Polymer−Surfactant Interactions Studied by Titration Microcalorimetry

Langmuir 1996, 12, 4709-4717

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Polymer-Surfactant Interactions Studied by Titration Microcalorimetry: Influence of Polymer Hydrophobicity, Electrostatic Forces, and Surfactant Aggregational State Jan Kevelam,† Jan F. L. van Breemen,‡ Wilfried Blokzijl,§ and Jan B. F. N. Engberts*,† Department of Organic and Molecular Inorganic Chemistry and Department of Biophysical Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, and Unilever Research Laboratory, Section Liquid Detergents, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands Received March 29, 1996. In Final Form: June 24, 1996X Isothermal titration microcalorimetry has been applied to investigate the interactions between hydrophobically-modified water-soluble polymers and surfactants. The following polymers were used in this study: poly(sodium acrylate-co-n-alkyl methacrylate) (A), where n-alkyl ) C9H19, C12H25, and C18H37 (percentage of n-alkyl methacrylate to total monomer content ranging from 0 to 8), and poly(acrylamideco-n-alkyl methacrylate) (B), where n-alkyl ) C12H25 (percentage of lauryl methacrylate to total monomer content ranging from 0 to 5). The surfactants were a cyclic (mono-) n-dodecyl sodium phosphate (1) (CMP), a cyclic di-n-dodecyl sodium phosphate (2) (CDP), n-dodecyltrimethylammonium bromide (3) (DTAB), and di-n-dodecyldimethylammonium bromide (4) (DDAB). The following factors were found to influence the interactions between polymers and surfactants: electrostatic forces, polymer hydrophobicity (both the length of the hydrophobic moiety and the degree of hydrophobic modification), and the aggregational states of the amphiphilic molecules, which are micellar for the single-tailed surfactants and vesicular for the double-tailed amphiphiles. We provide evidence that, in the case of the single-tailed surfactants, individual amphiphilic molecules adsorb onto existing polymeric microdomains. This is in strong contrast with ‘classical’ polymer-surfactant interactions, where cooperative aggregation of single-tailed amphiphiles in the presence of homopolymers like poly(ethylene oxide) or poly(propylene oxide) was found at concentrations lower than the critical micelle concentration in pure water. In the case of vesicle-forming surfactants, the hydrophobic side chain of the polymer anchors into the bilayers of the vesicles. Non-hydrophobicallymodified polymers do not interact at all with the vesicle bilayers. Interestingly, the interactions between single-tailed surfactants and hydrophobically-modified polymers are governed by different factors than the binding of hydrophobically-modified polymers to vesicular bilayers. In the former case, the number and strength of existing (inter)polymeric associations is of importance, and it is particularly the length of the hydrophobic moieties that is decisive. However, for favorable polymer-bilayer interactions it is sufficient that the hydrophobic moieties are long enough to be able to anchor. If this is the case, the number of hydrophobic anchors per polymer molecule further determines the effectiveness of the interaction. Finally, it appears that electrostatic repulsions can be easily overcome by hydrophobic interactions, but added salt facilitates the interactions between equally charged polymers and surfactants.

Introduction Mixtures of water-soluble polymers and surfactants find numerous applications in industry. It is particularly the combination of a certain polymer with a specific surfactant at well-chosen concentrations that determines the fine tuning of the rheology of its aqueous solutions. Previous studies include measurements of the interactions between micelles, especially sodium dodecyl sulfate, and water-soluble neutral polymers like poly(ethylene oxide) (PEO), poly(propylene oxide) (PPO), and poly(vinylpyrrolidone) (PVP). The main features have been summarized in review articles.1 It has been proposed that hydrophobically-modified water-soluble polymers interact differently with micelleforming surfactants.2 Most probably, surfactant monomers can interact with hydrophobically-modified polymers3 but certainly not with PEO, PPO, or PVP, in which * To whom correspondence should be addressed. E-mail J.B.F. [email protected]. † Department of Organic and Molecular Inorganic Chemistry, University of Groningen. ‡ Department of Biophysical Chemistry, University of Groningen. § Unilever Research Laboratory. X Abstract published in Advance ACS Abstracts, September 1, 1996. (1) (a) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 85. (b) Goddard, E. D. J. Am. Oil Chem. Soc. 1995, 95, 1.

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cases only interactions with surfactant aggregates can occur. On the basis of these studies we contend that surfactant monomers adsorb onto existing interpolymeric associations of hydrophobic moieties in aqueous solution, thereby increasing both the number and the strength of the physical knots of the interpolymeric network.4-7 There is very recent evidence that, at the lowest surfactant concentrations, added surfactant increases the strength of the polymeric hydrophobic microdomains, whereas at higher concentrations (but still lower than the cmc) the number of domains is increased with increased surfactant concentration.7b In order to be able to predict the properties of aqueous mixtures of hydrophobically-modified polymers and surfactants, without the need for trial-and-error procedures, (2) (a) Laschewsky, A. Adv. Polym. Sci. 1995, 124, 1. (b) Hogen-Esch, T. E.; Amis, E. Trends Polym. Sci. 1995, 3, 98. (c) Glass, J. E., Ed. Polymers in Aqueous Media: Performance Through Association; American Chemical Society: Washington, DC, 1989. (d) Goddard, E. D., Ananthapadmanabhan, K. P., Eds. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (3) Effing, J. J.; McLennan, I. J.; van Os, N. M.; Kwak, J. C. T. J. Phys. Chem. 1994, 98, 12397. (4) Biggs, S.; Selb, J.; Candau, F. Langmuir 1992, 8, 838. (5) Wang, K. T.; Iliopoulos, I.; Audebert, R. Polym. Bull. 1988, 20, 577. (6) Hulde´n, M. Colloids Surf., A. 1994, 82, 263. (7) (a) Iliopoulos, I.; Wang, K. T.; Audebert, R. Langmuir 1991, 7, 617. (b) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909.

© 1996 American Chemical Society

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considerable scientific effort is currently being expended to understand the details of the relevant interaction processes. In this paper, we present, to our knowledge for the first time, a systematic microcalorimetric study on the interactions of hydrophobically-modified poly(sodium acrylate)s (A) and poly(acrylamide)s (B) with both singleand double-tailed analogues of sodium n-alkyl phosphate surfactants, i.e. cyclic (mono-) n-dodecyl sodium phosphate (CMP) (1) and cyclic di-n-dodecyl sodium phosphate (CDP) (2), and with the single- and double-tailed cationic surfactants n-dodecyltrimethylammonium bromide (DTAB) (3) and di-n-dodecyldimethylammonium bromide (DDAB) (4) (Chart 1).8 The poly(sodium acrylate)s (A) will be called PSA-CX(Y) to indicate the number of carbon atoms in the hydrophobic side chains of the polymer (X). The degree of hydrophobic modification is expressed as the fraction of all the monomeric units that possess a hydrophobic moiety, multiplied by 100, which gives the number Y. The hydrophobically-modified poly(acrylamide)s are copolymers of acrylamide and lauryl methacrylate of different composition and will therefore be named LMAM(Z), where Z is the degree of hydrophobic modification (vide supra). It will be shown that titration microcalorimetry is a generally applicable and powerful tool in the study of polymer-surfactant interactions. Evidence will be provided that electrostatic forces, polymer hydrophobicity (both the length of the hydrophobic moiety and the degree of hydrophobic modification), and the aggregational states of the amphiphilic molecules determine the interaction processes that occur between surfactants and hydrophobically-modified polymers. Experimental Section Materials. Poly(sodium acrylate)s (A) were obtained from National Starch. The pH of the supplied polymer solutions was adjusted to 9, so that the acidic groups were completely converted into their sodium salts. The polymers that were used in the microcalorimetric studies possess molecular weights of about 8000 (GPC) except for PSA-C12(8), which had a molecular weight of 3000. Poly(acrylamide)s (B) were synthesized according to a literature procedure.9 The raw polymeric material was dissolved in a minimum amount of water and precipitated in methanol. This procedure was repeated. The polymer was dissolved in water and freeze-dried. The complete removal of acrylic monomers was ascertained by 1H-NMR. The percentage of hydrophobic moieties incorporated into the polymer molecules was determined by measuring the intensity of the terminal alkyl chain CH3 peak as compared to the intensity of all the peaks in the 1H-NMR spectrum. It appeared that in all cases ap(8) Compare: (a) Bloor, D. M.; Holzworth, J. F.; Wyn-Jones, E. Langmuir 1995, 11, 2312. (b) Olofsson, G.; Wang, G. Pure Appl. Chem. 1994, 66, 527. (c) Brackman, J. C.; Engberts, J. B. F. N. Langmuir 1988, 6, 1266. (d) Persson, K.; Wang, G.; Olofsson, G. J. Chem. Soc., Faraday Trans. 1994, 90, 3555. (9) American Cyanamid Co. Brit. Pat. 764,409 (Dec 28, 1956).

Kevelam et al. proximately 50% of the hydrophobic monomer feed was incorporated into the polymer. CMP (1), CDP (2), and DDAB (4) were kindly supplied by A. Wagenaar. DTAB (3), 99%, was purchased from Merck and was used without further purification. Vesicle Preparation. Vesicles were prepared by sonicating approximately 3 mL of aqueous dispersions of CDP or DDAB (typically 10 mM) using a Branson Cell Disruptor B15 for 5 min (40% tip limit, 35% duty cycle). Electron Microscopy. A Jeol JEM EX1200 or Philips CM20 transmission electron microscope was used to characterize the vesicles, before and after the addition of polymer, by cryoelectron microscopy or by negative staining (1% uranyl acetate). In the case of cryoelectron microscopy, 5 µL of a vesicle solution (10 mM) was adsorbed onto bare copper grids (400 mesh) which were plunged into liquid ethane and then observed at about -170 °C using a Gatan cryotransfer specimen holder. The accelerating voltage was 80 kV (JEM) or 200 kV (CM20). In all cases, exposures were made using the minimum dose system of the microscope. The electron dose, required to obtain a good image, and thereby an acceptable specimen radiation damage, was kept to a minimum by developing the photographic plates (Agfa Scientia) in full-strength Kodak D19 for 12 min. Microcalorimetry. An Omega isothermal titration microcalorimeter (Microcal Inc., Northampton, MA) was used to measure the enthalpies of polymer-surfactant interaction. The operation principle is as follows. A certain volume of solution S1 is injected into the measuring cell containing solution S2. S1 interacts with S2, and the resulting enthalpic effect is measured by comparing the differences in energy that have to be added to the measuring cell and to the reference cell (containing pure water) in order to keep both cells at a constant temperature. In cases where the interaction enthalpy is small, corrections have to be made for the enthalpies of dilution of S1 and S2. In a typical experiment, a 250 µL stirred syringe was filled with surfactant solution. Each time, a few microliters were injected into the measuring cell containing the polymer solution (approximately 1% w/w). A plot was constructed of the enthalpy per injection versus the surfactant concentration, at approximately constant polymer concentration; the temperature was 30.0 ( 0.1 °C. In some cases, particularly when the anionic hydrophobically-modified poly(sodium acrylate)s were used, the observed interaction enthalpies had to be corrected for the enthalpies of dilution of the polymer by the added water, which were measured in separate experiments. Conductometry. The onset of micellization of the surfactant CMP was indicated by a clear break in the conductivity versus [CMP] plot. Conductivity was measured using a Wayne-Kerr Autobalance Universal Bridge B642 fitted with a Philips black platinum electrode PW 9512/01 (cell constant: 1.41 cm-1). The temperature was kept constant at 30.0 ( 0.1 °C using a Lauda R2 circulating water thermostat bath equipped with a magnetic stirring device. The solutions were thermostated for at least 15 min before starting the measurements. The surfactant concentration was varied by adding 10 µL aliquots of a concentrated solution to the contents of the cell; concentrations were corrected for volume changes.

Results and Discussion Vesicle Characterization. In order to certify that the sonication of aqueous dispersions of CDP and DDAB resulted in the formation of unilamellar vesicles, we studied these sonicated dispersions by cryoelectron microscopy.10,11 From Figure 1a it can be seen that unilamellar CDP vesicles with a narrow size distribution have been formed. The average diameter is 40 nm. Vesicles formed from DDAB are unilamellar as well (Figure 1b). These vesicles are rather small; the average diameter is 25 nm. The size distribution is monodisperse. (Cryo)electron Microscopic and Visual Inspection of Polymer-Surfactant Mixtures. In the concentration ranges employed no problems regarding vesicle stability (flocculation etc.) were encountered in the studies (10) Sein, A. Ph.D. Thesis, Groningen, 1995. (11) Dubochet, J.; Lepault, J.; Freeman, R.; Berriman, J. A.; Homo, J. C. J. Microsc. 1982, 128, 219.

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Figure 2. (a) Cryo-transmission electron micrograph of a 5 mM sonicated dispersion of CDP in the presence of 2.5 mM PSA-C12(4). Bar represents 100 nm. (b) Cryo-transmission electron micrograph of a 3.3 mM sonicated dispersion of CDP in the presence of 70 mM LMAM(3). Bar represents 200 nm.

Figure 1. (a) Cryo-transmission electron micrograph of a 5 mM sonicated dispersion of CDP. Bar represents 200 nm. (b) Cryo-transmission electron micrograph of a 7 mM sonicated dispersion of DDAB. Bar represents 50 nm.

of the interactions between negatively charged surfactants and the hydrophobically-modified poly(sodium acrylate)s or poly(acrylamide)s. Cryoelectron micrographs show that vesicles from CDP remain intact in the presence of these polymers (Figure 2). However, when the molecular weight of the poly(sodium acrylate)s is increased above ca. 8200, irrespective of the degree of hydrophobic modification, flocculation of the vesicles occurs. Therefore, this kind of colloidal instability is not due to a salt effect, since only polymers of high molecular weight are able to induce the flocculation phenomena. Moreover, bridging (the phenomenon that one polymer molecule is anchored into two vesicles, thereby pulling them together) cannot account for these observations, since even poly(sodium acrylate), having no hydrophobic moieties, can induce flocculation provided that the polymer molecular weight exceeds 8200. Finally, no traces of Mg2+ or Ca2+ could be detected in the polymer solutions (atomic absorption). Thus, it can be excluded that these fusogenic ions are responsible for the flocculation processes.12,13 Therefore, we attribute the phenomena described above to depletion flocculation.14 Depletion flocculation means that the vesicles are pushed together because they occupy the space that the polymer molecules need for sufficient configurational entropy. Cryoelectron micrographs show that the vesicles are indeed very close together in the flocculated dispersion. The cationic surfactants form precipitates with the poly(sodium acrylate)s already at very low concentrations, because the charge neutralization of both the polymer (12) Fonteijn, T. A. A. Ph.D. Thesis, Groningen, 1992. (13) Rupert, L. A. M.; Hoekstra, D.; Engberts, J. B. F. N. J. Colloid Interface Sci. 1989, 130, 271. (14) See: Napper, D. H. Polymeric Stabilization of Colloidal Dispersions; Academic Press: London, 1983.

Figure 3. Cryo-transmission electron micrograph of a 5 mM sonicated dispersion of CDP in the presence of 60 mM poly(sodium acrylate) of molecular weight 250 000. Bar represents 200 nm.

and the surfactant renders the molecules water-insoluble. This appears from microcalorimetric experiments (not presented here) showing that the initially large (ca. 20 kJ‚mol-1) endothermic heat effects vanish completely at a surfactant-to-polymer ratio where charge neutralization has occurred. Many examples of associative phase separation in polyelectrolyte-oppositely-charged surfactant mixtures have been reported.15-18 We did not identify the complexes formed between DDAB and hydrophobically-modified poly(sodium acrylate)s by cryoelectron microscopy. However, using negative staining techniques, we could show that a dense network of polymer and partially destroyed vesicles has been formed (Figure 4). (15) Bahadur, P.; Dubin, P. L.; Rao, Y. K. Langmuir 1995, 11, 1951. (16) Dubin, P. L.; Chew, C. H.; Gan, L. M. J. Colloid Interface Sci. 1989, 128, 566. (17) McQuigg, W.; Kaplan, J. I.; Dubin, P. L. J. Phys. Chem. 1992, 96, 1973. (18) Goddard, E. D. Colloids Surf. 1986, 19, 301.

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Figure 4. (a) Transmission electron micrograph of a 1.4 mM sonicated dispersion of DDAB (negative staining, potassium tungstate). The striations could not be observed in the cryoelectron micrographs nor when uranyl acetate was used as the staining agent. However, no visually attractive pictures could be obtained when using the latter staining agent; therefore, potassium tungstate was preferred. Bar represents 50 nm. (b) Transmission electron micrograph of a 35 mM sonicated dispersion of DDAB in the presence of 1.3 mM poly(sodium acrylate) of molecular weight 8000 (negative staining, UAc). Bar represents 100 nm.

The stability of DDAB vesicles in aqueous solutions of hydrophobically-modified poly(acrylamide)s also appears to be rather low. This is remarkable, since these polymers are uncharged. The origin of the instability of DDAB vesicles might be tentatively attributed to their small size, which makes them more sensitive to depletion flocculation by low molecular weight polymers than the (larger) vesicles formed from CDP, which do not flocculate in solutions containing hydrophobically-modified poly(acrylamide)s. Microcalorimetry. I. CMP and HydrophobicallyModified Poly(acrylamide)s. Figure 5a shows the micellization of CMP in water and in the presence of 160 mM hydrophobically-modified poly(acrylamide). Several microcalorimetric studies of micellization of surfactant molecules in aqueous solutions have been described in the literature.19 At surfactant concentrations below the cmc, micelles break up in the cell. At surfactant concentrations higher than the cmc, micelles are only diluted in a solution of micelles. Since this heat effect is more exothermic than the breaking up of micelles, micellization of CMP in pure water is weakly exothermic, by 0.7 kJ‚mol-1. For CMP in pure water, the transition occurs at 2.8 mM. This value corresponds well with the cmc of 3.0 mM determined by conductometry. In the presence of poly(acrylamide) the cmc hardly changes, but the enthalpy of micellization becomes more exothermic. A (19) Bach, J.; Blandamer, M. J.; Burgess, J.; Cullis, P. M.; Soldi, L. G.; Bijma, K.; Engberts, J. B. F. N.; Kooreman, P. A.; Kacperska, A.; Rao, K. C.; Subha, M. C. S. J. Chem. Soc., Faraday Trans. 1995, 91, 1229 and references therein.

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Figure 5. (a) Plot of the enthalpy of transfer of CMP from water into 160 mM hydrophobically-modified poly(acrylamide): [CMP]syringe ) 30 mM; (2) LMAM(0); (b) water; ([) LMAM(1.5); (1) LMAM(3); (9) LMAM(5). (b) Plot of the enthalpy of transfer of CMP from water into 40 and 160 mM hydrophobically-modified poly(acrylamide): [CMP]syringe ) 30 mM; (9) LMAM(1.5), 40 unit mM; (b) LMAM(3), 40 mM; (2) LMAM (1.5), 160 mM; (1) LMAM(3), 160 mM.

likely explanation is that the surfactant monomers have a higher enthalpy: during the breakup of micelles, surfactant monomers are released into the solution. They are hydrated at the cost of the hydration of the polymer, or vice versa,20 leading to an endothermic effect. Since the transition occurs at the same concentration as in water, and is still cooperative, there is no indication for direct polymer-micelle interactions. Clearly, the hydrophobically-modified poly(acrylamide)s do interact with the surfactant. In accordance with the model proposed by Hulde´n,6 a typical binding isotherm is observed. CMP micelles disintegrate into monomers, which adsorb onto the hydrophobic domains which exist due to the aggregation of hydrophobic side groups (compare Persson et al.8d). The monomer-polymer interaction enthalpy is exothermic, since existing aggregates are stabilized. However, the exothermicity decreases rapidly as the surfactant concentration increases. This is due to increasing electrostatic repulsions between the bound ionic surfactants. Finally, the CMP-water curve is approached at higher surfactant concentrations for LMAM(5) than for LMAM(3) and LMAM(1.5), since it is the hydrophobic side chains that are responsible for the interaction. Figure 5b graphically depicts the influence of polymer concentration on the polymer-surfactant interaction. The solid lines apply to LMAM(1.5). It is clear that saturation (20) Compare: Arnold, K.; Pratsch, L.; Gawrisch, K. Biochim. Biophys. Acta 1983, 728, 121.

Polymer-Surfactant Interactions

Figure 6. (a) Plot of the enthalpy of transfer of CMP from water into 160 mM hydrophobically-modified poly(sodium acrylate): [CMP]syringe ) 10 mM; (b) PSA-C9(4); (9) poly(sodium acrylate); (1) water; (2) PSA-C12(8); ([) PSA-C18(4). (b) Plot of the enthalpy of transfer of CMP from water into 160 mM hydrophobically-modified poly(sodium acrylate): [CMP]syringe ) 30 mM; (b) water; (9) PSA-C12(8).

occurs at a much lower surfactant concentration if the concentration of LMAM(1.5) in the cell is 40 mM instead of 160 mM. This follows from the observation that at the lower polymer concentration the enthalpy of interaction approaches the enthalpy of dilution into water (about 0.7 kJ/mol) already at a CMP concentration of 2 mM, whereas in the presence of 160 mM polymer the CMP-water curve is not yet approached at the end point, where the CMP concentration is 6 mM. In the same figure, the curves corresponding to CMPLMAM(3) interaction are represented by dotted lines. Clearly, saturation in 40 mM LMAM(3) occurs at exactly twice the concentration required for saturation in the presence of 40 mM LMAM(1.5). This is what one would expect, since it is the polymer hydrophobes that are responsible for the adsorption process. In all cases saturation occurs at a surfactant-to-polymer hydrophobe ratio of 2:1, in accordance with the picture provided by Biggs et al.4 II. CMP and Hydrophobically-Modified Poly(sodium acrylate)s. The polymer-surfactant interactions described above are not complicated by salt effects. However, such effects do occur for interactions between CMP and poly(sodium acrylate-co-n-alkyl methacrylate). Direct interactions between the poly(sodium acrylate) and CMP are excluded because of the unfavorable electrostatic repulsions. Still, as can be seen from Figure 6a, the critical micelle concentration of CMP in the presence of 160 mM poly(sodium acrylate) is one order of magnitude lower than

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the cmc in pure water. Similarly, it has been observed that, in the presence of poly(sodium acrylate), SDS molecules aggregate cooperatively at concentrations much lower than the cmc in pure water.21,22 A reasonable explanation implies that due to the presence of electrolytes the counterion binding to the micelles increases, so that the electrostatic repulsions between the headgroups are diminished.23 Micellization in the presence of nonyl-modified poly(sodium acrylate) is slightly more favorable than that in the presence of poly(sodium acrylate). The cmc decreases from 0.4 to 0.3 mM, and the enthalpy of micellization becomes more exothermic by 0.7 kJ/mol. The interactions closely resemble those for the binding of SDS to PEO. That is, the association of surfactant molecules in the presence of polymer is cooperative; the postmicellar region is characterized by an exothermic interaction enthalpy whereas the initial interactions are accompanied by endothermic heat effects.8a These observations strongly point to a conventional stabilization of CMP micelles by nonyl-modified poly(sodium acrylate). The picture becomes completely different when the polymer contains lauryl or stearyl side chains. In those cases, no transition was observed that might point to a cooperative micellization process. Instead, noncooperative adsorption of monomers to the polymeric microdomains occurs. This process is exothermic. The magnitude of the heat effect is not so strongly dependent on surfactant concentration as in the case of the interactions with hydrophobically-modified poly(acrylamide)s because the latter macromolecules are charged. On the other hand, interactions of CMP with PSA-C12(8) are less exothermic than those with LMAM(3), even though the latter polymer contains less hydrophobic side chains per mole. The reason is that there is no electrostatic barrier to the polymer-surfactant interaction. Interestingly, adsorption of surfactant monomers onto PSA-C18(4) is more exothermic than the interaction of CMP with PSA-C12(8) in the same concentration region, although the latter polymer contains more hydrophobes per mole. The explanation is twofold. On one hand, the molecular weight of PSA-C12(8) is, on average, 3000, whereas the molecular weight of PSA-C18(4) is about 8000. The tendency to form interpolymeric associations is intrinsically larger for the higher molecular weight polymer. On the other hand, the length of the side chain is shorter for the PSA-C12(8). According to Hulde´n6 this is a reason to assume that the formation of interpolymer microdomains is less favored for PA-C12(Z) than for PAC16(Z). In short, the surfactant molecules find more and/ or stronger interpolymer physical cross-links onto which they can adsorb in a solution of PSA-C18(4) than in a solution containing PSA-C12(8), which is expressed in a more exothermic heat of interaction. Finally, the picture presented above is corroborated by the most recent statistical-mechanical model on polysoaps and their interactions with surfactants.24 Interestingly, there is indeed a limit to the amount of surfactant that can be adsorbed: the heat effects become less exothermic if the surfactant concentration is increased beyond a critical value (Figure 6b). Figure 7 provides strong evidence that interaction between surfactant monomers and hydrophobically-modified polymers indeed occurs. In the foregoing experiments, including those reported in the literature,8 the concentration of added surfactant was always higher than the cmc, (21) Maltesh, C.; Somasundaran, P. Colloids Surf. 1992, 69, 167. (22) Binana-Limbele´, W.; Zana, R. Colloids Surf. 1986, 21, 483. (23) Lindman, B.; Wennerstro¨m, H. Top. Curr. Chem. 1980, 87, 1. (24) Borisov, O. V.; Halperin, A. Langmuir 1995, 11, 2911.

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Figure 7. Plot of the enthalpy of transfer of CMP from water into 160 mM hydrophobically-modified polymer: [CMP]syringe ) 2 mM; (9) water; ([) LMAM(0); (1) poly(sodium acrylate); (0) PSA-C12(8); (b) PSA-C18(4); (2) LMAM(3).

so that mostly micelles were injected into the polymer solution, and it was assumed that they disintegrated into monomers. In the present experiment, the syringe only contains surfactant monomers, since the surfactant concentration lies well below the cmc. It is most significant that now the same trends for the interactions between individual CMP molecules and hydrophobically-modified polymers are obtained: the interactions are most favorable in the case of the neutral hydrophobically-modified poly(acrylamide) LMAM(3), and interactions with PSA-C18(4) are accompanied by more exothermic heat effects than those with PSA-C12(8). The titrations of surfactant monomers into solutions of poly(acrylamide) or poly(sodium acrylate) are accompanied by only small enthalpic effects that find their origin in simple medium effects. For example, the charges of the CMP molecules, which are organic salts, are shielded in a poly(electrolyte) solution. Debye-Hu¨ckel considerations predict that the endothermic heat effects that occur upon dilution of CMP into water largely disappear when the organic salt is titrated into an electrolyte solution. This was borne out by experiment. III. CDP and Hydrophobically-Modified Poly(sodium acrylate)s. Figure 8a illustrates the interactions between vesicles of CDP and hydrophobically-modified poly(sodium acrylate)s. The dilution of CDP vesicles into water is endothermic, as expected on the basis of Debye-Hu¨ckel considerations. Also in this case, the endothermic heat effect completely disappears if the CDP vesicles are injected into a poly(sodium acrylate) solution. If one in 25 monomeric units contains a nonyl side chain, the enthalpic effects become more exothermic by 0.5 kJ‚mol-1. However, the shape of the curve, which is essentially a straight horizontal line, suggests that no direct binding takes place. On the contrary, a binding isotherm is obtained for the interaction of CDP vesicles with PSA-C18(4). Interestingly, the interaction enthalpy becomes twice as exothermic if the number of hydrophobic anchors per mole is increased by a factor of two (PSA-C12(8)). This is surprising, since the length of the hydrophobe is smaller for PSA-C12(8) than for PSA-C18(4). Indeed, the trends observed for the efficiency of hydrophobically-modified polymer-vesicle interactions appear to be the exact opposite of those for the interactions between hydrophobically-modified polymers and micelle-forming surfactants. The explanation may be as follows. Firstly, there are almost no free monomers present in solutions of vesicles; neither can they be easily liberated in a fast

Figure 8. (a) Plot of the enthalpy of transfer of CDP vesicles from water into 160 mM hydrophobically-modified poly(sodium acrylate): [CDP]syringe ) 10 mM; ([) water; (9) poly(sodium acrylate); (b) PSA-C9(4); (2) PSA-C18(4); (1) PSA-C12(8). (b) Plot of the enthalpy of transfer of CDP vesicles from water into 160 mM hydrophobically-modified poly(sodium acrylate), in the presence and in the absence of 0.1 M sodium chloride: [CDP]syringe ) 10 mM; (9) poly(sodium acrylate) + salt; (b) PSAC12(8) + salt; (2) poly(sodium acrylate); (1) PSA-C12(8).

dynamic equilibrium, as is the case for micelles. It seems impossible for vesicles to adhere to interpolymeric junctions in the way surfactant monomers can, simply because the vesicles are too large. From the point of view of the polymer molecules, micelles are small objects, but vesicles are large: the hydrodynamic radius of an SDS micelle is 2.5 nm,25a the radius of gyration of a random coil of a polymer of molecular weight 10 000 is ca. 5 nm,25b and the average CDP vesicle radius is 20 nm. Therefore it is more logical to describe polymer-vesicle interactions in terms of the anchoring of polymer hydrophobes into the vesicles bilayers, as has been proposed throughout the literature.26 This is the reason why the polymer containing the largest number of hydrophobic side chains interacts most favorably with the vesicles, as long as the hydrophobes are capable of anchoring. Moreover, we suggest that the polymer having the smallest tendency to form interpolymeric microdomains, which is PSA-C12(8), might bind most easily to the vesicle bilayers just because a breakup of hydrophobic microdomains is required before interaction with the vesicle can occur: anchoring results in less hydrophobes available for domain formation. (25) (a) van Os, N. M.; Haak, J. R.; Rupert, L. A. M., Eds. PhysicoChemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: Amsterdam, 1993. (b) Brandrup, J.; Immergut, E. H. Polymer Handbook; Wiley: New York, 1989. (26) van de Pas, J. C. Ph.D. Thesis, Groningen, 1993 and references therein.

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The saturation conditions correspond with a situation where one vesicle is bound to as many as 104 polymer hydrophobes of PSA-C18(4). This ratio can be obtained as follows: the mean vesicle radius R is 20 nm, so that the average outer surface area amounts to 5 × 10-15 m2 ()4πR2). The inner surface area is 4π(20 - 2 × 12 × 0.14)2 ) 3.5 × 10-15 m2. The total surface area is 8.5 × 10-15 m2. The hydrated ionic radius of a phosphate headgroup is ca. 4 Å, which makes the effective headgroup area 5 × 10-19 m2. Therefore, on average, one vesicle is composed of (8.5 × 10-15/5 × 10-19) ) 17 000 CDP molecules. At saturation, the number of polymer hydrophobes ()160/25 ) 6.4 mM) is similar to the number of CDP molecules (≈2 mM): both are in the millimolar range. However, the number of CDP vesicles is much lower than the number of CDP molecules, by a factor of ca. 17 000. Thus it is estimated that about (6.4/2) × 17 000 ) 54 000 polymer chains bind to a vesicle having a radius of only 20 nm. This cannot be true, and it is more logical to assume that a few polymers are directly anchored into the vesicle bilayer, while they are at the same time connected to the other polymers which are involved in hydrophobic microdomains. As is shown in Figure 8b, the efficiency of the interactions between hydrophobically-modified poly(sodium acrylate) and CDP vesicles is increased by the addition of salt: according to the criteria mentioned above, in the absence of salt, saturation occurs at a surfactant concentration of 1.75-2 mM, whereas in the presence of salt the interaction process is still going on in the same surfactant concentration region. The saturation concentration is increased because electrostatic repulsions between vesicles and polymers are diminished due to charge neutralization. IV. CDP and Hydrophobically-Modified Poly(acrylamide)s. Microcalorimetric data for the interactions between CDP vesicles and hydrophobically-modified poly(acrylamide)s are shown in Figure 9a. There is no direct interaction of the vesicles with poly(acrylamide). The interactions with LMAM(1.5) are weak. This is anticipated on the basis of the origin of the hydrophobic effect. The concept of hydrophobic interactions is not completely clear, but there is strong evidence that the cooperativity of these interactions is caused by the extensive hydrophobic hydration shells that form around hydrophobic moieties in water; when the concentration of hydrophobic molecules is increased, suddenly the number of water molecules is insufficient to independently hydrate the hydrophobic chains and the hydrophobic groups aggregate. Just because these hydrophobic hydration shells are very large, cooperative aggregation of, for instance, surfactant molecules can indeed occur at very low surfactant concentrations (i.e., at the cmc).27 Following the same line of reasoning, the CDP concentration can be relatively high before the anchoring processes start to occur. The polymers LMAM(3) and LMAM(5) do interact significantly with the vesicles. The heat effects are exothermic. Their magnitudes are comparable with the heat effects accompanying the interactions between CMP and hydrophobically-modified poly(acrylamide)s. The interaction enthalpies are much more exothermic than those observed for the hydrophobically-modified poly(sodium acrylate)s, which is consistent with the view that electrostatic forces oppose the binding of surfactants to polymers bearing the same charge. Finally, electrostatic forces influence the interactions of CDP vesicles with hydrophobically-modified polymers much more strongly than the interactions of individual CMP molecules with (27) Blokzijl, W.; Engberts, J. B. F. N. Angew. Chem., Int. Ed. Engl. 1993, 32, 5145.

Langmuir, Vol. 12, No. 20, 1996 4715

Figure 9. (a) Plot of the enthalpy of transfer of CDP vesicles from water into 160 mM hydrophobically-modified poly(acrylamide): [CDP]syringe ) 10 mM; ([) water; (9) LMAM(0); (b) LMAM(1.5); (2) LMAM(3); (1) LMAM(5). (b) Plot of the enthalpy of transfer of CDP vesicles from water into 40 and 160 mM hydrophobically-modified poly(acrylamide): [CDP]syringe ) 10 mM; (9) LMAM(1.5), 40 unit mM; (2) LMAM(1.5), 160 unit mM; (b) LMAM(3), 40 mM; (1) LMAM(3), 160 mM.

these polymers. In the case of CDP, the polymers have to approach the vesicles, which are essentially macroions. From an electrostatic point of view, this is much more difficult to achieve than the adsorption of one surfactant monomer to a (small) polymeric microdomain. The influence of the polymer concentration on polymervesicle interactions is graphically depicted in Figure 9b. Firstly, comparison of the plots for 40 mM LMAM(1.5) and 160 mM LMAM(1.5) indicates that the CDP vesicles do interact with the LMAM(1.5) polymer at the higher concentration, but only weakly. Secondly, saturation behavior can be observed for the binding of CDP vesicles to LMAM(3), when the concentration of the polymer in the cell is 40 mM. The enthalpy of interaction approaches zero at a CDP concentration of 1 mM. This means that an equilibrium is reached where more than 104 hydrophobes appear to bind to one vesicle. As has been discussed earlier, it is highly unlikely that all these hydrophobes are directly anchored into the bilayer. V. DTAB and Hydrophobically-Modified Poly(acrylamide)s. We finally consider the interactions between cationic surfactants and hydrophobically-modified poly(acrylamides). It is known from the literature1 that cationic surfactants interact much more weakly with PEO, PPO, or PVP than with anionic surfactants. The reason is still obscure. A popular explanation involves the notion that the hydration shell overlap between cationic surfactants and polymers is not favorable. Another explanation involves the notion that the headgroups of cationic

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Kevelam et al.

modified poly(acrylamide)s. As anticipated, the enthalpy of binding is most exothermic for LMAM(5) and diminishes as the degree of hydrophobic modification decreases. Interestingly, the magnitudes of the heat effects observed are only -2 kJ‚mol-1 at most, whereas, in the case of CMP, heat effects of -7 kJ‚mol-1 have been observed. Furthermore, in the case of DTAB, a sigmoidal curve is observed which is typical for micellization. However, the line shapes indicate that the cooperativity of the aggregation process decreases with increasing hydrophobic modification of the polymer. This might hint at the adsorption of DTAB monomers onto hydrophobic microdomains, and, as is shown below, this process does indeed occur. Presumably, there are multiple equilibria occurring simultaneously. Finally, the interaction enthalpy is endothermic in the postmicellar region. All these experimental facts indicate that the interactions between cationic micelle-forming surfactants and hydrophobically-modified poly(acrylamide)s are much less favorable than those between hydrophobically-modified poly(acrylamide)s and anionic micelle-forming surfactants. Moreover, the interactions between hydrophobically-modified poly(acrylamide)s and cationic surfactants resemble the classical SDS-PEO interactions where micelles are stabilized by the polymer through shielding their interfaces from bulk water. Therefore, it is not surprising that no saturation behavior is observed (Figure 10b). Micellization of DTAB in a 40 mM solution of hydrophobically-modified poly(acrylamide) clearly resembles micellization in water more closely than micellization in 160 mM hydrophobicallymodified poly(acrylamide). From Figure 10c it can be seen that DTAB monomers indeed adsorb onto hydrophobicallymodified poly(acrylamide) but that the heat effects are small. We note that the syringe contained only DTAB monomers and no micelles. Conclusions

Figure 10. (a) Plot of the enthalpy of transfer of DTAB from water into 160 mM hydrophobically-modified poly(acrylamide): [DTAB]syringe ) 153 mM; (9) LMAM(0); (b) water; (1) LMAM (1.5); (2) LMAM(3); ([) LMAM(5). (b) Plot of the enthalpy of transfer of DTAB from water into 40 and 160 mM hydrophobically-modified poly(acrylamide): [DTAB]syringe ) 153 mM; (9) water; (b) LMAM(3), 40 unit mM; (2) LMAM(3), 160 mM. (c) Plot of the enthalpy of transfer of DTAB monomers from water into 160 mM hydrophobically-modified poly(acrylamide): [DTAB]syringe ) 10 mM; (b) water; (9) LMAM(6).

surfactants are usually quite bulky, so that the hydrophobic core of the micelle is already well-shielded from water and the driving force (hydrophobic interactions) for the conventional polymer-micelle interaction is reduced. As can be seen from Figure 10a, micellization of DTAB in water is rather similar to micellization in a 160 mM solution of poly(acrylamide). Thus, there is no evidence for direct interactions of DTAB micelles or monomers with LMAM(0). On the contrary, DTAB monomers do adsorb onto microdomains formed in solutions of hydrophobically-

On the basis of our work, the following conclusions regarding the interactions of surfactants with hydrophobically-modified polymers can be drawn: (1) The interactions between anionic micelle-forming surfactants and hydrophobically-modified water-soluble polymers are fundamentally different from those between these surfactants and uncharged polymers like PEO, PPO, and PVP. In the latter case, the driving force is the stabilization of the whole micellar aggregate. In the case of hydrophobically-modified polymers, surfactant monomers preferentially adsorb onto interpolymeric microdomains, thereby strengthening the interpolymeric interactions. (2) Anionic surfactants interact with anionic hydrophobically-modified polymers, despite the unfavorable electrostatic forces. Interactions with neutral hydrophobically-modified polymers are, however, accompanied by more exothermic heat effects. (3) The interactions between hydrophobically-modified polymers and vesicles are essentially different from those between these polymers and micelles. In the latter case polymer-surfactant interactions are promoted by the existence of interpolymeric associative complexes in solution; one might propose that the polymer with the smallest tendency to form aggregates with other polymers binds most efficiently to the vesicles because no concomitant breakup of microdomains is required. This implies that also the length of the hydrophobic chain might be of importance for the interactions with vesicles, but in a different way than for micelle-forming surfactants: the chain length should be long enough to anchor into the bilayer28 but not so long that the formation of interpoly-

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meric microdomains competes significantly with binding to the vesicles.

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Acknowledgment. We wish to thank Mr. A. Wagenaar who kindly provided the surfactants CMP, CDP, and DDAB. Mr. M. Collavo (University of Padova) synthesized the hydrophobically-modified poly(acrylamide)s when he visited the University of Groningen as

an Erasmus student. We wish to thank Prof. A. D. R. Brisson (Department of Biophysical Chemistry, Interfacultary Institute for Electron Microscopy) for helpful comments on the electron micrographs. Ing. J. Hommes is gratefully acknowledged for performing the Mg2+ and Ca2+ trace analyses. Finally, J.K. and J.B.F.N.E. express their gratitude toward Unilever Research for financial support.

(28) Compare: Iliopoulos, I.; Olsson, U. J. Phys. Chem. 1994, 98, 1500.

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