Swelling of Poly(acrylamide) Gels with Pendant Poly(ethylene oxide

Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, and Université P. et M. Cu...
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Swelling of Poly(acrylamide) Gels with Pendant Poly(ethylene oxide) Chains in Solutions of Ionic Surfactant and Salt Olof Rose´n,† Lennart Piculell,*,† and Dominique Hourdet‡ Physical Chemistry 1, Center for Chemistry and Chemical Engineering, Lund University, P.O. Box 124, S-221 00 Lund, Sweden, and Universite´ P. et M. Curie, CNRS URA 278, Laboratoire de Physicochimie Macromole´ culaire, ESCPI, 10 rue Vauquelin, 75231 Paris Cedex 05, France Received August 11, 1997. In Final Form: November 24, 1997 The swelling in water and in ionic surfactant solutions of a polymer gel based on poly(acrylamide) (PAm) with pendent poly(ethylene oxide) (PEO) chains (0-2.6 mol %) was investigated. In pure water, the swelling increased linearly with the content of PEO side chains. The anionic surfactants sodium dodecyl sulfate (SDS) and sodium octylbenzenesulfonate (SOBS) both bound to the PEO side chains above a critical association concentration (cac). For SOBS, binding isotherms to the gels were obtained, and the cac values for the surfactant in PEO solutions were determined by NMR, both at varying concentrations of added NaCl. Both surfactants affected the swelling of the copolymer gels similarly. When the surfactant concentration in the swelling medium was increased at low concentrations of added NaCl, a substantial swelling occurred at the cac, and the volume continued to increase up to a concentration just above the critical micellization concentration (cmc) for the free surfactant in the swelling medium. At higher concentrations of surfactant the gels started to deswell. At high contents of added NaCl (ca. 0.5 M and above), the swelling isotherm changed: The gels instead began to shrink at the cac, indicating a crosslinking of the PEO chains by the surfactant micelles. The gel volume went through a minimum when the ratio of PEO chains to bound micelles was approximately 2. At higher concentrations, the gel continued to swell until it was saturated with surfactant. The amount of bound surfactant at saturation increased with large amounts of added salt. The experiments in the presence of salt clearly showed that the binding of surfactant to the gels continues even at surfactant concentrations exceeding the cmc in the swelling medium.

Introduction Research on covalently cross-linked polymer gels that are sensitive to small changes in the environment, “responsive gels”, is currently very active and increasing.1 One of the more recent developments is to control the gel volume by introducing surfactants in the “swelling medium”,2-18 utilizing the well-documented interactions between polymers and surfactants.19 Among a number of possibilities, one is to use ionic surfactants and a gel * To whom correspondence should be addressed. † Lund University. ‡ Universite ´ P. et M. Curie. (1) Dusek, K. Responsive Gels. Advances in Polymer Science; SpringerVerlag: New York, 1993; Vol 109. (2) Zhang, Y.-Q.; Tanaka, T.; Shibayama, M. Nature 1992, 360, 142. (3) Inomata, H.; Goto, S.; Otake, K.; Saito, S. Langmuir 1992, 8, 687. (4) Inomata, H.; Goto, S.; Saito, S. Langmuir 1992, 8, 1030. (5) Piculell, L.; Hourdet, D.; Iliopoulos, I. Langmuir 1993, 9, 3324. (6) Kokufuta, E.; Zhang, Y.-Q.; Tanaka, T.; Mamada, A. Macromolecules 1993, 26, 1053. (7) Wada, N.; Kajima, Y.; Yagi, Y.; Inomata, H.; Saito, S. Langmuir 1993, 9, 46. (8) Moe, S. T.; Skjåk-Bra¨k, G.; Elgsa¨ter, A.; Smidsro¨d, O. Macromolecules 1993, 26, 3589. (9) Tam, K. C.; Ragaram, S.; Pelton, R. H. Langmuir 1994, 10, 418. (10) Saranj, A.; Yoshida, M.; Omichi, H.; Katakai, R. Langmuir 1994, 10, 2954. (11) Chu, B.; Yeh, F.; Sokolov, E. L.; Starodoubtsev, S. G.; Khokhlov, A. R. Macromolecules 1995, 28, 8447. (12) Kokufuta, E.; Nakaizuma, S.; Ito, S.; Tanaka, T. Macromolecules 1995, 28, 1704. (13) Sakai, M.; Satoh, N.; Tsujii, K.; Zhang, Y.-Q.; Tanaka, T. Langmuir 1995, 11, 2493. (14) Phillippova, O. P.; Hourdet, D.; Audebert, R.; Khokhlov, A. R. Macromolecules 1996, 29, 2822. (15) Isogai, N.; Gong, J. P.; Osada, Y. Macromolecules 1996, 29, 6803. (16) Kokufuta, E.; Suzuki, H.; Sakamoto, D. Langmuir 1997, 13, 2627. (17) Rose´n, O.; Piculell, L. Polym. Gels Networks 1997, 5, 185.

based on a nonionic polymer, to which the surfactant binds in the form of micelles. The most commonly investigated system of this type is based on poly(N-isopropylacrylamide) (p-NIPA) gels and sodium dodecyl sulfate (SDS).2,4,6,9,13,16,17 Water-swollen p-NIPA gels display a collapse on increasing the temperature beyond the lower consolute temperature of the linear polymer in water. The effect on the temperature-induced volume phase transition of p-NIPA gels on the addition of ionic surfactant has been thoroughly investigated,2-4,6,9,12,13,16 and the collapse temperature was found to increase on addition of ionic surfactant. The addition of some different salts gave a decrease in the transition temperature.3 However, when an ionic surfactant was present, the addition of salt in the concentration range 0-1 M did not affect the transition temperature.2 Nonionic gels showing the same type of behavior have also been made by chemical cross-linking of cellulose derivatives.17,20 An early demonstration of gel-surfactant interactions was made with a gel based on poly(acrylamide) (PAm) containing a small fraction of “macromonomers” carrying pendent poly(ethylene oxide) (PEO) side-chains.5 The PAm backbone is quite inert to changes in the environment such as temperature, addition of ionic surfactant, or salt. PEO, on the other hand, is well-known to associate with ionic surfactants above a certain concentration, the socalled critical association concentration (cac), where the (18) Piculell, L.; Thuresson, K.; Ericsson, O. Faraday Discuss. 1995, 101, 307. (19) Goddard, E. D.; Ananthapadmanbhan, K. P. Interactions of Surfactants with Polymers and Proteins; CRC Press: Ann Arbor, MI, 1993. (20) Harsh, D. C.; Gehrke, S. H. J. Controlled Release 1991, 17, 175.

S0743-7463(97)00901-3 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/27/1998

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surfactant starts to micellize on the polymer chain.21-31 Moreover, when linear PAm and PEO are mixed in an aqueous solution, they show a segregative phase behavior.32 This incompability in solution gives a swelling when the two polymers are copolymerized in a gel. In the present investigation, we have extended the previous preliminary study5 by making gels with larger degrees of substitution by PEO chains (up to 2.6 mol % of the repeating units), and by studying their interactions with two different anionic surfactants, sodium dodecyl sulfate (SDS) and sodium octylbenzenesulfonate (SOBS) in the presence and the absence of varying amounts (up to 1 M) of simple salt (NaCl). We have also obtained the binding isotherm of SOBS to the gels and, independently, measured the critical micelle concentration (cmc) of SOBS in water and the cac of SOBS in PEO solutions. The aim of this work is to obtain a better understanding of the swelling behavior of the gels, as a function of increasing surfactant concentration, in terms of the surfactant binding, both in the presence and in the absence of simple salt. Experimental Section Materials. Acrylamide (AAm, Sigma), N,N′-methylenebis(acrylamide) (BIS, Sigma), ammonium persulfate (APS, Sigma), and N,N,N′,N′-tetraethylenediamine (TEMED, Sigma) were used without further purification. Poly(ethylene oxide) monoacrylate (PEOMA) macromonomers with a relative molecular mass of 6800 g/mol33 were prepared according to the functionalization method of ω-methyl-R-amino polyether described by Gnanov and Rempp.34 SDS (BDH), SOBS (Tokyo Kasei), and NaCl (BDH) were used as obtained without further purification. D2O for the NMR experiments was obtained from Dr Glaser AG, Basel. The purity of D2O was 99.8% (0.2% H2O). MilliQ filtered water was used throughout. Gel Preparation. Gels were prepared by standard radical polymerization with APS (0.44 mg/mL of H2O) as initiator, BIS (2.5 mg/mL of H2O) as cross-linker, and TEMED (5 mg/mL of H2O) as accelerator. The sample solutions were degassed with nitrogen and the polymerization was allowed to proceed for 1 h at room temperature in 1.2 mm (i.d.) glass tubes. All gels were prepared with the same total molar amount of AAm + PEOMA (0.7 mol/kg of H2O) in the reaction bath. All gels produced were optically clear. The molar ratio PEOMA:AAm in the reaction bath was varied between 0% and 10%. Approximately 54% of the PEOMA monomer was not functionalized,33 and some of the monomers did not incorporate for other reasons. To estimate the fraction of PEOMA incorporated in the network, a precisely determined amount (about 0.6 g) of gel from each sample was allowed to equilibrate in 2.5 mL of water for 2 weeks. A precisely determined amount (about 1 mL) of the dialysis medium was then evaporated, and the remaining solid was dissolved in D2O. The amount of PEOMA not incorporated into the gel was analyzed from the 1H NMR spectra of these solutions, using an internal standard method, with the peak area from PEOMA and the peak area from the small amount of H2O (0.2%) in D2O as an internal standard. A series of samples with known amounts of PEOMA was used to obtain a standard curve. The estimated amount of (21) Jones, M. J. Colloid Interface Sci. 1967, 23, 36. (22) Schwunger, M. J. J. Colloid Interface Sci. 1973, 43, 491. (23) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (24) Moroi, Y.; Akisada, H.; Saito, M.; Matuura, R. J. Colloid Interface Sci. 1977, 61, 233. (25) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1982, 43, 1529. (26) Cabane, B.; Duplessix, R. Colloid Surf. 1985, 13, 19. (27) Tondre, C. J. Phys. Chem. 1985, 89, 510. (28) Cabane, B.; Duplessix, R. J. Phys. (Paris) 1987, 48, 651. (29) Gao, Z.; Wasylishen, R. E.; T., K. J. C. J. Phys. Chem. 1991, 95, 462. (30) Brown, W.; Fundin, J. Macromolecules 1992, 25, 7192. (31) Su¨ss, D.; Cohen, Y.; Talmon, Y. Polymer 1995, 36, 1809. (32) Perrau, M. B.; Iliopoulos, I.; Audebert, R. Polymer 1989, 30, 2112. (33) Hourdet, D.; L’Alloret, F.; Audebert, R. Polymer 1997, 38, 2535. (34) Gnanov, Y.; Rempp, P. Makromol. Chem. 1987, 188, 2111.

Rose´ n et al. PEO chains incorporated was 26% of the amount added in the synthesis. This result is similar to the result (22%) obtained in the previous study.5 Swelling and Binding Isotherms. The gels with a diameter of 1.2 mm were cut into about 1.2 mm long rods which were immersed in a large excess of water for 2 days, with one change of water, to wash away residual chemicals. In the swelling experiments with SDS, four rods were immersed in vials containing 10 mL of aqueous solution of SDS and/or NaCl. Owing to the large volume of the swelling medium, the equilibrium amount of SDS bound to the gel was always negligible, and the equilibrium concentration of SDS in the swelling medium, Cf, was therefore equal to the initial concentration (before immersing the gels). The gels were allowed to equilibrate for 3 days at 35 °C to reach the equilibrium swelling. A temperature of 35 °C was used to exceed the Krafft temperature of SDS at high salt concentrations. The equilibrium diameter, D, of each gel sample was measured with a video camera calibrated with a 0.1 mm scale. Each data point represents the average of ca. 10 measurements of D, showing a variation of less than 5%. Gel swelling ratios are given as V/V0 ) (D/D0),3 where D0 is the initial diameter of the gel (1.2 mm). In the experiments with SOBS, a precisely determined amount (ca. 200 mg) of a gel was put in a vial. A solution of SOBS and/or NaCl of 5 times the weight of the gel was then added, and the gel was allowed to equilibrate for 3 days in this swelling medium at 60 °C. The higher temperature was chosen because of the higher Krafft temperature for SOBS than for SDS. The gel diameters were measured as in the experiments with SDS. The amount of free SOBS in the solution was then analyzed with a UV spectrophotometer. The equilibrated solutions were first diluted about 10 times, where after the absorbance of SOBS was measured at 261.1 nm and the concentration was calculated from a calibration curve. cmc and cac Measurement. The values of cmc for SOBS, and cac for SOBS in PEO solutions, were measured at various salt concentrations at 60 °C by 1H NMR using the changes of the chemical shift35,36 of the methylene group closest to the benzene ring in the SOBS molecule. This peak is seen at 2.96 ppm in the NMR spectrum. When micelles are formed, the region around the methylene group is changed from a hydrophilic to a hydrophobic environment, which gives a change in the chemical shift. SOBS, together with PEO and/or NaCl as appropriate, was dissolved in D2O, and the measurements were made with a superconducting Bruker DMX 100 MHz spectrometer.

Results Effects of Modification by PEO. The swelling of a pure PAm gel, without any modification, was studied in solutions of pure water, 10 mM SDS, 1 M NaCl, and a mixed solution containing 1 M NaCl and 10 mM SDS at 35 °C. The gel swelling was the same in all these solutions, showing that neither surfactant nor salt, alone or together, affect the swelling of a pure PAm gel. Figure 1 shows the swelling in pure water of the gels as a function of the degree modification by PEO chains. The swelling shows a linear dependence on the degree of modification. The previous study5 showed the same trend, but the effect was less pronounced, since the degree of modification (e0.22 mol %) chains was much lower. This work shows that the amount of incorporated PEO chains can be at least as high as 2.6 mol %. The large additional swelling on modification can have three different origins. Trivially, the swelling should increase since the total polymer concentration in the gel increases on replacing small AAm monomers with large macromonomers. There may also be an effect of the difference in polymer architecture; the changes in configurational entropy upon gel swelling should be different (35) Wennerstro¨m, H.; Lindman, B. Topics of Current Chemistry; Springer-Verlag: New York, 1980; Vol. 87, Chapter 5. (36) Wennerstro¨m, H.; Lindman, B. Rev. Sect. Phys. Lett. 1987, 52, 18.

Gel Swelling

Figure 1. Swelling ratios of PEO-PAm gels in water at room temperature, as a function of the molar percentage of PEO side chains incorporated in the gel.

for a side chain (with one dangling end) compared to an active chain (with both ends terminating in a cross-link) in the network. Finally, there should be a contribution from short-range chain-chain interactions. As mixtures of linear PAm and PEO display a segregative phase behavior,32 they experience mutual repulsion, and a gel network made of a mixture of the polymers should therefore display an additional swelling compared to the pure polymer gel. The trivial effect of increasing the concentration seems to dominate the effect seen in Figure 1, but there are also significant contributions from other mechanisms. This we infer from estimates of the mass swelling ratio m/m0, where m0 and m are the masses of the completely dried gel and the swollen gel, respectively. The mass swelling ratios were obtained from the volume swelling ratios V/V0 by assuming a density of 1 g/cm3 for all swollen gels, using the known concentrations of the ingredients in the gel synthesis, and taking into account the fact that only 26% of the macromonomers in the reaction bath were incorporated into the gels. For the nonmodified gel and the 2.6 mol % modified gel, we thus obtain mass swelling ratios of 38 and 56, respectively. This means that there is roughly a 50% extra increase in the mass of absorbed water per unit mass of polymer for the most modified gel due to additional repulsive chain-chain interactions and/ or differences in the gel architecture. Swelling Isotherms in SDS Solutions. Swelling isotherms for gels with different degrees of PEO modification were obtained as functions of the SDS concentration in solutions containing different constant concentrations of added NaCl. All the features reported below were found for all degrees of modification; the features were thus quite reproducible and significant. Since, naturally, the effects were largest for the gel with the highest degree of modification, only these results are reproduced in the figures below. Figure 2 shows the swelling isotherm for a PEO-PAm gel as a function of the free SDS concentration in salt-free solutions at ambient temperature. The qualitative features are the same as have been reported and discussed previously, both for the same type of gel with a lower degree of modification5 and for gels based on ethyl hydroxyethyl cellulose (EHEC).17,18 Above the cac and until the cmc of the free surfactant is reached (8 mM for SDS), there is a great swelling of the gel, signifying the continuous binding of surfactant, making the gel increasingly more ionic. The additional gel swelling is due to the osmotic pressure excerted by the

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Figure 2. Swelling ratios of PEO-PAm gels with 2.6 mol % PEO side chains at 35 °C as a function of the SDS concentration in a salt-free swelling medium.

counterions of the surfactant.38 For the present gel, the surfactants associate only with PEO but not with PAm. The binding of SDS, and the swelling, thus increases with an increasing degree of modification. At surfactant concentrations far above the cmc, there is a significant deswelling, caused by the increased surfactant concentration in the swelling medium (the relative difference in concentration of ions inside and outside the gels decreases). The high degree of surfactant swelling of the present gels makes it possible to conclude that the maximum swelling occurs at a surfactant concentration (10 mM) which is significantly higher than the cmc (8 mM) of SDS, a feature that was not noted previously. This implies that there may be a significant additional binding of the surfactant even when free micelles have formed in the swelling medium. This point will be substantiated and discussed further below. The addition of salt to the surfactant-gel system produces some interesting changes in the swelling isotherms, as demonstrated by the isotherms at 35 °C in Figure 3. The arrows in the figure show the cmc’s for the free SDS molecules at the respective salt concentration. These values were calculated from the relation19

log(cmc) ) - 3.550 - log(cmc + Cs) × 0.712 (1) Here Cs is the concentration of the added salt. The numerical values in eq 1 were obtained from a linear leastsquares fit to the data from Gunnarsson et al.39 At 10 mM added salt the swelling isotherm is similar to the swelling isotherm for the salt-free case. The only differences are that the gel volume is somewhat lower throughout the whole surfactant concentration range and that the SDS concentration where the gel starts to swell, as well as the SDS concentration where the gel volume is at its maximum, is shifted to lower SDS concentrations. All these effects are expected consequences of the additional screening in excess salt. At 0.1 M salt the gel starts to swell at about 1 mM SDS, but the swelling increases to concentrations much higher than the cmc, and at still higher SDS concentrations, the gel volume seems to reach a plateau value instead of going through a maximum as for the salt-free case. Qualitatively, the absence of the shrinking at high surfactant (37) Schild, H. G.; Tirell, D. A. Langmuir 1991, 7, 665. (38) Flory, P. J. Principles of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (39) Gunnarsson, G.; Jo¨nsson, B.; Wennerstro¨m, H. J. Phys. Chem. 1980, 84, 3114.

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a

b Figure 3. Swelling ratios of PEO-PAm gels with 2.6 mol % PEO side chains at 35 °C as a function of the SDS concentration at the following constant concentrations of NaCl: 0 (filled circles), 10 mM (open squares), 0.1 M (diamonds), 0.5 M (open circles), and 1 M (filled squares). Arrows denote the cmc values for SDS at the various salt concentrations.

concentrations can be understood to be a result of the high levels of screening salt; the salt concentration in this isotherm is always higher than or equal to the surfactant concentration. At even higher salt concentrations, 0.5 and 1 M, the binding isotherm goes through a minimum at low SDS concentrations. Similar effects have previously been seen for EHEC gels on addition of SDS in the presence of salt.17 The SDS concentrations where the gels start to decrease in volume correspond well to the cmc values for SDS at 0.5 and 1 M salt. For 0.5 M salt the gel volume has its minimum at ca. 1 mM SDS. The swelling at higher SDS concentrations seems to level off at about 3 mM SDS, similarly as for 0.1 M salt, with a small tendency of further swelling at higher Cf. For 1 M salt, the trends are the same as for 0.5 M but more pronounced. There is a much broader minimum and the value where the swelling is leveling off is reached at a higher SDS concentration. The tendency of further swelling is also more pronounced. Interactions with SOBS. To understand the features of the swelling isotherms with SDS, we performed similar and additional experiments with the surfactant SOBS. SOBS is similar to SDS in many ways: The counterion is the same, and the cmc at room temperature (ca.11 mM40) is close to the cmc of SDS. The advantage with SOBS is that its concentration can be easily analyzed by UV absorption, making it possible to conveniently measure binding isotherms. One disadvantage, though, is that SOBS has a higher Krafft point. To exceed the Krafft point at high salt contents, the swelling experiments with SOBS therefore had to be performed at 60 °C. The PEOPAm gel does not show any volume change at 60 °C compared to 35 °C, as seen when comparing the swelling values at zero surfactant content (Figures 3 and 5a below). This allows us to use the binding and swelling isotherms with SOBS to understand also the swelling isotherms with SDS. The cmc for SOBS and the cac for SOBS in PEO solutions with and without salt were measured at 60 °C by 1H NMR. Figure 4a shows a representative plot of the determination of cmc by this method, and Figure 4b shows the variation of the cmc and the cac with the content of NaCl. As the (40) Munkerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1971.

Figure 4. (a) 1H NMR determination of the cmc value for SOBS in a salt-free aqueous solution at 60 °C. Inverse SOBS concentration as a function of the chemical shift of the methylene group closest to the benzene ring in the SOBS molecule. The H2O peak is set to 0 Hz. (b) Dependence of the cmc (circles, solid line) and cac (squares, dashed line) values (in PEO solutions) at 60 °C of SOBS on the total concentration of added NaCl + monomeric surfactant, as determined by 1H NMR measurements.

salt concentration is increased, the cmc and cac values approach each other, and at 0.5 and 1 M of NaCl the values are almost the same. As will be shown below, however, the absence of a significant difference in the micellization concentration in the presence and in the absence of PEO may not be taken as an indication that there is no binding. In Figure 5a swelling isotherms for the PEO-PAm gel as functions of the equilibrium external SOBS concentration are shown for solutions containing constant levels of 0.1, 0.5, or 1 M NaCl. The isotherms are similar for SOBS as for SDS. As the cmc for SOBS differs from the cmc for SDS, the typical minima, the large increases, and the leveling off of the swelling isotherms are shifted to higher concentrations in the SOBS case. The inserted figure shows the same data plotted in linear form. The linear plot clearly shows that the swelling levels off at high concentrations; there is a large concentration range where the swelling changes very little. Figure 5b shows the corresponding binding isotherms. The inserted figure is, again, the same data plotted in linear form. Note that both the swelling and the surfactant binding were measured on exactly the same gels in one experiment; at a given salt content, the values of the swelling (Figure 5a) and the surfactant binding (Figure 5b) quoted at a given free surfactant concentration thus refer to the same gel. Therefore, all conclusions regarding the correlation between surfactant binding and gel volume changes are independent of any possible quantitative

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Figure 5. (a) Swelling ratios of PEO-PAm gels with 2.6 mol % PEO side chains as a function of the SOBS concentration at the following constant concentrations of NaCl: 0.1 M (filled circles), 0.5 M (open squares), and 1 M (diamonds). (b) Binding isotherms for PEO-PAm gels and with 2.6 mol % PEO side chains as a function of the SOBS concentration at the following constant concentrations of NaCl: 0.1 M (filled circles), 0.5 M (open squares), and 1 M (diamonds).

uncertainties in the free surfactant concentration. A comparison between the two data sets thus clearly shows that the onset of a significant volume change (swelling or shrinking) is the result of a significant binding of surfactant and that the leveling off of the swelling at high concentrations also correlates with a similar leveling off of the surfactant binding. As the salt concentration is increased, the concentration where the binding starts decreases. These onset concentrations are in good agreement with the corresponding cac values from the NMR measurements, shown in Figure 4b. The concentration where the gel becomes saturated with SOBS seems to be the same (within the accuracy of the experiment) for all salt concentrations. The amount of bound SOBS at saturation is lowest in 0.1 M NaCl, but no significant difference is seen for 0.5 and 1 M salt. The largest difference between the binding and the swelling isotherms is that while the former are monotonic, the swelling isotherms are nonmonotonic in the presence of high concentrations of salt. Thus, in the latter cases, the surfactant binding first results in a decrease, then in an increase in the gel swelling. Discussion The addition of salt to ionic surfactants induces some major changes. In addition to lowering the cmc,39,41 Figure 4, added salt can also induce micellar growth. At (41) Hayashi, S.; I., S. J. Phys. Chem. 1980, 84, 744.

salt concentrations above 0.4 M SDS micelles are known to grow into prolates at room temperature.42-45 At high salt concentrations the micelle size is also temperature sensitive. In 0.5 M NaCl at 30 °C the micelles are small and spherical up to concentrations of about 20 mM of SDS.43 So in 0.5 M NaCl and at 35 °C the micelles should be small and spherical up to at least 20 mM, but in 1 M salt there is probably some micellar growth above 20 mM surfactant. This will be discussed below. The effect of increasing the salt concentration on the cac for linear PEO and SDS is well investigated.25,26,31 As the salt concentration is increased, the cac is decreased, just as the cmc. For PEO and SOBS this is also the case as shown above. Moreover, when more salt was added, the relative difference between cmc and cac was shown above to decrease, and at 1 M salt there was no measurable difference. It is also known that there is a larger binding of SDS to PEO when salt is added.26 This is because the repulsion between neighboring bound micelles decreases in salt solutions because of the screening effect. The SDS micelles associated to the PEO have the same aggregation numbers as free micelles (ca. 60), and the bound micelles do not grow even at high salt concentrations; they remain small and spherical up to a NaCl concentration of 0.8 M.25 With this background the swelling and binding isotherms in Figures 3, 5a, and 5b can be understood. Let us first look at the swelling and binding isotherms for SOBS in 0.1 M salt. The swelling starts at about 3 mM of SOBS. This corresponds well to the cac in Figure 4b and to the binding isotherm in Figure 5b, as already noted. Above this concentration there is a large swelling and a large binding up to about 10 mM SOBS. The cmc for the free surfactant is about 5 mM in this case, which means that there is a binding to the gel even above the cmc for the free surfactant. Note that the maximum swelling is larger for 0.1 M salt compared to 0.5 and 1 M salt (Figure 5a), although the amount of bound SOBS is lower (Figure 5b). This is in contrast to the results of Zhang et al.,2 showing that the addition of salt to a p-NIPA gel with 2 wt % SDS did not give any marked change in the gel volume. The binding isotherm reaches a plateau at ca. 40 mol of SOBS/mol of PEO. If we assume that the aggregation number for a bound SOBS micelle is similar to that for SDS (cf. the measured hydrodynamic radii of 24 and 20 Å for free SOBS46 and SDS26 micelles, respectively), this corresponds to a reasonable saturation value of the order of 1 micelle per PEO side chain. Both in 0.5 and 1 M salt the swelling isotherms show a decrease in swelling at the (salt-dependent) onset of surfactant binding, and the gel volume goes through a minimum when the free SOBS concentration is around 2 mM. The latter concentration corresponds to a binding of ca. 20 SOBS molecules per PEO chain; cf. Figure 5b. This means that the gel starts to swell roughly when the number of bound micelles exceeds one micelle per two PEO chains. This suggests that the decrease in the swelling is an effect of physical cross-linking between PEO chains by shared micelles. (The same explanation has been given previously for a similar minimum in gel volume (42) Missel, P. J.; Mazer, N. A.; Benedek, G. B.; Young, C. Y. J. Phys. Chem. 1980, 84, 1044. (43) Hayashi, S.; Ikeda, S. J. Phys. Chem. 1980, 84, 4. (44) Mazer, N. A.; Benedek, G. B.; Carey, M. C. J. Phys. Chem. 1976, 80, 1075. (45) Young, Y. Y.; Missel, P. J.; Mazer, N. A.; Benedek, G. B. J. Phys. Chem. 1978, 82, 1375. (46) Kamenka, N.; Puyal, M.; Brun, B.; Haouche, G.; Lindman, B. Tracer Self-Diffusion of Surfactant Association; Mittal, K. L., Lindman, B., Eds.; Plenum Press: New York, 1982; Vol. 1, p 359.

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that occurs on adding SDS to EHEC gels in the presence of salt).17 However, as the binding increases, the number of shared micelles should decrease so that ultimately, at saturation (see below), all cross-links have vanished. It seems reasonable that the concomitant changeover from contraction to expansion occurs when the number of micelles in the PEOPAm gels exceeds 1 micelle per 2 PEO chains. The saturation of the gels, both in 0.5 and in 1 M salt, is reached at about 80 SOBS molecules per PEO chain, corresponding to roughly 2 micelles per PEO chain. This insensitivity of the plateau level binding to the salt concentration is not a trivial effect of a “total” screening of the electrostatic repulsions; a significant increase in the screening even at these high levels of salt is evidenced by both the decrease in the cac and the significantly lower degree of swelling as the higher salt concentration is increased from 0.5 to 1 M. The fact that the saturation value of surfactant binding is the same in both 0.5 and 1 M NaCl, and very nearly twice that in 0.1 M NaCl, is suggestive: This agrees with a model where the saturation binding to the side chains increases in a discrete step, corresponding to the aggregation number of a micelle (ca. 40 surfactant molecules), when the salt content is increased. It is also of interest to compare the binding of SOBS to PEO with previously obtained values for SDS. The molecular weight of the PEO side chain is 6800 g/mol, which corresponds to 155 ethylene oxide monomers. This gives a value of ca. 0.5 SOBS molecules per ethylene oxide monomer at high salt contents. Cabane et al.26 obtained the same maximum number of bound SDS monomers per ethylene oxide monomer in the presence of 0.6 M NaBr. We believe that the swelling isotherms with SDS may be understood in the same terms as the swelling in SOBS. In SDS, however, we also have to explain the significant increase in swelling also at high surfactant concentrations in the presence of 1 M salt. One possible explanation (cf. above) is that there is significant micellar growth, and thus increased surfactant binding, when the surfactant concentration increases at these high levels of added salt.

Rose´ n et al.

swelling of the gel at 2.6 mol % of PEO side chains comes from additional chain-chain repulsions and/or the pendant chain architecture of the modified gels. Large effects on the equilibrium swelling of the modified gel in ionic surfactant solutions correlate well with the binding of an ionic surfactant. This holds both for the onset of swelling/shrinking at low surfactant concentrations and for the leveling off of the swelling at high surfactant concentrations. Moreover, the onset of binding agrees very well with the cac for the binding of the surfactant to the linear polymer chain in solution, as measured in independent experiments. At high surfactant concentrations in low salt or saltfree solutions a deswelling of the gel commences at surfactant concentrations just above the cmc of the free surfactant. This deswelling vanishes when the concentration of added salt is larger than the highest level of added surfactant, supporting the previous interpretation that the deswelling is due to the increased concentration of ions in the swelling medium as the surfactant concentration is increased. There is a significant increase in the binding of surfactant to the gel with increasing surfactant concentration even above the concentrations when there are free surfactant micelles in the swelling medium. This is particularly obvious in the presence of salt, when the cac and cmc values may become almost equal. The binding of surfactant micelles to the polymer chains of the gel can give rise to some extra physical cross-links, which give rise to a contraction of the gel in the presence of large amounts of added salt. The binding isotherms imply that these cross-links start to vanish when the average number of polymer chains per mixed micelle decreases below the value of 2. The swelling of the surfactant-saturated gel in the presence of excess salt decreases continuously when the salt content increases. In contrast, the plateau level of binding of surfactant to the PEO side chains changes in discrete steps, possibly corresponding to the aggregation number of a polymer-bound micelle, as the salt content is varied.

Conclusions The most significant findings in the present study may be summarized as follows. High levels of PEO side chains increase the swelling of PEO-modified PAm gels in water beyond the trivial effect of increasing the polymer concentration. A 50% extra

Acknowledgment. This work was supported by the Swedish Research Council for Engineering Sciences (TFR). We thank Magnus Nyde´n for help with the NMR measurements and Ilias Iliopoulos for helpful discussions. LA970901U