Polymer-surfactant Systems Research Articles

Evaluation of Rheological and Thermal Properties of a New Fluorocarbon Surfactant–Polymer System for EOR Applications in High‐Temperature and High‐Salinity Oil Reservoirs

AbstractThermal stability and rheological properties of a novel surfactant–polymer system containing non‐ionic ethoxylated fluorocarbon surfactant was evaluated. A copolymer of acrylamide (AM) and 2‐acrylamido‐2‐methylpropane sulfonic acid (AMPS) was used. Thermal stability and surfactant structural changes after aging at 100 °C were evaluated using TGA, 1H NMR, 13C NMR, 19F NMR and FTIR. The surfactant was compatible with AM–AMPS copolymer and synthetic sea water. No precipitation of surfactant was observed in sea water. The surfactant was found to be thermally stable at 100 °C and no structural changes were detected after exposure to this temperature. Rheological properties of the surfactant–polymer (SP) system were measured in a high pressure rheometer. The effects of surfactant concentration, temperature, polymer concentration and salinity on rheological properties were studied for several SP solutions. At low temperature (50 °C), the viscosity initially increased slightly with the addition of the surfactant, then decreased at high surfactant concentration. At a high temperature (90 °C), an increase in the viscosity with the increase in surfactant concentration was not observed. Overall, the influence of the fluorocarbon surfactant on the viscosity of SP system was weak particularly at high temperatures and high shear rate. Salts present in sea water reduced the viscosity of the polymer due to a charge shielding effect. However, the surfactant was found to be thermally stable in the presence of salts.

Comparing the spectral properties of pyrene as free molecule, label and derivative in some colloidal systems

This paper investigates the behavior of pyrene as free molecule (probe), label and derivative in some colloidal systems. Aqueous octaethylene glycol mono n-dodecyl ether (C12E8) with or without poly(acrylic acid) (PAA), and a hydrophobically modified polyacrylate are considered. The effect of C12E8 on PAA labeled with pyrene is also studied. As a probe, pyrene has a typical behavior in the absence or presence of PAA. The labeled PAA has the peak corresponding to the sandwich excimer even in the absence of surfactant. The pyrene derivative is 4-(N′,N′-diphenyl-hydrazine)-3,5-dinitrobenzoic acid 2-oxo-2-pyren-2-yl-ethyl ester. Its fluorescence emission is low and bathochromically shifted by 9nm with respect to pyrene. The low fluorescence emission of derivative is due to its self-quenching. The data reveal that pyrene probe is located in the free or polymer-bound micelles. The labels on PAA create hydrophobic domains and shrink the polymer coil. By surfactant addition, labeled pyrene is solubilized in micelles palisade layer, closer to the ethylene oxide (EO) chains. The derivative is also located into the micellar palisade layer, in the proximity of the EO groups. The fluorophore behavior is discussed in relation to the critical points of surfactant and polymer–surfactant systems.

Solubilization of octane in electrostatically-formed surfactant–polymer complexes

Polymers can be used to modulate the stability and functionality of surfactant micelles. The purpose of this study was to investigate the solubilization of an octane oil-in-water emulsion in mixtures of an anionic polymer (carboxymethyl cellulose) and anionic sodium dodecylsulphate (SDS), nonionic polyoxyethylene sorbitan monooleate (Tween 80) and cationic cetyltrimethylammonium bromide (CTAB) surfactant micelles using dynamic light scattering, microelectrophoresis and turbidity measurements. The results showed that the addition of anionic carboxymethyl cellulose accelerated octane solubilization in cationic CTAB and CTAB–Tween 80 micelles, but did not affect the solubilization behaviors of micelles that were nonionic and anionic. The surfactant–polymer interactions were also studied using isothermal titration calorimetry (ITC) to characterize different physiochemical interaction regions depending on surfactant concentration in surfactant–polymer systems. Upon octane solubilization in CTAB–carboxymethyl cellulose mixtures, shape transitions of polymer–micelle complexes may have taken place that altered light scattering behavior. Based on these results, we suggest a mechanism for oil solubilization in electrostatically-formed surfactant–polymer complexes.

Effect of viscosity and interfacial tension of surfactant–polymer flooding on oil recovery in high-temperature and high-salinity reservoirs

This study aims to analyze the influence of viscosity and interfacial tension (IFT) on oil displacement efficiency in heterogeneous reservoirs. Measurement of changes in polymer viscosity and IFT indicates that viscosity is influenced by brine salinity and shearing of pore media and that IFT is influenced by salinity and the interaction between the polymer and surfactant. High concentrations (2,500 and 3,000 mg/L) of polymer GLP-85 are utilized to reduce the effect of salinity and maintain high viscosity (24 mPa s) in formation water. After shearing of pore media, polymer viscosity is still high (17 mPa·s). The same polymer viscosity (17 mPa·s) is utilized to displace oil, whose viscosity is 68 mPa·s, at high temperature and high pressure. The IFTs between surfactant DWS of 0.2 % in the reservoir water of different salinities and crude oil droplet are all below 10−2 mN/m, with only a slight difference. Surfactant DWS exhibits good salt tolerance. In the surfactant–polymer (SP) system, the polymer solution prolongs the time to reach ultra-low IFT. However, the surfactant only has a slight effect on the viscosity of the SP system. SP slugs are injected after water flooding in the heterogeneous core flooding experiments. Recovery is improved by 4.93–21.02 % of the original oil in place. Furthermore, the core flooding experiments show that the pole of lowering the mobility ratio is more significant than decreasing the IFT of the displacing agent; both of them must be optimized by considering the injectivity of the polymer molecular, emulsification of oil, and the economic cost. This study provides technical support in selecting and optimizing SP systems for chemical flooding.

Polymer-Surfactant Interactions and the Effect of Tail Size Variation on Micellization Process of Cationic ATAB Surfactants in Aqueous Medium

The interactions between oppositely charged surfactant-polymer systems have been studied using surface tension and conductivity measurements and the dependence of aggregation phenomenon over the polyelectrolyte concentration and chain length of cationic ATAB surfactants, cetyltrimethyl ammonium bromide (CTAB), tetradecyltrimethyl ammonium bromide (TTAB), and dodecyltrimethyl ammonium bromide (DTAB) have been investigated. It was observed that cationic surfactants induce cooperative binding with anionic polyelectrolyte at critical aggregation concentration (cac). The cac values of ATAB surfactants in the presence of anionic polyelectrolyte, sodium carboxy methyl cellulose (NaCMC), are considerably lower than their critical micelle concentration (cmc). After the complete complexation, free micelles are formed at the apparent critical micelle concentration (acmc), which is slightly higher in polyelectrolyte aqueous solution than in pure water. Among the cationic surfactants (i.e., CTAB, TTAB, and DTAB), DTAB was found to have least interaction with NaCMC. Surfactants with longer tail size strongly favor the interaction, indicating the dependence of aggregation phenomenon on the structure, morphology, and tail length of the surfactant.

The Role of Surfactants in Mechanical Degradation of Drag-Reducing Polymers

Turbulent drag reduction behavior of mixed polymer–surfactant systems (anionic polymer/cationic surfactant; nonionic polymer/cationic surfactant; nonionic polymer/anionic surfactant) was studied in a pipeline flow loop to explore the role of surfactants in mechanical degradation of polymers. The polymers investigated were nonionic polyethylene oxide (PEO) and anionic polyacrylamide (PAM). The surfactants studied were cationic octadecyltrimethylammonium chloride (OTAC) and anionic sodium dodecyl sulfate (SDS). The pipeline flow results obtained for PAM/OTAC mixtures support the idea that the coiling of polymer molecules does not protect the polymer molecules against shear degradation. The addition of oppositely charged cationic surfactant (OTAC) to anionic polymer (PAM) results in coiling of polymer molecules. The coiled polymer molecules undergo faster mechanical degradation than stretched polymer molecules. The addition of surfactant (cationic OTAC or anionic SDS) to nonionic polymer PEO increases the resistance of polymer molecules against shear degradation. This is reflected in the pipeline flow results. The effect is more significant for anionic surfactant (SDS) than for cationic surfactant (OTAC) especially at high concentrations of surfactant; the smaller size of the headgroup of anionic surfactant monomers allows them to have a greater influence on the polymer molecules. These results support the idea that extended polymer chains are more resistant to mechanical degradation as compared with coiled polymer molecules. The polymer chains undergo extension due to repulsion between the neighboring surfactant micelles attached to the backbone of the polymer chains.

Rheological Study on ATBS‐AM Copolymer‐Surfactant System in High‐Temperature and High‐Salinity Environment

Experimental studies were conducted to evaluate the rheological properties of surfactant‐polymer (SP) system. This SP system consists of a copolymer of acrylamide (AM) and acrylamido tertiary butyl sulfonate (ATBS) and sodium dodecyl sulphate (SDS) surfactant. Effects of surfactant concentration, temperature, polymer concentration, and salinity on rheological properties of SP system were investigated by means of oscillation and shear measurements. Comparison with classical partially hydrolyzed polyacrylamide (HPAM) was made. For the same temperature range, the viscosity drop for HPAM was about four times higher than the viscosity drop for ATBS‐AM copolymer. In deionized water, viscosity of both polymers and SP systems was very high as compared to viscosity in saline water. Viscosity reduction of ATBS‐AM copolymer was higher for salts having divalent cations. The SP system showed precipitation in presence of divalent cations. It worked well with monovalent cations even at relatively high salinities. The addition of 0.1% surfactant to the polymer resulted in a 60% decrease in the viscosity. Some interfacial rheological experiments were also carried out to investigate the behaviors on the interface between SP solutions and oil. Addition of 0.1% surfactant showed a 65% decrease in G′ at SP solution‐oil interface. SP system consisting of ATBS‐AM and SDS showed better performance at high temperature compared to HPAM‐SDS system. Due to precipitation, the SP system should be restricted to environment having low divalent cations.

Interactional behaviour of surface active ionic liquids with gelling biopolymer agarose in aqueous medium

Interactions of surfactant-like ionic liquids (ILs), 1-butyl-3-methylimidazolium octylsulfate [C4mim][C8OSO3] and 3-methyl-1-octylimidazolium chloride [C8mim][Cl] with the neutral biopolymer agarose (Ag) has been investigated in aqueous media through an array of techniques, viz. tensiometry, isothermal titrimetric calorimetry (ITC), turbidimetry, dynamic light scattering (DLS), conductometry, scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). Tensiometric and calorimetric profiles of the studied systems showed three transitions corresponding to the critical aggregation concentration C1(cac), the saturation concentration C2(sc), and the critical micelle concentration C3(cmc). The tensiometric behaviour of both the systems is found to be more like charged polymer–surfactant systems because of the large number of electron rich oxygens in the Ag chain. Interactions in the Ag–[C4mim][C8OSO3] system were found to be weaker when compared with the Ag–[C8mim][Cl] system because of higher cation–anion interactions in the former as evidenced by low a van’t Hoff's factor (∼1.0) of [C4mim][C8OSO3] in the studied concentration regime. ITC measurements showed that both the ILs interact exothermically below 6 mmol kg−1 with Ag and endothermically thereafter until C3(cmc) is reached. Turbidimetric and DLS profiles have shown coacervate formation of Ag with both the ILs. The complex formation has been verified by SEM images, which have shown that Ag nanoplates change into spherical complexes upon interaction with the ILs. FTIR spectra indicated hydrogen bonding between IL ions and Ag in various concentration regimes. The study can be useful in understanding the importance of ion–dipole, hydrogen bonding and hydrophobic interactions in Ag sol–gel systems, which are often used as matrices for electrophoresis of charged molecules.

Comparative studies on enhanced oil recovery by alkali–surfactant and polymer flooding

Chemical flooding methods are now getting importance in enhanced oil recovery to recover the trapped oil after conventional recovery. In the present study, a comprehensive study has been carried out on alkali, surfactant and polymer flooding. The chemicals with different compositions and combinations were used to recover the oil after conventional water flooding. It has been observed that increase in concentration of alkali, surfactant and polymer increases the additional recovery, but beyond a certain limit, the increase in recovery is only marginal. A series of flooding experiments using the combination of the above methods have been performed with additional recoveries more than 25 %. An analysis has been made on the relative cost of the different chemical slugs injected and the corresponding additional oil recovery. Based on the analysis, an optimum composition of the alkali–surfactant–polymer system has been recommended.

Mixed aggregate formation in gemini surfactant/1,2-dialkyl-sn-glycero-3-phosphoethanolamine systems

An evaluation of the physical interactions between gemini surfactants, DNA, and 1,2-dialkyl-sn-glycero-3-phosphoethanolamine helper lipid is presented in this work. Complexation between gemini surfactants and DNA was first investigated using surface tensiometry where the surface tension profiles obtained were found to be consistent with those typically observed for mixed surfactant–polymer systems; that is, there is a synergistic lowering of the surface tension, followed by a first (CAC) and second (CMC) break point in the plot. The surfactant alkyl tail length was observed to exhibit a significant effect on the CAC, thus demonstrating the importance of hydrophobic interactions during complexation between gemini surfactants and DNA. The second study presented is an investigation of the mixing interactions between gemini surfactants and DOPE using Clint’s, Rubingh’s, and Motomura’s theories for mixed micellar formation. The mixing interactions between the 16–3–16/16–7–16/16–12–16/16–7NH–16 gemini surfactants and DOPE were observed to be antagonistic, where the strength of antagonism was found to be dependent upon the gemini surfactant spacer group and the solution composition.

Drag reduction in turbulent pipeline flow of mixed nonionic polymer and cationic surfactant systems

AbstractTurbulent drag reduction behaviour of a mixed nonionic polymer/cationic surfactant system was studied in a pipeline flow loop to explore the synergistic effects of polymeric and surfactant drag reducing additives. The nonionic polymer used was polyethylene oxide (PEO) at three different concentrations (500, 1000, and 2000 ppm). The surfactant used was cationic octadecyltrimethylammonium chloride (OTAC) at concentration levels of 1000 and 2500 ppm. Sodium salicylate (NaSal) was used as a counter‐ion for the surfactant at a molar ratio of 2 (MR = Salt/OTAC = 2).Relative viscosity and surface tension were measured for different combinations of PEO and OTAC. While the relative viscosities demonstrated a week interaction between the polymer and the surfactant, the surface tension measurements exhibited negligible interaction.The pipeline results show a considerable synergistic effect, that is, the mixed polymer–surfactant system gives a significantly higher drag reduction (lower friction factors) as compared with pure polymer or pure surfactant. The addition of surfactant to the polymer always enhances drag reduction. However, the synergistic effect in mixed system is stronger at low polymer concentrations and high surfactant concentrations. © 2011 Canadian Society for Chemical Engineering

Effect of cationic surfactant addition on the drag reduction behaviour of anionic polymer solutions

AbstractAlthough extensive research work has been carried out on the drag reduction (DR) behaviour of polymers and surfactants alone, little progress has been made on the synergistic effects of combined polymers and surfactants. In this work, the interactions between drag‐reducing anionic polymer (copolymer of acrylamide and sodium acrylate, referred to as PAM) and drag‐reducing cationic surfactant (octadecyltrimethylammonium chloride, OTAC) are studied. Solutions are prepared using both deionised (DI) water and tap water. The measurement of the physical properties such as electrical conductivity and viscosity are used to determine the surfactant–polymer interactions.The addition of surfactant to the polymer solution has a significant effect on the properties of the system. The critical micelle concentration (CMC) of the mixed surfactant–polymer system is found to be different from that of the surfactant alone. With the addition of surfactant to a polymer solution, a substantial decrease in the viscosity occurs. The observed changes in the viscosity of mixed polymer–surfactant system are explained in terms of the changes in the extension of polymeric chains, resulting from polymer–surfactant interactions. The anionic PAM chains tend to collapse upon the addition of cationic OTAC.The pipeline flow behaviour of PAM/OTAC mixtures is found to be consistent with the bench scale results. The DR ability of PAM is reduced upon the addition of OTAC. At low concentrations of PAM, the effect of OTAC on the DR behaviour is more pronounced. The DR behaviour of polymer solutions is strongly influenced by the nature of water (DI or tap). © 2011 Canadian Society for Chemical Engineering

Interaction between two homologues of cationic surface active ionic liquids and the PEO-PPO-PEO triblock copolymers in aqueous solutions

The interaction of nonionic triblock copolymers of poly(ethyleneoxide) (PEO) and poly(propyleneoxide) (PPO) (PEOnPPOmPEOn) with a series of cationic surface-active ionic liquids in aqueous solutions have been investigated. The cationic surface-active ionic liquids include 1-alkyl-3-methylimidazolium bromide (CnmimBr, n = 8, 10, 12, 14, 16) and N-alkyl-N-methylpyrrolidinium bromide (CnMPB, n = 12, 14, 16). For different polymer-surfactant systems, the critical aggregation surfactant concentration (cac), the surfactant concentration to form free micelles (C m), and the saturation concentration of surfactant on the polymer chains (C 2) were determined using isothermal titration microcalorimetry (ITC) and conductivity measurements. The structure of the formed aggregates depended strongly on the hydrophobicity of the surfactant and the ratio of polymer/surfactant concentration. For C8mimBr, there were not any micelle-like surfactant–polymer clusters detected in the solution, and only micelles appeared. For other surfactants, the polymer–surfactant aggregates were formed in the solution, which was verified by the appearance of a broad endothermic peak in the ITC thermograms. The intensity of polymer–surfactant interaction increased with the hydrophobicity of the surfactants and the polymers but was not affected by the surfactant headgroups.

Effect of polymer–surfactant interaction on its turbulent drag reduction

Turbulent drag reduction (DR) efficacy of ionic poly(acrylic acid) and sodium dodecyl sulfate (SDS) complex system regarding polymer–surfactant interaction was examined under a turbulent flow in a rotating disk apparatus, in which the DR efficacy indicates how the torque is being reduced with a tiny amount of additives under a turbulent flow at a fixed rotational speed. Both addition of SDS and effect of pH on the ionic polymer were found to increase the polymer chain dimensions via a conformational structural change, thus enhancing the DR efficacy. Polymer–surfactant system also shows that there exists a critical polymer concentration at which the drag reduction becomes a maximum, and then above the critical concentration, the DR efficacy decreases more rapidly than that of pure polymeric systems. The dependence of drag reduction on the turbulent strength from the rotation speed change is also observed that the drag reduction increases with the increase of rotation speed regardless of polymer and surfactant concentration, implying that the complex interactions between the polymer and the surfactant molecules plays a critical role.

Acid–base equilibrium of neutral red in aqueous nonionic surfactant–polymer systems

The interactions of an aqueous phenazinium dye, viz., neutral red with nonionic Tween surfactants in the presence of some nonionic polymers, viz., PVA, PEG-200, PEG-400 and PEG-600 have been investigated. The equilibrium constants of the interaction of the dye with the micelles in the absence and in the presence of the polymers have been determined by a partition equilibrium (PE) method based on UV–VIS spectroscopy. The distributions of the two forms of the dye between the bulk water and the micelles have been computed from the equilibrium constants by an iterative method using a FORTRAN program. The thermodynamic parameters of the binding of the dye with the micelles and the polymers have been evaluated, which indicate the dye–surfactant associations to be entropy driven. The apparent pK as of the dye have been predicted from the concentrations of the acid and base forms of the free and bound dye as well as determined directly by spectrophotometric method. The predicted pK as agreed well with the experimental results in the micellar systems as well as in the aqueous surfactant–polymer systems indicating predictability of the acid–base behavior of the dye by the present method.

Interfacial Dilational Properties of Acidic Model Oil and Chemical Flooding Systems

The interfacial dilational properties of lauric acid model oil and chemical flooding systems have been investigated by means of longitudinal method. The influences of acid on the dilational viscoelasticity of partly hydrolyzed polyacrylamide (HPAM) and anionic surfactants (4,5-diheptyl-2-propylbenzene sulfonate 377 and Gemini surfactant C12COONa-p-C9SO3Na) have been expounded. The results show that lauric acid concentration is an important factor in controlling the dilational properties of mixed-adsorption layer. The addition of small amounts of lauric acid to solutions of anionic surfactant appears to increase the dilational modulus, while the superfluous addition could decrease the dilational modulus mainly due to the weakening of strong interaction among long alkyl chains of surfactant molecules. The polymer plays different roles in influencing the nature of adsorbed layers at different lauric acid concentrations: at low concentration, the addition of polymer perhaps weakens the hydrophobic interactions of lauric acid molecules, which results in decreasing the dilational modulus of adsorbed layer. At high concentration, the addition of the polymer increases the dilational modulus due to hydrophobic interactions between polymer and lauric acid molecules. At the same time, the mixed adsorbed layer shows low frequency due to the slow diffusivity of polymer molecules. The addition of lauric acid to polymer-surfactant system will further increases diffusion-exchange process between the interface and the bulk, which consequently leads to the decrease in dilational modulus.

A theoretical analysis of the surface tension profiles of strongly interacting polymer–surfactant systems

Measurements of the surface tension of static polymer–surfactant mixtures as the concentrations of polymer and surfactant are varied produce interesting and surprising profiles. It is well known how the critical points in the surface tension profile of a weakly interacting system relate to the formation of complexes in the bulk. However the critical points in the profiles of strongly interacting systems are much less well understood, and an important open question is what conditions are required for the formation of a peak. Here, using a model for the surface tension developed in a previous article (Bell et al., Langmuir 23 (2007) 6042 [ 11]), we apply asymptotic techniques to show explicitly how the structure of the surface tension profile and the critical concentrations depend on the relative stability of the underlying polymer–surfactant complexes. We derive the interesting result that none of the critical concentrations is at the critical aggregation concentration (CAC). We also identify a criterion for the appearance of a peak in the surface tension profile, expressed in terms of the physical and chemical parameters of the system.

Intermolecular association and supramolecular structures of PNVF–LiPFN and PVP–LiPFN complexes in the aqueous phase

A new polymer–surfactant system, the poly(N-vinylformamide) (PNVF)–lithium perfluorononanoate (LiPFN) system, has been studied by a combination of the surface tension method and two-dimensional 1H-19F heteronuclear Overhauser effect NMR spectroscopy (2D 1H-19F HOESY). Using the surface tension method, we found that the critical aggregation concentration (cac) of LiPFN in the presence of PNVF is ca. 2 mM. In addition, the association behavior between LiPFN and PNVF is similar to that between poly(vinylpyrrolidone) (PVP) and LiPFN. The supramolecular structure of the PNVF–LiPFN complex in the aqueous phase is revealed by means of the 2D 1H-19F HOESY experiment. On the basis of the intermolecular cross-relaxation between the PNVF protons and the LiPFN fluorines, we could conclude that the PNVF chain do penetrate into the LiPFN aggregate. The semi-quantitative analysis of the internuclear distance indicates that the PNVF chain is not located at the center core of the LiPFN aggregate, but the PNVF chain thread itself through the surface shell of the LiPFN aggregate instead. PNVF protons are nearest to the fluorines next to the carboxyl group, suggesting that the interaction between PNVF and LiPFN is mainly due to the dipole-ionic attraction. Moreover, we have reinvestigated the supramolecular structure of PVP–LiPFN complex and have found that the PVP chain also penetrates into the LiPFN aggregate. In contrast with the PNVF–LiPFN complex, the PVP chain in the PVP-LiPFN complex is rather close to the middle part of the LiPFN molecules which constitute the polymer-bound aggregate. This indicates that the formation of PVP–LiPFN complex involves more or less fluorophilic interactions.

Partition equilibrium of phenol red in aqueous polymer–surfactant system: Determination of critical aggregation concentration

Acid–base equilibrium of phenol red (PR), a sulphonephthalein dye, was studied spectrophotometrically in aqueous media containing water soluble nonionic polymers viz., polyvinyl alcohol (PVA) and polyethylene glycol (PEG) in the presence of an anionic surfactant, sodium dodecyl sulfate (SDS). A partition equilibrium method was utilized to determine the equilibrium constant of partition of the PR between micellar pseudo phase and aqueous phase and the critical aggregation concentration (CAC) of SDS in buffered aqueous systems containing the neutral polymers. The pH dependent association constants, K ass of PR with SDS–PVA, and SDS–PEG system increase in the order PEG 200 < PEG 400 < PEG 600 < PVA. The value of K ass was found to be greater in Tris buffer system than in phosphate buffer system of same pH. Interaction of the dye with the aqueous SDS–PVA and SDS–PEG systems is found to be endothermic and entropy oriented.

Topology change by screening the electrostatic interactions in a polymer–surfactant system

We show that a neutral polymer (PEG) induces a topology transition of the bilayers of an ionic surfactant system (SDS–hexanol–brine), provided that the electrostatic interactions between membranes are screened. Hexanol is used as a cosurfactant in order to get a lamellar or a sponge phase, depending on the cosurfactant/surfactant mass ratio. Using brine as solvent, the addition of polymer triggers a transformation between flat or saddle-like bilayers into vesicles. This modification is not observed in pure water because of the electrostatic repulsion between membranes. The effect can be understood in terms of the modification of the membrane Gaussian modulus due to polymer adsorption and of the entropy gain of the adsorbed polymer when the membrane bends to form a vesicle.