Langmuir 1998, 14, 5795-5801
5795
Responsive Polymer Gels Based on Hydrophobically Modified Cellulose Ethers and Their Interactions with Ionic Surfactants Olof Rose´n, 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 October 28, 1997. In Final Form: July 16, 1998 Responsive gels made by chemically cross-linking two modified cellulose ethers, separately and in mixtures, have been investigated with respect to their swelling in aqueous solutions containing anionic or cationic surfactants and/or added salt. One of the polymers (catHM-HEC) is a salt-sensitive and strongly hydrophobically associating (hydroxyethyl)cellulose modified with cationic hydrophobic side chains. The other polymer, ethyl(hydroxyethyl)cellulose (EHEC), is a temperature-sensitive, weakly self-associating polymer. Both polymers bind ionic surfactants, and it was found that the response of the cross-linked gels to the content of surfactant or salt in the swelling medium reflected the expected surfactant binding isotherms. A maximum in swelling was generally observed when the surfactant concentration in the swelling medium was close to the critical micelle concentration (cmc). The collapse/swelling behavior of pure catHM-HEC gels on addition of cationic and anionic surfactants showed a close correspondence to the previously studied phase behavior of non-cross-linked catHM-HEC in mixtures with the same surfactants. Mixed gels of EHEC and catHM-HEC in different proportions showed a behavior intermediate between those of the pure polymer gels. It was thus possible to customize gels with respect to their responses to different influences.
Introduction Polymer gels that respond to changes in the external conditions with a volume change, often referred to as responsive gels or environmentally sensitive gels, have attracted much interest in recent years.1 The most studied nonionic responsive gel is the poly-(N-isopropylacrylamide) (p-NIPA) gel, because of its temperature-induced volume phase transition. The effects on the transition temperature of different additives such as cosolvent,2 salt,3,4 or surfactant4-10 and of the introduction of ionic groups11 have been well investigated. Another example of a synthetic copolymer gel that responds to ionic surfactants is a polyacrylamide-based gel with pendant poly(ethylene oxide) chains (a PEO-PAm gel).12 A very recent study on PEO-PAm gels shows that a complex behavior (an initial collapse followed by a reswelling) may be obtained if the ionic surfactant is added to a swelling medium also containing salt.13 During the last years there has been increasing interest in gels based on natural polymers,14,15 partly because these come from renewable sources and partly because they * To whom correspondence should be addressed. (1) Dusek, K., Ed. Responsive Gels; Advances in Polymer Science, Vol. 109; Springer-Verlag: New York, 1993. (2) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (3) Inomata, H.; Goto, S.; Saito, S. Langmuir 1992, 8, 1030. (4) Inomata, H.; Goto, S.; Otake, K.; Saito, S. Langmuir 1992, 8, 687. (5) Zhang, Y.-Q.; Tanaka, T.; Shibayama, M. Nature 1992, 360, 142. (6) Tam, K. C.; Ragaram, S.; Pelton, R. H. Langmuir 1994, 10, 418. (7) Sakai, M.; Satoh, N.; Tsujii, K.; Zhang, Y.-Q.; Tanaka, T. Langmuir 1995, 11, 2493. (8) Kokufuta, E.; Zhang, Y.-Q.; Tanaka, T.; Mamada, A. Macromolecules 1993, 26, 1053. (9) Kokufuta, E.; Nakaizuma, S.; Ito, S.; Tanaka, T. Macromolecules 1995, 28, 1704. (10) Kokufuta, E.; Suzuki, H.; Sakamoto, D. Langmuir 1997, 13, 2627. (11) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392. (12) Piculell, L.; Hourdet, D.; Iliopoulos, I. Langmuir 1993, 9, 3324. (13) Rose´n, O.; Piculell, L.; Hourdet, D. Langmuir 1998, 14, 777.
can be used in foods and for medical purposes.1 Gehrke and co-workers have studied gels based on slightly hydrophobic cellulose derivatives such as the temperature responsive (hydroxypropyl)cellulose (HPC), alone17,18 or mixed with the salt sensitive carboxymethyl cellulose (CMC).19 In our laboratory we have studied gels based on chemically cross-linked ethyl(hydroxyethyl)cellulose (EHEC; cf. Figure 1a).20 Like p-NIPA and HPC, EHEC has a lower consolute temperature (LCST) in water, and the gel volume is sensitive to the influence of temperature, salt, and added surfactants.20 Modified celluloses are necessarily much more complex, chemically, than gels made from synthetic homopolymers; the substitution pattern varies for process reasons as well as for statistical reasons. Indeed, one may convince oneself that no two EHEC molecules of a given batch should be exactly alike.21 For the possible uses of such complex polymers in responsive gels, it thus seems important to clarify if they display some generic behavior, independent of fine chemical detail. For the nonionic EHEC gel and its interactions with ionic surfactants, we have already shown that this is indeed the case: EHEC gels in solutions of an anionic surfactant, with and without salt, follow the same swelling/collapse trends as do the chemically simpler p-NIPA or PEO-PAm gels.13,20 The generality of these trends, and their simple interpretation in terms of the surfactant binding, also demonstrated that gel experi(14) Moe, S. T.; Skjåk-Bræk, G.; Elgsæter, A.; Smidsrød, O. Macromolecules 1993, 26, 3589. (15) Amiya, T.; Tanaka, T. Macromolecules 1987, 20, 1162. (16) Reference deleted in revision. (17) Kabra, B. G.; Gehrke, H. S.; Spontak, R. J. Macromolecules 1998, 31, 2166. (18) O’Connor, M. S.; Gehrke, H. S. J Appl. Polym. Sci. 1997, 66, 1279. (19) Harsh, D. C.; Gehrke, S. H. J. Controlled Release 1991, 17, 175. (20) Rose´n, O.; Piculell, L. Polymer Gels Networks 1997, 5, 185. (21) Samii, A. A.; Karlstro¨m, G.; Lindman, B. Langmuir 1991, 7, 653.
S0743-7463(97)01168-2 CCC: $15.00 © 1998 American Chemical Society Published on Web 09/12/1998
5796 Langmuir, Vol. 14, No. 20, 1998
Rose´ n et al.
Figure 1. Schematic pictures of the polymers: (a) EHEC; (b) catHM-HEC.
ments are quite useful in the study of polymer-surfactant interactionssa field of considerable current interest.22 In the same spirit, we have, in the present work, extended our studies to even more complex surfactantbinding gels based on a (hydroxyethyl)cellulose (HEC) modified with cationic hydrophobic tails (Figure 1b). This polymer will henceforth be referred to as catHM-HEC. CatHM-HEC has an interesting and rather complex phase behavior in mixtures with ionic surfactants and/or salt,23-26 which nevertheless, may be rationalized quite simply in terms of the binding isotherms of the surfactants and the salt sensitivity of the polymer.25,26 As will be shown below, the surfactant/salt sensitivity of catHM-HEC conveys interesting environmentally sensitive properties to the corresponding cross-linked gels, which can be understood by similar arguments. For comparison, we have made additional studies of the EHEC gels. Finally, we have also made mixed polymer gels of EHEC and catHM-HEC, to illustrate the possibility of combining salt-sensitive and temperature-sensitive properties in the same gel by simply cross-linking the mixture. Both EHEC and catHM-HEC are essentially HEC derivatives (Figure 1), but the differing additional modifications result in different final properties. EHEC (Figure 1a) has extra ethyl groups, which make the polymer slightly hydrophobic. Because of the hydrophobicity, the polymer is weakly self-associating, which results in an enhanced viscosity in a water solution and an ability to associate with ionic surfactants.27-35 The hydrophobicity also brings the LCST of the polymer down to below 100 °C in water. The LCST of the EHEC used in this work (22) Goddard, E. D., Ananthapadmanabhan, K. P., Eds. Interactions of Surfactants with Polymers and Proteins; CRC Press: Boca Raton, FL, 1993. (23) Goddard, E. D.; Leung, P. S. Colloids Surf. 1992, 65, 211. (24) Goddard, E. D.; Leung, P. S. Langmuir 1992, 8, 1499. (25) Guillemet, F.; Piculell, L. J. Phys. Chem. 1995, 99, 9201. (26) Piculell, L.; Guillemet, F.; Thuresson, K.; Shubin, V.; Ericsson, O. Adv. Colloid Interface Sci. 1996, 63, 1. (27) Piculell, L.; Thuresson, K.; Ericsson, O. Faraday Discuss. 1995, 101, 307. (28) Cabane, B.; Lindell, K.; Engstro¨m, S.; Lindman, B. Macromolecules 1996, 29, 3188. (29) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Langmuir 1986, 2, 536. (30) Carlsson, A.; Karlstro¨m, G.; Lindman, B. J. Phys. Chem. 1989, 93, 3673. (31) Carlsson, A.; Karlstro¨m, G.; Lindman, B.; Stenberg, O. Colloid Polym. Sci. 1988, 266, 1031. (32) Carlsson, A.; Karlstro¨m, G.; Lindman, B. Colloid Surf. 1990, 47, 147. (33) Thuresson, K.; So¨derman, O.; Hansson, P.; Wang, G. J. Phys. Chem. 1996, 100, 4909.
is ca. 65 °C.20,36 EHEC is not very sensitive to salt; ca. 3 M of added NaCl is required to bring down the LCST of the EHEC used in this work to room temperature.37 The catHM-HEC used here (Figure 1b) is an N,Ndimethyl-N-dodecyl derivate of HEC. The added side chains make the polymer both hydrophobic and ionic. Molecules of catHM-HEC self-associate via a micelleformation of their hydrophobic groups, and the polymer is water-soluble only due to its charge. This makes the catHM-HEC solutions very sensitive to salt; the addition of 15 mM NaCl is sufficient to cause phase separation.25 On the other hand, catHM-HEC solutions are rather insensitive to temperature changes. Added surfactant molecules bind to the micellar aggregates formed by the hydrophobic side chains of catHM-HEC, thus forming mixed micellar aggregates.25,26 No surfactant binding occurs to the HEC chain itself.38 The salt sensitivity of catHM-HEC causes problems in connection with the chemical cross-linking, since NaOH is needed to catalyze the reaction. To overcome this problem, we have here added surfactant to the reaction medium in order to solubilize the hydrophobic tails and keep the polymer in solution. Experimental Section Materials. EHEC was supplied by Akzo Nobel Surface Chemistry AB, Stenungsund, Sweden. The degrees of substitution of hydroxy(ethyl) and ethyl groups were MSEO ) 1.8 and DSethyl ) 0.6-0.7, respectively, referring to the average numbers of substituents per sugar unit of the polymer. The catHM-HEC, Quatrisoft LM200 (from Amerchol), contains 0.2 mmol of N,Ndimethyl-N-dodecyl chains (with chloride counterions) per gram dry polymer, which corresponds to ca. 5 side chains per 100 glucose units. Both polymers have molecular weights of about 100 000 g/mol. The polymers were purified by Ultrasette filtration with a cutoff of 10 K and then freeze-dried. NaOH (analytical grade from Eka Nobel) and divinyl sulfone (DVS) from Sigma were used in the cross-linking of the polysaccharides. Sodium dodecyl sulfate (SDS) and NaCl were obtained from BDH and used without purification. Cationic alkyltrimethylammonium bromide surfactants (CnTAB) were obtained from TCI-EP and used without further purification. Millipore-filtered water was used in all experiments. (34) Thuresson, K.; Nystro¨m, B.; Wang, G.; Lindman, B. Langmuir 1995, 11, 3730. (35) Thuresson, K.; Lindman, B. J. Phys. Chem. B 1997, 101, 6460. (36) Thuresson, K.; Karlstro¨m, G.; Lindman, B. J. Phys. Chem. 1995, 99, 3823. (37) Thuresson, K.; Nilsson, S.; Lindman, B. Langmuir 1996, 12, 2412. (38) Tanaka, R.; Meadows, J.; Williams, P. A.; Phillips, G. O. Macromolecules 1992, 25, 1304.
Polymer Gels Based on Cellulose Ethers Gel Preparation. The polysaccharides were cross-linked with DVS in alkaline solution.17-20,39 The EHEC and catHMHEC gels were made from stock 2.5% (w/w) solutions of the respective polymer mixed in the desired proportions and diluted to a final concentration of 1%. C12TAB (50 mM) was added to the solutions in order to keep catHM-HEC efficiently solubilized when NaOH was added; cf. above. NaOH (6 mM) was added to catalyze the cross-linking reaction. DVS (85 mM) 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 1.2 mm (i.d.) glass tubes. The gels were cut into approximately 1.2 mm long rods, which were immersed in a large excess of water (with three changes) over 6 days to wash away the C12TAB and other residual chemicals. On the sixth day, the conductivity was measured and found to be the same as for the MilliQ water (ca. 1 µS/cm), which indicates that the surfactants had been efficiently washed away. Except for the lower NaOH concentration, the elevated temperature, and the addition of C12TAB, the gel preparation procedure was the same as in our previous study,20 where we also demonstrated that the DVS cross-links did not influence the binding of surfactant to the gels. This was inferred from the finding that the swelling of a cross-linked gel based on HEC, which does not bind surfactants, was not affected by the presence of 5 or 10 mM SDS in the swelling medium. Swelling Experiments. In the swelling experiments, 4-8 gel rods were immersed in vials containing 8 mL of aqueous solutions of surfactant and/or salt. Owing to the much larger volume of the swelling medium (a factor of 100 or more) compared to the volume of the gels, the fraction of the surfactant bound to the gels was always negligible, except at the lowest concentration of SDS (50 µM) in the experiments with the catHM-HEC gels. The equilibrium concentration of surfactant in the swelling medium was therefore (with the single exception just mentioned) equal to the initial concentration Cs (before immersion of the gels). The gels were allowed to equilibrate for 3 days at room temperature to reach the equilibrium degree of swelling. For the determination of the temperature dependence of the gel swelling, the temperature of the vials containing the gels was controlled using a circulating water bath. The temperature was raised in steps of 5 °C, and after each temperature change, the gels were allowed to equilibrate for 2 h. Those gels that collapsed on increasing temperature did so on a much shorter time scale. The gel swelling ratios are given as V/V0 ) (D/D0),3 where V is the volume and D is the diameter of the gel rod, and the subscript 0 denotes the corresponding value at the preparation stage (D0 ) 1.2 mm). The diameters of the gels were measured with a video camera calibrated with a 0.1 mm scale. Each data point represents the average of ca. six measurements of D on the 4-8 gel rods in a vial, typically showing a variation of ca. 5% (corresponding to a 15% variation in the gel volume).
Results and Discussion Temperature Dependence. In Figure 2 the temperature dependences of the gel volumes are shown for pure EHEC and catHM-HEC gels and for mixed gels in the proportions (by weight) of 3:1 and 1:1 (EHEC:catHMHEC). For the batch of EHEC used in the gels, the cloud point in pure water varies between 63 and 65 °C in the concentration range 0.5-2%.36,40 The corresponding EHEC gels have previously been found to collapse around the same temperature interval.20 The pure EHEC gel in this work confirms the latter result. As the volume transition is continuous for EHEC gels, the transition temperature has here been taken as the inflection point of the curve,19 giving a value near 65 °C. We note in passing that the similarity of the results from this and the previous study, where no surfactant was present during the cross-linking reaction, indicates that the surfactant has been efficiently washed away from the (39) Balazs, E. A. U.S. Patent no. 4582865, 1986. (40) Joabsson, F.; Rose´n, O.; Thuresson, K.; Piculell, L.; Lindman, B. J. Phys. Chem. B, 1998, 102, 2954.
Langmuir, Vol. 14, No. 20, 1998 5797
Figure 2. Gel volumes in pure water for different gel compositions as functions of temperature: pure EHEC (circles), pure catHM-HEC (squares), and mixed EHEC:catHM-HEC gels in the proportions 3:1 (open squares) and 1:1 (triangles). Solid lines are guides for the eye only.
Figure 3. Gel volumes versus the NaCl concentration in the swelling medium for gels of pure EHEC (circles), pure catHMHEC (squares), and EHEC:catHM-HEC in the proportion 1:1 (triangles). Solid lines are guides for the eye only.
EHEC gel in the purification step of the present study. Even small amounts of bound ionic surfactant would give a significant increase in the transition temperature of an EHEC gel.20 The volume of the catHM-HEC gel is not affected by temperature changes. Mixed gels give an intermediate behavior, where the volume transition becomes larger when the EHEC content is increased. The transition of a mixed gel occurs at slightly higher temperatures than for pure EHEC, but the effect is not large (ca. 10 °C for the 1:1 gel). Such a shift is generally observed for LCST gels when ionic groups are incorporated11,19 and, also, for EHEC gels with added ionic surfactant.20 The fact that the effect here is small is probably related to the rather modest degree of ionic substitution of catHM-HEC. This interpretation is further supported by the observation that the swelling of the pure, ionic catHM-HEC gel at low temperatures (below the collapse transition) is only slightly larger than that of the EHEC gel. Addition of Salt. The addition of NaCl to the pure EHEC gel has no large effect on the gel volume until quite high concentrations are reached. This is seen in Figure 3, where there is a decrease in the volume from the salt free case up to ca. 5 mM of salt. Then the volume is unchanged up to about 1 M of salt, when a further decrease in volume sets in. At 3 M of salt the gel is totally collapsed.
5798 Langmuir, Vol. 14, No. 20, 1998
Figure 4. Swelling isotherm for gels of different compositions in SDS solutions: pure EHEC gel (circles), EHEC:catHM-HEC in the proportions 1:1 (triangles), and pure catHM-HEC gel (squares). The symbols in brackets give the swelling in pure water, and the arrow denotes the cmc of SDS. Solid lines are guides for the eye only.
The initial decrease is not understood, but it could indicate the presence of small amounts of charge, of unknown origin, on the EHEC gel. A similar small decrease occurs on adding small amounts of ionic surfactant;20 cf. also below. The volume collapse between 2 and 3 M of salt occurs because the LCST of the linear EHEC is lowered to room temperature at these high salt concentrations.37 For the pure catHM-HEC gel the addition of salt has a stronger effect. The volume decreases continuously until about 40 mM salt. Above this concentration there is no further collapse of the gel. When the two polymers EHEC and catHM-HEC are mixed in a gel in the proportions 1:1, the effect of salt gives a gel volume change that is intermediate between those of the two pure polymer gels. Gels with the proportions 3:1 and 1:3 (not shown for clarity) gave results in between those of the 1:1 mixture and the respective pure gels, thus confirming that the salt sensitivity of a mixed gel can be varied continuously between the limits given by the pure gels. Addition of SDS. The effect of adding SDS to gels from pure EHEC, pure catHM-HEC, and a 1:1 mixture are shown in the swelling isotherms in Figure 4. The major differences in the responses of EHEC and catHMHEC occur at low surfactant concentrations (below ca. 10 mM). In order that these differences may be clearly seen, the surfactant concentration in Figure 4 is given on a logarithmic scale. Again, the mixed gel shows a behavior intermediate between those of the pure gels. The swelling isotherm for the pure EHEC gel reproduces our previous results on EHEC,20 and similar trends have also been found for nonionic gels based on p-NIPA20 or PEO-PAm.12,13 At very low concentrations of SDS, there is a small decrease in the gel volume with added surfactant. This decrease is similar to that found for NaCl in Figure 3, and it may thus be a general salt effect. A significant swelling of the gel starts around the so-called critical association concentration (cac) for SDS and EHEC, which is the concentration where SDS starts to micellize on the EHEC chain, turning the gel effectively into an ionic gel. For the EHEC used here, cac is approximately 2 mM.33 A maximum in the gel volume occurs at a surfactant concentration just above the critical micelle concentration (cmc) for the surfactant in the swelling medium (8 mM). At higher SDS concentrations there is a decrease in gel volume, which is attributed to the fact that further added
Rose´ n et al.
surfactant mainly acts as an added salt, which generally shrinks an ionic gel. The fact that the maximum occurs not at, but slightly above, the cmc has been attributed to the fact that there is no distinct saturation of the binding to the polymer, but the binding continues, albeit to a lesser extent, even after the cmc.13 In pure water, the catHM-HEC gel has approximately the same volume as the EHEC gel. However, a very small amount of added SDS is enough to induce a collapse of the oppositely charged catHM-HEC gel. The collapse corresponds to the associative phase separation (i.e., the formation of a phase concentrated in both polymer and surfactant) that occurs for the linear (non-cross-linked) polymer on addition of SDS. Studies on the latter mixture have shown that SDS binds to the micelles formed by catHM-HEC at very low concentrations and that a phase separation sets in slightly before the global charge equivalence point, where the number of moles of added SDS equals the number of moles of polymer charges.25 For the catHM-HEC used here, and considering the volumes of the gels and of the swelling medium (cf. Experimental Section), the equivalence point would correspond to a concentration of ca. 10 µM of SDS in the swelling medium. The lowest finite concentration investigated here is 50 µM, where the gels are already collapsed. At higher SDS concentrations there is a reswelling of the catHM-HEC gel which, again, agrees nicely with the previous phase studies of the solutions: A “redissolution” of the concentrated phase occurred when the free SDS concentration was 7 mM,25 a concentration that coincides with the large volume increase for the catHM-HEC gel in Figure 4. As was explained in connection with the phase studies of catHM-HEC and SDS,25 the reswelling/redissolution is a consequence of a massive, cooperative binding of SDS to the catHM moieties, which occurs as the concentation of free SDS approaches the bulk cmc. This cooperative part of the surfactant binding isotherm is quite general and may be seen as essentially a self-assembly of the SDS micelles, aided by the presence of the hydrophobes of the catHM-HEC. The cooperative binding has two consequences, which both contribute to the swelling/ redissolution: First, the amount of charged groups on the gel increases rapidly. Second, the physical polymerpolymer cross-links, created by the hydrophobic association of the catHM moieties into micelles, become progressively dissolved as the average number of catHM moieties in a mixed micelle rapidly decreases due to the dilution with SDS. Finally, as for the EHEC gel, and for the same reason, Figure 4 shows that a maximum in the swelling of the catHM-HEC-containing gels occurs around the bulk cmc of SDS: The SDS binding must level off as free micelles are created in the solution. We note that the maximum swelling is lower for catHM-HEC than for EHEC; this cannot be predicted by the simple arguments about the nature of the surfactant binding given above. A prediction of the maximum degree of swelling would require more detailed knowledge about the gel structure, and the degree of surfactant binding at saturation. Such investigations are, however, beyond the scope of the present investigation, where the focus is on the generic swelling/collapse behavior, and on the possibilities to tune this response by choice of surfactant, addition of salt, or choice of polymer (mixture) in the gel. We will now digress for a moment to make some detailed comparisons between the earlier phase studies and the present gel swelling studies of the catHM-HEC/SDS systems, to illustrate the nature and the consequences of
Polymer Gels Based on Cellulose Ethers
Langmuir, Vol. 14, No. 20, 1998 5799
Figure 6. Swelling isotherm for a pure catHM-HEC gel at different SDS concentrations in the absence of NaCl (circles), at 10 mM of NaCl (open squares), and at 50 mM NaCl (triangles). Arrows denote the cmc for SDS at the respective salt concentrations, and the symbols in brackets give the swelling in pure salt solution. Solid lines are guides for the eye only.
Figure 5. Comparisons between phase separation and gel swelling experiments for pure catHM-HEC at different concentrations of free SDS: (a) relative volume of the concentrated phase in a phase-separated mixture (open circles) compared with the relative swelling of a cross-linked gel (filled circles); (b) binding isotherm for the phase-separated mixtures. Data for the phase-separated mixtures are taken from ref 25. Solid lines are guides for the eye only.
the surfactant binding isotherm. Figure 5a shows the early part of the swelling isotherm for catHM-HEC in SDS, now plotted on a linear concentration scale, together with data from ref 25 on the swelling of the concentrated phase in mixtures of SDS/catHM-HEC. The horizontal axis gives the free SDS concentration Cs,f which, for the gels, is the concentration in the swelling medium, Cs, and, for the solutions, the SDS concentration in the dilute phase of the phase-separated system. In the latter systems, the dilute phase may be viewed as a swelling medium for the concentrated phase, which contains most of the polymer. All data pertain to systems with an excess of SDS relative to the number of cationic hydrophobes of catHM-HEC. This means that SDS dominates in the mixed micelles formed by the surfactant molecules and the catHM moieties. Both experiments in Figure 5a show the same trends, which can be understood in terms of the binding isotherm of SDS to catHM-HEC (data from ref 25) displayed in Figure 5b: At low SDS concentrations, there is a rapid increase in the binding of SDS with increasing surfactant concentration, which gives rise to a corresponding rapid swelling of both the gel and the polymer bottom phase. Above ca. 1 mM free SDS, the binding starts to level off. This anticooperativity in the binding occurs because it becomes increasingly more difficult to add more SDS to the increasingly negatively charged mixed micelles. The swelling is here also less pronounced. As might be expected, the relative increase in the swelling in this region is smaller for the chemically cross-linked gel than for the concentrated solution phase of the linear polymer.
The final, cooperative, branch of the binding isotherm, which was discussed in connection with Figure 5a above, was obviously not accessible in the solution studies, since redissolution occurred, and the solutions became monophasic. This is why this part of the binding isotherm is absent in Figure 5 and was only conjectured in ref 25. The gel experiments in Figure 4, however, clearly demonstrate that the cooperative binding occurs. Gel experiments are thus a nice complement to phase studies for the study of polymer/surfactant interactions, since they show changes that occur under conditions where the corresponding solutions would be monophasic. This point will be further illustrated below. Addition of SDS in the Presence of Salt. Figure 6 shows how the swelling of a pure catHM-HEC gel varies with the SDS concentration in a swelling medium that, in addition, contains a constant concentration of simple salt. Examples for 10 and 50 mM NaCl are compared with the results for pure water. At low and at high SDS concentrations, the added salt in the swelling medium leads to a shrinking of the gel, a normal salt effect. However, the trend is reversed at intermediate SDS concentrations. From the different curves in Figure 6 it is evident that the addition of salt to an initially salt-free swelling medium containing a constant SDS concentration in the range 2-6 mM leads to an initial swelling of the gel, before the normal deswelling sets in. This complicated behavior is due to the salt-induced shift of the cmc of an ionic surfactant. Added salt lowers the cmc and, consequently, the related cooperative branch of the binding isotherm of the surfactant. The same reverse salt effect was found in the phase behavior of the linear polymer, where the redissolution of the concentrated phase occurred at a free surfactant concentration of 5 mM in 10 mM NaCl, as compared to 7 mM in the salt-free solution.25 A redissolution concentration of 5 mM compares favorably to the SDS concentration at the inflection point of the swelling isotherm in 10 mM NaCl in Figure 6. At 50 mM of NaCl, the surfactant-induced reswelling sets in even earlier. As already pointed out, the level of the gel swelling at high SDS concentrations (above the cmc) decreases with added salt. Moreover, at the highest salt concentration, there is no longer a maximum in the swelling, but rather a leveling off. The flattening of the maximum with increasing salt is rather expected, since the contribution from added surfactant to the total electrolyte concentra-
5800 Langmuir, Vol. 14, No. 20, 1998
Figure 7. Swelling isotherm for a gel of pure EHEC in solutions of the cationic surfactants C16TAB (circles), C14TAB (squares), and C12TAB (triangles). Vertical arrows denote the bulk cmc values (from ref 42). Solid lines are guides for the eye only.
tion, which gives rise to the screening and the deswelling, becomes less and less significant when more and more salt is already present. Addition of Cationic Surfactant. Figure 7 shows swelling isotherms for pure EHEC gels on addition of cationic surfactants. The features are the same as for SDS. In particular, the results for the different alkyl chain lengths confirm that the maximum swelling occurs above, but close to the bulk cmc of the surfactant. A longer alkyl chain (a lower cmc) thus gives an earlier swelling. However, the results in Figure 7 also show that the maximum swelling is larger: C12TAB gives only a small volume change, C14TAB a more pronounced swelling, and the swelling with C16TAB is much larger, even larger than with SDS (Figure 4). The differences in swelling at the maximum should at least partly be due to differences in the concentration of free surfactant, which acts as a screening electrolyte. As we have seen, the surfactant concentration Cs at the maximum is roughly equal to the cmc, and the cmc is higher for a surfactant with a shorter alkyl chain. However, it is also possible that the maximum degree of binding is higher for a surfactant with a longer alkyl chain. This would also contribute to an increased swelling. We note that Wada et al. also found, in another gel/surfactant system, that the cmc of the surfactant was crucial for the swelling.41 Figure 8 shows the corresponding experiments for pure catHM-HEC gels with five homologous cationic surfactants, with cmc values as indicated in the figure. A clear pattern is again seen, which, however, is different in important respects from the one in Figure 7. For surfactants with short alkyl chains, the variation of the gel swelling on increasing the surfactant concentration is markedly nonmonotonic, beginning with a gradual shrinking before a reswelling sets in at a concentration close to the bulk cmc. Note also that the results more or less coincide for all the different surfactants in the initial shrinking regime. As in the previous cases, this behavior reflects the expected binding isotherm of the surfactant. At concentrations far below the cmc, the binding of a cationic surfactant to the micelles formed by the catHM moieties is weak. The effect of added cationic surfactant (41) Wada, N.; Kajima, Y.; Yagi, Y.; Inomata, H.; Saito, S. Langmuir 1993, 9, 46. (42) Mukerjee, P.; Mysels, K. J. Critical Micelle Concentrations of Aqueous Surfactant Systems; National Bureau of Standards: Washington, DC, 1971.
Rose´ n et al.
Figure 8. Swelling isotherm for a gel of pure catHM-HEC gel in solutions of the cationic surfactants C16TAB (filled circles), C14TAB (open squares), C12TAB (triangles), C10TAB (open circles), and C8TAB (filled squares). Vertical arrows denote bulk cmc values (from ref 42). Solid lines are guides for the eye only.
is therefore initially very similar to that of simple NaCl, i.e., a continuous, salt-induced deswelling. As the bulk cmc is approached, however, the cooperative surfactant binding sets in, just as for SDS, resulting in an increased charge and, eventually, a solubilization of the mixed micellar cross-links connecting the catHM-HEC chains. A pronounced collapse of the gel before the reswelling occurs only for cationic surfactants with a chain length of C12 or shorter. For C14TAB and C16TAB, the minima preceding the maxima at the respective cmc’s are quite shallow, or insignificant. The same trends have been observed in the phase behavior of linear catHM-HEC mixed with the various cationic surfactants:24,26 No phase separation occurred with C14TAB and C16TAB, but there was an associative phase separation followed by a redissolution for C12TAB, C10TAB, and C8TAB. Moreover, the width of the phase separation region increased with a decreasing length of the surfactant alkyl chain. Concluding Remarks We have here illustrated the possibility of designing tunable responsive gels based on two surfactant-binding cellulose derivatives and their mixtures. We have succeeded in chemically cross-linking the salt-sensitive catHM-HEC by adding surfactants to the reaction medium to solubilize the hydrophobic moieties on the polymer. The catHM-HEC gel showed interesting responsive properties with surfactants, and it should be suitable also for incorporation of proteins, because of its cationic and hydrophobic character. Indeed, preliminary experiments in our laboratory confirm that catHM-HEC gels bind the anionic protein β-lactoglobuline. It was found possible to mix catHM-HEC with EHEC in all proportions in 1% gels, and the resulting mixed gels proved to have properties intermediate between those of the pure polymer gels. The reason that such a simple blending rule holds is probably that the backbones of the two polymers are so similar. Nevertheless, as the catHMHEC gel and the EHEC gel respond differently to different influences (temperature and salt), the mixing of these polymers gives a possibility to customize gels, made from natural polymers, that respond in some desired fashion to changes in temperature, salt content, or surfactant concentration. The polymers used to make the gels in this study are complex cellulose derivatives, with a large heterogeneity
Polymer Gels Based on Cellulose Ethers
in their substitution patterns. Still, their qualitative swelling behavior in solutions of ionic surfactants, with or without added salt, may be predicted by relatively simple arguments. This is because most of the behavior is governed by the surfactant properties: The overall features of the polymer/surfactant binding isotherm are determined by the cmc of the surfactant, and thus show quite universal features. The important properties to know about the polymer are its charge and its salt sensitivity. In the present work, this point is illustrated particularly clearly in the results for catHM-HEC with short-chain cationic surfactants in Figure 8. Finally, we conclude that experiments on chemically cross-linked gels provide valuable information on polymer/ surfactant interactions, which are complementary to those obtained from the phase diagrams for the non-cross-linked
Langmuir, Vol. 14, No. 20, 1998 5801
systems. The occurrence of phase separation phenomena, and the developments of the phases on changing temperatures or composition, provide invaluable information on the interactions in the systems. However, phase studies are insensitive to changes occurring in single-phase solutions. The present gel studies emphasize the fact that the processes leading to transitions between monophasic and biphasic solutions continue also inside the monophasic region. Acknowledgment. We thank Ingegerd Lind for valuable technical assistance. This work was supported by the Swedish Research Council for Engineering Sciences (TFR). LA971168+
