Simple Gel Swelling Experiments Distinguish between Associating

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Simple Gel Swelling Experiments Distinguish between Associating and Nonassociating Polymer-Surfactant Pairs Jesper Sjo¨stro¨m and Lennart Piculell* Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden Received November 29, 2000. In Final Form: February 28, 2001 Simple experiments, based on volume changes of cross-linked polymer gels immersed in surfactant solutions, can be used to distinguish between associating and nonassociating nonionic polymer/ionic surfactant pairs. For an associating pair, the cooperative binding of the surfactant at the critical association concentration gives rise to a significant increase in the gel volume. Here, we study gels based on hydroxyethyl cellulose (HEC). Alkyl sulfate surfactants are shown to bind to HEC, but only for tail lengths longer than 8 carbons. No binding is found for cationic surfactants. All tested surfactants bind to hydrophobically modified HEC, but the extent of gel swelling (and, presumably, surfactant binding) is much larger for those surfactants that also bind to nonmodified HEC. We also study how the swelling equilibria are affected by added salt and demonstrate that valuable information may be gained from transient volume changes as the gels approach swelling equilibrium.

Introduction It is a well-known experimental fact that many ionic surfactants associate cooperatively to “slightly hydrophobic” nonionic water-soluble polymers.1-3 The association is best viewed as a micelle formation at the polymer, that occurs at a critical association concentration (cac) that is lower than the critical micelle concentration (cmc) of the pure surfactant. The relative lowering of the cmc (i.e., the difference (cmc - cac)/cmc)) is a measure of the additional stabilization of the micelle that results from its binding to the polymer. Many classical methods, such as measurements of surface tension, electrical conductivity, viscosity, solubilization of oil-soluble additives, or NMR, have been used to detect the cac’s. Most of these methods rely on the observation of a significant lowering of the cmc in the presence of the polymer. However, in instances where the difference between the cmc and the cac is small, there may remain an uncertainty as to whether a certain polymer-surfactant pair displays association or not. A classical example, where conflicting conclusions appear in the literature, concerns the pair hydroxyethyl cellulose (HEC) and the anionic surfactant sodium dodecyl sulfate (SDS). Here, some studies indicate an interaction,4-7 whereas others do not.8-15 In a recent study, we * To whom correspondence should be addressed. (1) Kwak, J. C. T. Polymer-Surfactant Systems; Marcel Dekker: New York, 1998. (2) Lindman, B.; Thalberg, K. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; Chapter 5. (3) Goddard, E. D. Colloids Surf. 1986, 19, 255. (4) Goddard, E. D.; Hannan, R. B. In Micellization, Solubilization and Microemulsions; Mittal, K. L., Ed.; Plenum Press: New York, 1977; Vol. 2, p 835. (5) Sivadasan, K.; Somasundaran, P. Colloids Surf. 1990, 49, 229. (6) Tanaka, R.; Meadows, J.; Phillips, G. O.; Williams, P. A. Carbohydr. Polym. 1990, 12, 443. (7) Piculell, L.; Nilsson, S.; Sjo¨stro¨m, J.; Thuresson, K. In Associative Polymers in Aqueous Media; Glass, J. E., Ed.; ASC Symposium Series, Vol. 765; American Chemical Society: Washington, DC, 2000; Chapter 19. (8) Goddard, E. D.; Phillips, T. S.; Hannan, R. B. J. Soc. Cosmet. Chem. 1975, 26, 461. (9) Ohbu, K.; Hiraishi, O.; Kashiwa, I. J. Am. Oil Chem. Soc. 1982, 59, 108.

have established the binding between SDS and HEC using a number of methods.7 There were significant effects in viscosity, but the effects were nevertheless small and could easily have been overlooked. NMR measurements of surfactant self-diffusion, on the other hand, showed substantial binding. However, the binding could also be clearly evidenced by a rather more simple experiment, where the polymer was cross-linked chemically and the swelling of the resulting gel, immersed in surfactant solutions at increasing surfactant concentration, was measured. The qualitative features of the resulting surfactant swelling isotherms have been established for different classes of associating polymer-surfactant pairs in previous studies in our laboratory. We have studied the effect of ionic surfactants (mostly SDS but also other anionic and cationic surfactants) on gels based on N-isopropylacrylamide (NIPA)16 and acrylamide with pendant chains of poly(ethylene oxide) (PEO)17,18 and on several gels based on HEC: HMHEC,7 EHEC,16,19,20 HMEHEC,19 cat-HEC,21 and cat-HMHEC.20-22 Here, HM ) hydrophobically modified, E ) ethyl modified, and cat ) cationically modified. Other laboratories have also studied slightly hydrophobic nonionic polymer gels immersed in solutions with ionic surfactants, but these studies have mainly focused on thermal effects and, specifically, the effect of surfactants on the thermal collapse of gels based on NIPA and its (10) Dualeh, J. A.; Steiner, C. A. Macromolecules 1990, 23, 251. (11) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304. (12) Goddard, E. D. J. Colloid Interface Sci. 1992, 152, 578. (13) Ka¨stner, U.; Hoffmann, H.; Do¨nges, R.; Ehrler, R. Colloids Surf., A 1996, 112, 209. (14) Hoffmann, H.; Ka¨stner, U.; Do¨nges, R.; Ehrler, R. Polym. Gels Networks 1996, 4, 509. (15) Panmai, S.; Prudhomme, R. K.; Peiffer, D. G. Colloids Surf., A 1999, 147, 3. (16) Roseń, O.; Piculell, L. Polym. Gels Networks 1997, 5, 185. (17) Piculell, L.; Hourdet, D.; Iliopoulos, I. Langmuir 1993, 9, 3324. (18) Roseń, O.; Piculell, L.; Hourdet, D. Langmuir 1998, 14, 777. (19) Piculell, L.; Thuresson, K.; Ericsson, O. Faraday Discuss. 1995, 101, 307. (20) Roseń, O.; Sjo¨stro¨m, J.; Piculell, L. Langmuir 1998, 14, 5795. (21) Sjo¨stro¨m, J.; Piculell, L. Colloids Surf., A 2001, 183-185, 429. (22) Sjo¨stro¨m, J.; Piculell, L. Langmuir 2000, 16, 4770.

10.1021/la0016726 CCC: $20.00 © 2001 American Chemical Society Published on Web 05/31/2001

Simple Gel Swelling Experiments

Figure 1. Schematic equilibrium swelling isotherm for a nonionic polymer gel immersed in a quasi-infinite solution of an ionic surfactant that binds to the polymer. The cartoons illustrate the molecular situation (surfactant micellization, counterion concentration) in the gel and in the external solution at different regions of surfactant concentration; see text.

analogues.23-35 Studies have also been performed on other polymers with a reverse solubility, such as poly(acryloylL-pronlin alkyl esters)36-38 or poly(vinylcaprolactam).39,40 Only a few of these studies also include equilibrium swelling isotherms.31,33,35,39 Figure 1 illustrates, schematically, the generic features of the equilibrium swelling isotherm for a gel, based on a slightly hydrophobic nonionic polymer, that is immersed in a large volume of an otherwise salt-free solution containing an ionic surfactant that binds to the polymer. At low surfactant concentrations, where no micelles are formed, the surfactant distributes evenly between the gel and the external solution, and the volume remains unaffected. At the cac, however, a sharp increase in swelling with increasing surfactant concentration commences as polymer-bound micelles are formed inside the gel. This increase is due mainly to the osmotic pressure (23) Zhang, Y.-Q.; Tanaka, T.; Shibayama, M. Nature 1992, 360, 142. (24) Inomata, H.; Goto, S.; Saito, S. Langmuir 1992, 8, 1030. (25) Wada, N.; Kajima, Y.; Yagi, Y.; Inomata, H.; Saito, S. Langmuir 1993, 9, 46. (26) Kokufuta, E.; Zhang, Y.-Q.; Tanaka, T.; Mamada, A. Macromolecules 1993, 26, 1053. (27) Tam, K. C.; Ragaram, S.; Pelton, R. H. Langmuir 1994, 10, 418. (28) Kokufuta, E.; Nakaizumi, S.; Ito, S.; Tanaka, T. Macromolecules 1995, 28, 1704. (29) Sakai, M.; Satoh, N.; Tsujii, K.; Zhang, Y.-Q.; Tanaka, T. Langmuir 1995, 11, 2493. (30) Wu, C.; Zhou, S. J. Polym. Sci., Part B 1996, 34, 1597. (31) Kokufuta, E.; Suzuki, H.; Sakamoto, D. Langmuir 1997, 13, 2627. (32) Suzuki, H.; Kokufuta, E. Colloids Surf., A 1999, 147, 233. (33) Murase, Y.; Onda, T.; Tsujii, K.; Tanaka, T. Macromolecules 1999, 32, 8589. (34) Murase, Y.; Tsujii, K.; Tanaka, T. Langmuir 2000, 16, 6385. (35) Sayil, C.; Okay, O. Polym. Bull. 2000, 45, 175. (36) Safranj, A.; Yoshida, M.; Omichi, H.; Katakai, R. Langmuir 1994, 10, 2954. (37) Yoshida, M.; Safranj, A.; Omichi, H.; Miyajima, M.; Katakai, R. Radiat. Phys. Chem. 1995, 46, 181. (38) Yoshida, M.; Asano, M.; Omichi, H.; Kamimura, W.; Kumakura, M.; Katakai, R. Macromolecules 1997, 30, 2795. (39) Makhaeva, E. E.; Thanh, L. T. M.; Starodoubtsev, S. G.; Khokhlov, A. R. Macromol. Chem. Phys. 1996, 197, 1973. (40) Gao, Y.; Au-Yeung, S. C. F.; Wu, C. Macromolecules 1999, 32, 3674.

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contribution from the small counterions which, for electroneutrality reasons, follow the bound surfactant micelles into the gel interior. After a swelling maximum, occurring at a concentration slightly above the cmc, the gel starts to deswell. The reason for the deswelling is that surfactant added above the cmc mainly ends up in “free” micelles (not bound to the polymer). These free ionic micelles distribute (as would an added simple salt) between the gel interior and the outside solution. The net effect is a reduced osmotic pressure difference between the two compartments and a shrinking of the gel. In the present context, we are interested in the swelling isotherm primarily as an instrument by which we may detect surfactant binding and, in relevant cases, extract cac values. We note that for the systems investigated to date (NIPA/SDS,16,33 PEO/SDS,17 HEC/SDS,7 and EHEC/ SDS16) the cac values obtained from the gel swelling isotherm have been in excellent agreement with cac values obtained, by other methods, for the corresponding linear polymer in solution. Moreover, this agreement has been insensitive to the amount of cross-linker used in the gels, within the quite wide range that has so far been explored experimentally.7,16 The question as to whether there is binding of surfactants to the polymer backbone is interesting also when the polymer is “hydrophobically modified” by the grafting of a small proportion of strongly hydrophobic units, “hydrophobes”, to the backbone. Mixed micellization between the hydrophobes and surfactants is well-known, but many studies overlook the fact that the association may also involve the polymer backbone. This binding may have large consequences both for the total amount of surfactant bound and for the mixed micellar cross-linking between polymer chains. Indeed, some of our previous studies have indicated that the total amount of bound surfactant is rather similar for modified and nonmodified polymers in cases where binding occurs also to the backbone.7,19 In the present study, we demonstrate that simple gel swelling experiments are quite potent to distinguish between associating and nonassociating polymer-surfactant pairs. We use HEC as an example and find that certain surfactants bind, whereas others do not. All tested surfactants bind to HMHEC, but the gel experiments clearly indicate that the extent of binding to HMHEC is much larger for those surfactants that also bind to the nonmodified backbone. The latter conclusion also seems to hold for cat-HMHEC. We also discuss effects of added salt on the swelling equilibria and how studies of the volume changes with time can be used to improve the sensitivity of the gel swelling method. Experimental Section Materials. HEC and HMHEC with the commercial names Natrosol 250 GR and Natrosol Plus grade 330 CS, respectively, were obtained from Aqualon. According to the manufacturer, both samples have a molecular mass of ca. 250 000 g/mol. HMHEC contains grafted C16-alkyls at an amount corresponding to 0.54 mM alkyl chains in a 10 g/L aqueous solution of the polymer.41 Cationic hydrophobically modified HEC (cat-HMHEC, the chloride salt of an N,N-dimethyl-N-dodecyl derivate of hydroxyethyl cellulose) with the commercial name Quatrisoft LM200 was obtained from Amerchol Inc. The substitution degree of cationic groups corresponds to 2.9 mM cationic groups in a 10 g/L aqueous solution of cat-HMHEC.21 See Figure 2 for a schematic figure of the polymers. Divinyl sulfone (DVS, from Sigma) and NaOH (from Eka Nobel) were used in the cross-linking of the polymers. (41) Nilsson, S.; Thuresson, K.; Hansson, P.; Lindman, B. J. Phys. Chem. B 1998, 102, 7099.

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Sjo¨ stro¨ m and Piculell

Table 1. Surfactants Used and cac Values Obtained in This Study systematic name sodium hexadecyl sulfate sodium tetradecyl sulfate sodium dodecyl sulfate

sodium decyl sulfate sodium octyl sulfate sodium dodecyl-di(ethylene oxide)-sulfate sodium 4-n-octyl benzene sulfonate sodium deoxycholate hexadecyl trimethylammonium bromide tetradecyl trimethylammonium bromide dodecyl trimethylammonium bromide dodecylammonium chloride

abbreviation SHS STS SDS

SDeS SOS SD-(EO)2-S SOBS NaDC C16TAB C14TAB C12TAB C12AC

added NaCl (mM) 0 0 0 10 30 60 100 200 500 1000 0 0 0 0 0 0 0 0 0

expt temp

cac with HEC (mM)

cmc (mM)a

45 °C 25 °C room temp room temp 25 °C 25 °C 25 °C

0.3 1.3 6.0 4.5 2 1 0.7

0.5 2.05 8.1 5.3 3.1

35 °C 35 °C room temp room temp room temp 25 °C room temp 30 °C room temp room temp room temp

0.2 0.1 30 10

manufacturer Merck BDH BDH

1.5 0.9 33 133 3b ≈11 2-5c 0.9 3.5 16 15

Merck Merck Kao Chemicals GmbH TCI Merck Merck Fluka Chemika TCI-GR Lancaster

a Values from ref 67 unless otherwise specified. b Contains according to the product description some impurities of sodium tetradecyldi(ethylene oxide)-sulfate. From the molecular weight of 385 g/mol, stated by the supplier, we estimate the ST-(EO)2-S content to be approximately 25%. The cmc is therefore expected to be about 2.5 mM. c From ref 53.

Figure 2. Schematic picture of HEC and its hydrophobic derivatives. MSEO ) hydroxyethyl groups per repeating anhydroglucose unit. DS ) hydrophobic groups per repeating anhydroglucose unit.

procedure for polymer purification is a notable advantage of the gel swelling method compared to experiments using linear polymers. Gel Swelling Experiments. The washed gel rods were immersed in flat-bottom vials each containing one gel rod and 5 mL of aqueous surfactant solution. Owing to the much larger volume of the swelling medium compared to the volume of the gel, the fraction of surfactant bound to the gels was negligible. The swelling is given as V/m, where V is the gel volume and m the mass of linear polyelectrolyte that the gel piece contained at synthesis. V/m was calculated as (d/d0)3/C0, where d0 is the inner diameter of the glass tubes and d is the diameter of the gel immersed in the solution. Both these diameters were measured in the vials by a video camera calibrated with a 0.1 mm scale with the help of an image computer program.44 Reproducibility measurements on a large number of gel rods at different positions in the vials gave a maximum variation in d of ca. 5%, corresponding to a 15% uncertainty in the gel volume. Prior to measurements of the equilibrium swelling degrees, the gel rods were allowed to equilibrate for at least 3 days. The equilibration occurred at the temperatures specified in Table 1. Owing to an elevated Krafft point, some experiments were performed at temperatures higher than room temperature. In a kinetic experiment, a gel cylinder was initially equilibrated in pure water. This initial “preparation solution” was then removed using a Pasteur pipet. When 5 mL of a different “equilibration solution” was added to the vial, a stopwatch was started. Images were then captured automatically at constant time intervals using the movie function in the computer program.44 These experiments were performed at 25 °C and without stirring.

NaCl was obtained from Riedel-de Hae¨n. The surfactants used are presented in Table 1. All chemicals were used without purification. Millipore filtered water was used in all experiments. Gel Preparation. Chemical gels were made by cross-linking the HEC derivatives with DVS in alkaline solutions.7,16,20,21,42 The concentration of polysaccharide at synthesis (C0) was 20 g/L. The amount of DVS was 0.3 mL/g polymer for HEC and 0.1 mL/g polymer for HMHEC. NaOH (20 mM) was added to the polymer solution. The cat-HMHEC gels were the same as batch 1 in ref 22. The amount of DVS was 0.1 mL/g cat-HMHEC. NaOH (6 mM) and 50 mM C12TAB were added to the cat-HMHEC reaction solution; the surfactant was included to solubilize this salt sensitive43 polyelectrolyte during the synthesis, and the low concentration of NaOH was selected to maintain as low an ionic strength as possible. DVS was added during stirring, and the cross-linking reaction was allowed to proceed for about 24 h in a water bath at 50 °C in glass capillaries with an inner diameter of 1.4 mm. The gels were cut into approximately 1.4 mm long rods. Residual chemicals (i.e., added C12TAB and salt impurities known to be present in the original polymer sample) were leached out by immersing the gel rods in a large excess of deionized water followed by Millipore water for at least a week. This simple

HEC and Alkyl Sulfates. Figure 3 shows equilibrium swelling isotherms for nonionic HEC gels immersed in solutions with increasing concentrations of anionic sodium alkyl sulfates. A similar isotherm for SDS has been reported elsewhere.7 For all the surfactants except sodium octyl sulfate (SOS), the swelling isotherms have the qualitative appearance shown in Figure 1. A sharp increase in swelling sets in at a quite well-defined surfactant concentration, taken as the cac. The cac values thus extracted from the various isotherms in Figure 3 are given in Table 1.

(42) Balazs, E. A.; Leshchiner, A. U.S. Patent No. 4,582,865, 1986. (43) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201.

(44) NIH Image VDM, version 1.58 VDM; National Institutes of Health: Bethesda, MD.

Results and Discussion


Figure 3. Equilibrium swelling isotherms for HEC gels immersed in solutions of sodium alkyl sulfates: SHS (filled circles), STS (open circles), SDS (filled squares), SDeS (open squares), and SOS (triangles). Arrows denote the cmc’s. The lines are guides to the eye only.

The data in Figure 3 illustrate the well-established fact that the cac, just like the cmc, decreases with increasing surfactant chain length.45-47 Moreover, as we have shown previously for EHEC gels swollen in solutions of alkyl trimethylammonium bromides,20 a longer alkyl length of the surfactant gives not only an earlier swelling but also a larger maximum swelling. The main reason for the latter effect is probably to be sought in the concentration of free (monomeric) surfactant at the swelling maximum. Free surfactant acts as an external salt, decreasing the degree of swelling of an ionic gel (cf. also below). At the maximum, the free surfactant concentration is very nearly equal to the cmc; therefore, a higher cmc implies a lower maximum degree of swelling. Another possible reason could be that the maximum degree of binding is higher for a surfactant with a longer alkyl chain. However, the fact that all the deswelling curves seem to follow a common line after the maxima indicates that the amount of surfactant molecules bound to the HEC gels is quite similar for all the binding surfactants of the homologous series. In contrast to the higher homologues, SOS induces no swelling of HEC gels, indicating no association. A possible alternative explanation would be that the extent of maximum swelling in this case has decreased beyond detection, owing to the high cmc of SOS. We regard the latter explanation as unlikely, however, because all experiments we have performed with associating polymersurfactant pairs consistently show a clear gel volume response at the cac, even when large amounts of added salt are present (cf. below). Rather, we conclude that there is no binding of SOS to HEC. This conclusion is supported by Figure 4, which shows log(cac) and log(cmc) plotted versus the number of carbon atoms in the surfactant tail. The two straight lines cross at about 8 carbon atoms, which is the number of carbon atoms in SOS. The molecular interpretation of this crossover is that as the hydrophobic tail length becomes shorter, the free energy gain in forming polymer-bound micelles (as compared to free micelles) becomes smaller and eventually disappears. That the (45) Arai, H.; Murata, M.; Shinoda, K. J. Colloid Interface Sci. 1971, 37, 223. (46) Shirahama, K.; Ide, N. J. Colloid Interface Sci. 1976, 54, 450. (47) Shirahama, K. In Polymer-Surfactant Systems; Kwak, J. C. T., Ed.; Marcel Dekker: New York, 1998; Chapter 4.

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Figure 4. The cac (to HEC, filled symbols) and cmc (open symbols) values for sodium alkyl sulfates versus the number of carbon atoms in the surfactant tail. The cac and cmc values are those listed in Table 1. The lines are exponential fits to the data.

Figure 5. Equilibrium swelling isotherms for HEC gels immersed in solutions of anionic surfactants: SOBS (circles), SD-(EO)2-S (squares), and NaDC (crosses). Arrows denote the cmc’s. The cmc of NaDC is 2-5 mM (ref 53). The line is a guide to the eye only.

interaction disappears at some point, when the hydrophobic tail is made shorter, has also been shown by equilibrium dialysis for the binding of sodium alkyl sulfates to PEO.46,47 HEC and Other Anionic Surfactants. Figure 5 shows equilibrium swelling isotherms for HEC gels and a selection of other anionic surfactants. Sodium dodecyldi(ethylene oxide)-sulfate (SD-(EO)2-S) is similar to SDS but contains an additional short hydrophilic “spacer” inserted between the alkyl chain and the sulfate moiety. By contrast, sodium 4-n-octyl benzene sulfonate (SOBS) contains a hydrophobic benzyl group in the corresponding position. The anionic sodium deoxycholate (NaDC), finally, is a biological surfactant (a bile salt) with a totally different chemical structure. It is included here for reasons of comparison and, also, for its potential interest in pharmaceutical applications. Of these alternative anionic surfactants, only SOBS binds, and the qualitative appearance of the swelling

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Figure 6. Equilibrium swelling isotherms for HEC gels immersed in solutions of cationic surfactants: C16TAB (circles), C14TAB (squares), C12TAB (triangles), and C12AC (crosses). Arrows denote the cmc’s. The line is a guide to the eye only.

isotherm is the same for SOBS as for the sodium alkyl sulfates. Interestingly, there is no swelling of the HEC gels immersed in solutions of SD-(EO)2-S. From this result, it can be concluded that there is no association between HEC and SD-(EO)2-S. Already 40 years ago, Saito showed, by viscosity measurements, that poly(vinylpyrrolidone) (PVP) interacts with SDS but not with sodium alkyl ethersulfates such as SD-(EO)4-S.48 Evidently, the same pattern is followed by HEC. As a tentative explanation, we suggest that the potential polymer adsorption “sites” on the micellar surface (i.e., regions of exposed hydrocarbon) are already occupied by the EO spacer groups. HEC and Cationic Surfactants. Equilibrium swelling isotherms for HEC gels in solutions of various cationic surfactants are shown in Figure 6. It is clear that none of the cationic surfactants associate with HEC at room temperature. This is especially noteworthy for C16TAB. In view of the trends shown in Figure 4, one might have expected an association for cationics with sufficiently long alkyl chains. By contrast, we previously found, using similar gel swelling experiments, that EHEC gels bound all tested alkyl trimethylammonium halides (C16TAB, C14TAB, and C12TAB).20 The unavoidable conclusion, that also was drawn by Wang and Olofsson,49,50 is that it is the ethyl substituents that cause the association between EHEC and alkyl trimethylammonium halides. In the literature, it has been speculated that the difference between sodium alkyl sulfates and alkyl trimethylammonium bromides, as regards their interactions with nonionic polymers, is due to differences in the size of the headgroup.2,49,51 This hypothesis is not so probable, or is at least not the whole explanation, in view of our result (Figure 6) that there is no association between HEC and C12AC, a cationic surfactant with a relatively small headgroup. Effect of Hydrophobic Modification: HMHEC. Figure 7 compares the swelling isotherms for HMHEC gels immersed in solutions of SDS or SD-(EO)2-S. The SDS isotherm has been reported elsewhere.7 In contrast (48) Saito, S. J. Colloid Sci. 1960, 15, 283. (49) Wang, G.; Olofsson, G. J. Phys. Chem. 1995, 99, 5588. (50) Wang, G.; Olofsson, G. J. Phys. Chem. B 1998, 102, 9276. (51) Ruckenstein, E.; Huber, G.; Hoffmann, H. Langmuir 1987, 3, 382.


Figure 7. Equilibrium swelling isotherms for HMHEC gels immersed in solutions of anionic surfactants: SDS (circles) and SD-(EO)2-S (squares). Arrows denote the cmc’s. The lines are guides to the eye only.

to unmodified HEC (see Figure 5), HMHEC binds SD(EO)2-S. This binding must therefore be entirely due to the hydrophobes of HMHEC. A new feature apparent in both swelling isotherms of Figure 7, in contrast to those in Figure 3, is a marked deswelling of the gels at low surfactant concentrations, before the cooperative binding/swelling sets in as the surfactant concentration approaches the cmc. For SD(EO)2-S, the magnitude of the deswelling (compared to the reference surfactant-free gel) is almost as large as that of the maximum swelling. This surfactant response at low concentrations has previously been explained as a consequence of the nature of surfactant binding to hydrophobically modified polymers.7 As has been noted repeatedly, surfactants join the hydrophobes of hydrophobically modified polymers in mixed micelles already at concentrations far below the surfactant cmc.7,52 Thus, no critical concentration of surfactant (no cac) is required for the onset of binding. The gel swelling isotherms in Figure 7 suggest that, at low amounts of bound surfactants, the mixed micelles serve as additional physical cross-links, which result in a shrinking of the gel. The normal ionic swelling mechanism sets in only at higher degrees of surfactant binding. Interestingly, and in contrast to the trend in Figure 3, the maximum swelling of HMHEC gels near the surfactant cmc is much larger in SDS solutions than in solutions of SD-(EO)2-S, despite the lower cmc of the latter surfactant. We conclude that this difference is due to the fact that SDS binds both to the backbone polymer (see Figure 3) and to the HM groups. Figure 8 shows that the effects of alkyl trimethylammonium bromides on the swelling of HMHEC are similar to that of SD-(EO)2-S; an initial gradual deswelling is followed by a rather modest but cooperative binding/swelling as the surfactant concentration approaches the cmc. These results thus confirm the trend that the presence/absence of an association to the main chain is important for the quantitative binding of surfactants to a hydrophobically modified polymer. Effect of Hydrophobic Modification: cat-HMHEC. Swelling isotherms for cat-HMHEC gels in solutions of cationic and anionic surfactants have been reported previously.20,22 In brief, all tested surfactants give rise to (52) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1.


Figure 8. Equilibrium swelling isotherms for HMHEC gels immersed in solutions of cationic surfactants: C16TAB (circles), C14TAB (squares), and C12TAB (triangles). Arrows denote the cmc’s. The lines are guides to the eye only.

Figure 9. Equilibrium swelling isotherms for cat-HMHEC gels immersed in solutions of anionic surfactants: STS (open circles), SD-(EO)2-S (filled circles), NaDC (filled squares), and SDS (open squares). Arrows denote the cmc’s. The lines are guides to the eye only.

a pronounced swelling of cat-HMHEC gels near their respective cmc’s. Prior to this swelling, a more or less pronounced deswelling occurs. As expected, the deswelling is large (gel collapse) for anionic (i.e., oppositely charged) surfactants. In the present context, we report new swelling isotherms for cat-HMHEC gels for the purpose of further illustrating differences between those surfactants that do and those that do not bind to the HEC backbone. Figure 9 compares the isotherms for the anionic surfactants SDS, sodium tetradecyl sulfate (STS), SD-(EO)2-S, and NaDC. (The swelling isotherms with SDS and STS have been reported elsewhere.22) The SD-(EO)2-S and NaDC isotherms in Figure 9 show that these amphiphiles bind to cat-HMHEC, but the swelling is, again, not as large as for the sodium alkyl sulfates. A further difference is that the alkyl sulfates, but not SD-(EO)2-S or NaDC, induce a steep swelling step just before the swelling maximum. The comparison suggests that the latter step corresponds to a binding of the alkyl sulfates to the HEC backbone.

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Figure 10. Equilibrium swelling isotherms for HEC gels immersed in solutions of SDS and varying amounts of NaCl: 0 mM (open circles), 10 mM (filled circles), 30 mM (open squares), 60 mM (filled squares), 100 mM (open triangles), 500 mM (filled triangles), and 1000 mM (crosses). Arrows denote the cmc’s at the various salt concentrations. The cmc values are taken from ref 67, except for the values at 60 mM and at high salt concentrations, which were obtained by extrapolation. The lines are guides to the eye only.

Figure 11. Equilibrium swelling isotherms for HMHEC gels immersed in solutions of SDS and varying amounts of NaCl: 0 mM (open circles), 10 mM (filled circles), and 100 mM (open triangles). Arrows denote the cmc’s at the various salt levels. The lines are guides to the eye only. The large circles indicate the equilibration solutions used for the studies of transient volume changes presented in Figure 13.

The maximum in the reswelling with added bile salt NaDC occurs at a much higher concentration than the cmc (2-5 mM) stated in ref 53. This indicates either that the real cmc is actually higher or that a significant binding of NaDC occurs also after the cmc. Effect of Added Salt. Swelling isotherms for HEC gels and HMHEC gels in SDS solutions at a number of different levels of added NaCl are shown in Figures 10 and 11. As shown in Figure 10, SDS binds to HEC at all salt levels in the investigated range (0-1 M NaCl). Importantly, even at high levels of salt the association is (53) Small, D. M. In The Bile Acids - Chemistry, Physiology, and Metabolism; Nair, P. P., Kritchevsky, D., Eds.; Plenum Press: New York, 1971; Vol. 1, Chapter 8.

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still detectable, although the features of the binding isotherm are different. The cac decreases with increasing salt concentration,3,18,54 just like the cmc of the pure surfactant. At low salt concentrations, the isotherms show the same features as in the absence of added salt, that is, a cooperative swelling at the cac, followed by a maximum swelling around the cmc and then a gradual deswelling. At 30 mM NaCl, the increase in swelling has become more gradual, the maximum level of swelling has decreased, and the peak in the isotherm has been replaced by a leveling off. The latter feature is due to the decreasing relative contribution of added surfactant to the total salt level in the presence of a significant amount of salt. At high levels of salt (60 mM and above), the isotherm changes shape. Now, the surfactant binding first gives rise to a deswelling, followed by a reswelling that commences at surfactant concentrations above the cmc. Very similar effects of added salt have previously been shown for the swelling isotherms of gels of poly(acrylamide) with pendant PEO side chains (PEO-PAm gels) in solutions of SDS or SOBS.18 In the latter study, it was shown, by direct measurements, that the cooperative binding of surfactant started exactly at the surfactant concentration where the gel volume started to change (increase or decrease, depending on the level of added salt). An ionic surfactant seems to give rise to two general effects when it associates to a nonionic polymer gel: (1) it adds ionic groups to the polymer gel, which induces gel swelling, and (2) it gives rise to an effective polymerpolymer attraction which induces a gel contraction. Presumably, the mechanism for the surfactant-induced attraction is a “micellar cross-linking”, that is, that a bound micelle is shared between two (or more) independent polymer chains. In addition to these effects of bound surfactant, the free ionic surfactant acts as a salt. The interplay between these three effects determines the overall surfactant effect (swelling or shrinking). Somewhere in the interval of 30-60 mM NaCl in the SDS solution, the cross-linking effect of the bound ionic surfactant becomes more important than the ionic effect, at lease initially (just after the cac). At high salt concentrations, more and more bound surfactant is required to overcome the cross-linking effect. In addition to the ionic effect, one additional effect may contribute to the gel reswelling at high surfactant levels: an attraction caused by micellar cross-linking should disappear at high levels of surfactant binding, when the binding of micelles approaches saturation. As shown in Figure 11, salt has a similar effect on HMHEC gels as on HEC gels with respect to the surfactant-induced volume changes. As in the binding isotherms in Figure 10, the results in Figure 11 show that there is a significant deswelling of the HMHEC gels before the cooperative binding also without added salt or with just 10 mM NaCl added. A swelling minimum before the cooperative binding of SDS has also been seen for EHEC gels.16,20 Kinetic Studies. In the measurements of equilibrium swelling, we typically observe a certain variation in gel swelling among different gel pieces. The main reason for this spread is probably that the gel pieces are located at different positions in the (optically imperfect) glass vials. However, by studying the volume change of a selected gel piece with time, it is possible to confirm volume changes that, in the usual equilibrium swelling experiment, would fall within the experimental error. This is because the (54) Hoffmann, H.; Huber, G. Colloids Surf. 1989, 40, 181.


Figure 12. Volume change with time for an HMHEC gel, initially equilibrated in pure water, when immersed in a solution of 100 mM NaCl (without SDS). The line is a guide to the eye only.

Figure 13. Transient volume changes of HMHEC gels, initially equilibrated in pure water, when immersed in solutions of 10 mM SDS (circles) or 10 mM SDS + 100 mM NaCl (triangles). The lines are guides to the eye only. The square root time scale on the x-axis was chosen in order that both the short- and the long-time behavior would be clearly visible; it bears no theoretical significance.

precision increases when the same gel piece is studied at a stationary position in the test tube. Figure 12 shows the transient volume change of an HMHEC gel piece in a 100 mM NaCl solution (without SDS). A small, but significant, monotonic deswelling is seen. This experiment confirms the salt trend seen at low or zero SDS in the equilibrium swelling isotherms for HMHEC gels in Figure 11. To follow the time dependence is, furthermore, important as a check that the equilibrium swelling has actually been reached within the experimental time; cf. the leveling off of the data beyond 300 min in Figure 12. The transient behavior on approach to equilibrium for gels can also be interesting in its own right. In particular, we have very recently reported that the volume changes of both cat-HEC gels21 and cat-HMHEC gels,22 preswollen in pure water, under certain circumstances could be nonmonotonic when the gels are immersed in surfactant solutions. This behavior was encountered in some cases when the surfactant concentration in the swelling medium corresponded to a reswollen gel at equilibrium. The transient behavior then reflected the nonmonotonic equi-


librium swelling isotherms, in that the gradual diffusion of surfactant into the gel first resulted in a more or less rapid deswelling, followed by a more gradual swelling of the gel to the equilibrium value. Figure 13 shows that similar nonmonotonic transient effects may appear for HMHEC gels, initially equilibrated in pure water, when they are immersed in SDS solutions. The experiments shown are for 10 mM SDS with or without added salt. The corresponding equilibrium swelling data are marked with large circles in Figure 11. With 100 mM added NaCl to the SDS solution, the transient volume change is still nonmonotonic. However, the minimum occurs later and it takes longer to reach equilibrium. Concluding Remarks We have illustrated that the gel swelling experiment is a simple and potent tool to study polymer-surfactant associations. Changes in the binding of the surfactant to the polymer are accompanied by a macroscopic change of the gel volume; the swelling behavior reflects the binding isotherm. Because the polymer is contained in a separate compartment, the gel, it can be studied in an effectively infinite bath of surfactant solution, where ctot is approximately equal to cfree. Thus, the response is immediately obtained as a function of the free surfactant concentration, which is the fundamental quantity. Moreover, purification of the polymer from low-molecular (salt) impurities is easily performed for a gel as a simple washing step. One further point to note is that the gels may serve as a useful model for other systems based on the same polymer. For instance, the response of the gel to a given solution environment should be similar to the response of a surface “coating”, based on the same polymer, to the same environment. Gel swelling experiments clearly distinguish between associating and nonassociating polymer-ionic surfactant pairs. In particular, it is generally straightforward to detect the onset of surfactant binding in cases where this occurs at a critical association concentration (cac). The method relies only on the general appearance of a volume change of the gel (positive in salt-free solutions or at low salt, negative in the presence of large amounts of salt) at the cac. The association can be detected for arbitrarily small differences between the cac and the cmc in the polymer-free solution. By studying the transient volume changes of gels, it is possible to detect even smaller changes that, in the usual equilibrium swelling experiment, would be within the experimental error. The HEC gels studied here were found to bind sodium alkyl sulfates with chain lengths >8 carbons and SOBS. By contrast, no binding occurred for the anionic surfactants

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SD-(EO)2-S or sodium deoxycholate nor for any of the tested cationic surfactants. HEC seems to have almost the same interaction pattern as PEO, which associates with SDS55-59 and other sodium alkyl sulfates46,60 but to a lesser extent with alkyl trimethylammonium halides,49,50,56,58,61-66 where long alkyl chains50,56,62,66 and/or high temperatures64 seem to be required for a binding to occur. This similarity seems reasonable, because HEC may be regarded as an EO-substituted cellulose. For a homologous series of surfactants associating to HEC, the following trends are seen in salt-free solutions: When the surfactant tail length is decreased, the relative difference between the cac and the cmc decreases, and the association eventually disappears when cac ≈ cmc. When the surfactant tail length is increased, the maximum swelling (which occurs close to the cmc) also increases. At surfactant concentrations above the swelling maximum, the swelling isotherms merge for all chain lengths that give association. The last two trends have been observed also for other cellulose ether-surfactant combinations in previous gel experiments.20-22 All tested surfactants bind to hydrophobically modified HEC. However, the gel experiments clearly indicate that the extent of binding to HMHEC is much larger for those surfactants that also bind to the nonmodified backbone. The latter conclusion also holds for HEC modified with hydrophobes bearing charges of opposite sign to that of the surfactant. Acknowledgment. This work was financed by the Center for Amphiphilic Polymers from Renewable Resources (CAP) at Lund University. L.P. also acknowledges support from the Swedish Research Council for Engineering Sciences (TFR). LA0016726 (55) Jones, M. N. J. Colloid Interface Sci. 1967, 23, 36. (56) Schwuger, M. J. J. Colloid Interface Sci. 1973, 43, 491. (57) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (58) Witte, F. M.; Engberts, J. B. F. N. Colloids Surf. 1989, 36, 417. (59) Zanette, D.; Lima, C. F.; Ruzza, A. A.; Belarmino, A. T. N.; Santos, S. d. F.; Frescura, V. L. A.; Marconi, D. M. O.; Froehner, S. J. Colloids Surf., A 1999, 147, 89. (60) Brackman, J. C.; Engberts, J. B. F. N. J. Colloid Interface Sci. 1989, 132, 250. (61) Zana, R.; Lianos, P.; Lang, J. J. Phys. Chem. 1985, 89, 41. (62) Shirahama, K.; Himuro, A.; Takisawa, N. Colloid Polym. Sci. 1987, 265, 96. (63) Brackman, J. C.; Engberts, J. B. F. N. Langmuir 1991, 7, 2097. (64) Anthony, O.; Zana, R. Langmuir 1994, 10, 4048. (65) Olofsson, G.; Wang, G. Pure Appl. Chem. 1994, 66, 527. (66) Mya, K. Y.; Jamieson, A. M.; Sirivat, A. Langmuir 2000, 16, 6131. (67) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1971.

Simple Gel Swelling Experiments Distinguish between Associating

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