other experiments carried out by Esumi showed that low molecular weight polyelectrolytes can be displaced from the metal oxide surface when surfactants are added to solution following prior polyelectrolyte adsorption12 and that there is an inverse relationship between polymer molecular weight and surfactant adsorption; as the molecular weight of poly(ethylene oxide) was increased, cationic surfactant adsorption on silica was reduced.13 Velegol showed that the specific counterions in solution (Br −, Cl −, etc.) can have

Materials. Sodium dodecyl sulfate (SDS) was produced by MP Biomedical, LLC., and was purchased through Fisher Scientific, its manufacturer stated purity was ∼99% and was used as received. SDS is an anionic surfactant with a sulfate headgroup and an alkyl chain length of 12 carbons. Dodecylpyridinium chloride (DPC) was obtained from the Aldrich Chemical Co. as a 98% purity solid powder and was used as received. DPC is a cationic surfactant with a pyridinium headgroup and an alkyl chain length of 12 carbons. Tergitol NP-15 (NP-15) is a nonionic ethoxylated nonylphenol surfactant that contains an average of 15 ethoxylate monomers, and was manufactured and provided by The Dow Chemical Company. Polydiallyl dimethylammonium chloride (PDADMAC) is a cationic polyelectrolyte produced by SNF, Inc. under the trade name of Floquat FL4540. It was received as a ∼21−24 wt % solution of PDADMAC in water. The polymer was used as received, but all stated PDADMAC concentrations are based on the assumption that the Floquat FL4540 is a 22.5 wt % active solution. Polystyrenesulfonate (PSS) is an anionic polyelectrolyte and was purchased from SigmaAldrich and produced by Aldrich Chemistry. The PSS has a typical molecular weight of 70 000, was procured as a 30 wt % solution in water, and was used as received. The silica used was procured from Evonik Industries under the trade name Aerosil 200. It contains >99.8 wt % SiO2, has a specific surface area of 200 ± 15 m2/g, an average primary particle size of 14 nm and is a fluffy, white powder. Aerosil 200 has a measured surface charge of −0.02 C/m2 at the pH and salt concentration used in this study.16 The alumina used was procured from Evonik Industries under the trade name Aeroxide Alu C. It contains >99.8 wt % Al2O3, has a specific surface area of 100 ± 15 m2/ g, and is a fluffy, white powder. Aeroxide Alu C has a measured surface charge of 0.05 C/m2 at the pH and salt concentration used in this study.10 Other materials used during this study are ACS grade sodium chloride (from Acros) and HPLC grade methanol (from Fisher Scientific). Experimental Methods. Surfactant and polymer adsorption solutions for each experimental data point were made with 0.15 wt % (0.025 M) NaCl to act as a swamping electrolyte. The swamping electrolyte is used so that the chemical potential of the surfactants can be considered independent of counterion concentration. Flat bottom test tubes were filled with 0.50g of silica/alumina and were then filled with 10 mL of surfactant/polymer solution and shaken until all silica/ alumina was wetted by the surfactant/polymer solution. Solution pH was adjusted to neutral (pH = 7) using 5 M NaOH. The tubes were then placed into a 30 °C water bath, shaken once a day and allowed to equilibrate for 4 days. After equilibration, the tubes were centrifuged for 1 h, and the aqueous supernatant was extracted from the test tubes, diluted, and surfactant concentrations were analyzed via either UV−vis spectrophotometry (at a wavelength of 274 nm) or High Performance Liquid Chromatography (HPLC) with conductivity and UV detectors.

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EXPERIMENTAL SECTION

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shown in Figure 1.17 Focusing on Figures 2 and 3, at equilibrium concentrations above 1000 μM for SDS and 100 μM for NP-15, the equilibrium adsorption for the solutions with the sacrificial agent was essentially equivalent with the equilibrium adsorption for the equivalent surfactant-only solutions. However, at equilibrium concentrations below those values, the polymer/surfactant isotherms noticeably deviate from the surfactant-only isotherms. One hypothesis that explains this behavior would be that the amount of polyelectrolyte required to saturate the surface adsorption sites was underestimated, and the sacrificial agent was able to prevent some surfactant adsorption at low concentrations, but at higher concentrations the SDS was able to adsorb onto the metal oxide surface, which then allowed for cooperative adsorption of more surfactant molecules via admicelle formation. A similar delay in adsorption can be observed in the DPC isotherms shown in Figure 4, although the surfactantonly and surfactant/polymer isotherms do not appear to converge in the tested range of surfactant/metal oxide ratios tested. Figure 4 also shows that adding additional sacrificial polyelectrolyte to the solution further retards surfactant adsorption throughout the tested range of surfactant concentrations. Esumi,18 Velegol,15 and Sastry19 observed similar delays in admicelle formation when studying simultaneous adsorption of a variety of polyelectrolytes and surfactants onto metal oxide surfaces.

RESULTS AND DISCUSSION The adsorption study began by completing adsorption kinetics studies to ensure that surfactant adsorption is complete after 96 h. Five-hundred milligrams of metal oxide and 10 mL of a 16 000 μM solution of surfactant were added to a series of vials, which were thoroughly mixed. Samples were taken every day for 7 days and supernatant surfactant concentrations determined. The results of these kinetics studies showed that adsorption reaches equilibrium after 1−2 days in most samples, and has reached a stable equilibrium by the intended sample date of 4 days after surfactant addition.

Figure 2. Adsorption isotherms of NP-15 (nonionic) on Aerosil 200 (anionic) with and without PDADMAC (cationic).

Figure 4. Adsorption isotherms of DPC (cationic) on Aerosil 200 (anionic) with and without PDADMAC (cationic).

Since it appeared that the initial estimate for the amount of polyelectrolyte required to saturate the metal oxide surface was too low, an additional series of adsorption experiments were performed where the initial concentration of surfactant was held constant while the concentration of polyelectrolyte was varied from 0.01 to 0.75 wt %. Additionally, the surfactant was added to the solution using two different methodologies: simultaneous addition and sequential addition. The simultaneous method of addition involved adding the polyelectrolyte and surfactant to the vial at the same time. The sequential addition method involved adding the polyelectrolyte to the metal oxide-filled vial, allowing the polymer to reach adsorption equilibrium on the metal oxide surface over 48 h, and subsequently adding a concentrated surfactant solution to the vial. These vials were then allowed to equilibrate for an additional 3 days. Figures 5−7 contain plots of equilibrium

Figure 3. Adsorption isotherms of SDS (anionic) on Aeroxide Alu C (cationic) with and without PSS (anionic).

Two adsorption isotherms were prepared for SDS/alumina and NP-15/silica and three isotherms were prepared for the DPC/silica system. One isotherm was measured using the surfactant only, while the remaining isotherms were prepared with the presence of polymer at either 0.15 or 0.45 wt %. The 0.15 wt % figure was determined by estimating the number of charged sites on the metal oxide surface and then calculating the polymer concentration that would be required to balance out the surface charges with an equal number of oppositely charged groups on the polymer molecules. Figures 2−4 contain all of these isotherms plotted on a log−log scale and can be compared to the idealized surfactant adsorption isotherm 6386

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reached at approximately 0.3 wt % polymer. For both the SDS and DPC solutions, surfactant adsorption is also noticeably lower when the polyelectrolyte was added sequentially and allowed to adsorb onto the alumina without having to compete with the surfactant for adsorption sites, although this behavior was not present for the nonionic NP-15. As seen in Figure 5, NP-15 adsorption was reduced from an average of 0.18 ± 0.03 μmol/mg without PDADMAC to 0.02 ± 0.01 μmol/mg for solutions with PDADMAC concentrations ≥0.30 wt %, or an 88% reduction in NP-15 adsorption. Similarly, SDS adsorption was reduced from an average of 0.43 ± 0.01 μmol/mg without PSS to 0.20 ± 0.02 μmol/mg for solutions with PSS concentrations ≥0.30 wt %, or a 53% reduction in SDS adsorption (Figure 6). Lastly, DPC adsorption was reduced from an average of 0.33 ± 0.01 μmol/mg without PDADMAC to 0.03 ± 0.01 μmol/mg when ≥0.15 wt % PDADMAC was added, and the DPC is added sequentially; a 91% reduction in surfactant adsorption.

Figure 5. Adsorption of 35 000 uM NP-15 solutions on Aerosil 200 as a function of PDADMAC concentration for both sequential (adding surfactant after polymer) and simultaneous addition of NP-15.

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CONCLUSIONS The tested polyelectrolytes appear to effectively prevent significant amounts of surfactant adsorption on metal oxides by blocking charged adsorption sites on the mineral surface. In some cases, surfactant adsorption was almost completely prevented at the concentrations tested, which were well above the critical micelle concentration. The polymers also appear to irreversibly adsorb onto the metal oxide surface due to multisite adsorption and kinetic limitations, which was confirmed by experimental results with the PSS only. Irreversible polyelectrolyte adsorption is in agreement with several cited literature sources as well.10 Additionally, the order of surfactant addition has a strong effect on the equilibrium surfactant adsorption. We hypothesize that when the polyelectrolyte and surfactant are exposed to the metal oxide simultaneously they compete for adsorption sites, with the smaller surfactant molecules being able to migrate to the mineral surface more quickly and interfere with the efficient adsorption of the polymers. Others have shown that simultaneous adsorption of surfactant and polyelectrolytes can have an effect on the surface conformation, which is the likely cause of the increased adsorption in the simultaneous addition experiments.20−23 Since a noticeably lower amount of surfactant adsorption is seen in the sequential addition experiments, an environmental remediation or enhanced oil recovery chemical flood would likely see greater benefits if the surfactant flood was preceded by a polyelectrolyte flood, rather than simply using a single combined flow of polyelectrolyte and surfactant. The results also indicate that, although the polyelectrolytes can prevent a significant amount of surfactant adsorption, some surfactant adsorption was seen in all of the tested samples, implying that the electrostatic adsorption mechanism is not the only method of adsorption in these systems and that hydrophobic−hydrophobic interactions between the polymers and surfactant molecules may play a role in the remaining adsorption. The above experiments, along with work completed by ShamsiJazeyi et al.24,25 demonstrate that the concept of using a polyelectrolyte as a sacrificial agent to prevent surfactant adsorption is reasonable, but further tests must be carried out to ensure that the mechanism described above translates effectively to practical situations. In particular, adsorption studies must be carried out on natural sand and rock samples with the more complicated surfactant formulations commonly

Figure 6. Adsorption of 50 000 uM SDS solutions on Aeroxide Alu C as a function of PSS concentration for both sequential (adding surfactant after polymer) and simultaneous addition of SDS.

Figure 7. Adsorption of 50 000 uM DPC solutions on Aerosil 200 as a function of PDADMAC concentration for sequential (adding surfactant after polymer) and simultaneous addition of DPC.

surfactant adsorption in μmol/mg of metal oxide as a function of polyelectrolyte concentration. It can be seen that in all of the three systems investigated, adsorption decreases with increasing polyelectrolyte concentration until an adsorption minimum is 6387

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used in chemical floods. This is especially important for understanding what effect the polyelectrolytes will have on surfactant adsorption onto clay minerals, which exhibit very different adsorption behaviors from simpler metal oxides.26 It is also unclear what effect increased salt concentrations will have on the proposed sacrificial agents, since electrostatic interactions tend to be strongly influenced by the salt concentration.27,28

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(18) Esumi, K.; Oyama, M. Simultaneous Adsorption of Poly(vinylpyrro1idone) and Cationic Surfactant from Their Mixed Solutions on Silica. Langmuir 1993, 9, 2020−2023. (19) Sastry, N. V.; Sequaris, J.-M.; Schwuger, M. J. Adsorption of Polyacrylic Acid and Sodium Dodecylbenzenesulfonate on Kaolinite. J. Colloid Interface Sci. 1995, 171, 224−233. (20) Fan, A.; Somasundaran, P.; Turro, N. J. Role of sequential adsorption of polymer/surfactant mixtures and their conformation in dispersion/flocculation of alumina. Colloids Surf., A 1999, 146, 397− 403. (21) Penfold, J.; Tucker, I.; Thomas, R. K. Polyelectrolyte Modified Solid Surfaces: The Consequences for Ionic and Mixed Ionic/ Nonionic Surfactant Adsorption. Langmuir 2005, 21, 11757−11764. (22) Esumi, K.; Masudo, A.; Otsuka, H. Adsorption of Poly(styrenesulfonate) and Ionic Surfactant from Their Binary Mixtures on Alumina. Langmuir 1993, 9, 284−287. (23) Harrison, I. M.; Meadows, J.; et al. Competitive Adsorption of Polymers and Surfactants at the Solid/Liquid Interface. J. Chem. Soc. Faraday Trans. 1995, 91, 3919−3923. (24) ShamsiJazeyi, H.; Hirasaki, G. J.; Verduzco, R. Sacrificial Agent for Reducing Adsorption of Anionic Surfactants. SPE Int. Symp. 2013, 1−16. (25) ShamsiJazeyi, H.; Verduzco, R.; Hirasaki, G. J. Reducing adsorption of anionic surfactant for enhanced oil recovery: Part I. Competitive adsorption mechanism. Colloids Surf., A 2013, http://dx. doi.org/10.1016/j.colsurfa.2013.10.042. (26) Amirianshoja, T.; Junin, R.; et al. A comparative study of surfactant adsorption by clay minerals. J. Petrol. Sci. Eng. 2013, 101, 21−27. (27) Li, N.; Zhang, G.; et al. Adsorption Behavior of Betaine-Type Surfactant on Quartz Sand. Energy Fuels 2011, 25, 4430−4437. (28) Velegol, S. B.; Fleming, B. D.; et al. Counter ion Effects on Hexadecyltrimethylammonium Surfactant Adsorption and Self-Assembly on Silica. Langmuir 2000, 16, 2548−2556.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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REFERENCES

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