Solubilization of Phenols in Anionic Polyelectrolyte Gels with

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Solubilization of Phenols in Anionic Polyelectrolyte Gels with Adsorbed Cationic Surfactant Matthew Baumgart, Michael Lindley, Drew Wright,† and Mark R. Anklam* Department of Chemical Engineering, Rose-Hulman Institute of Technology, 5500 Wabash Avenue, Terre Haute, Indiana 47803 Received January 19, 2005. In Final Form: March 16, 2005 Solubilization isotherms for various phenols in cetylpyridinium chloride (CPC)-polyelectrolyte gel aggregates have been determined in order to compare solubilization within these aggregates with that in free micelles and to examine the effects of gel chemistry and structure on solubilization. The isotherms describing solubilization are quite similar to those found for free surfactant in solution. Solutes that are more hydrophobic give rise to larger solubilization constants with trends similar to what is seen for hydrophobic effects in adsorption from aqueous solutions onto hydrophobic solids. The solubilization constants decrease as the fraction of solute in the aggregates increases, indicating that the solutes partition into the palisade region of the aggregates. Solubilization is found to be quite insensitive to changes in gel structure (cross-linker varying from 1% to 3%) and chemistry (poly(acrylic acid) versus poly(methacrylic acid) and neutralization from 50% to 100%). However, the switch from poly(acrylic acid) to poly(methacrylic acid) did give rise to a slight decrease in magnitude of the slope of the isotherm. The most significant factors appear to be the initial concentration of surfactant in solution and the ratio of surfactant solution to gel amount. A decrease in surfactant concentration (especially combined with an increase in solution volume) gives rise to a decrease in solubilization constants.

Introduction It is well known that micelles in aqueous surfactant solutions can solubilize organic compounds. Micellar solubilization has numerous applications, including separations, and one difficulty with separations is the retrieval of the micelles and the solutes within them. The separations may require cost-intensive procedures such as membrane separations,1,2 electrophoresis, and ultracentrifugation. Properly designed immobilization or containment of the micelles within larger particles would simplify retrieval and handling of the organic by allowing techniques such as filtration or packed bed “adsorption” to be used. One such method to immobilize micelles is to have them contained in polyelectrolyte gels. A simplified illustration of such a micelle is shown in Figure 1(a). The research presented here is a study of solubilization of phenols within micellar aggregates contained in hydrophilic, polyelectrolyte gels: poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMA). The study includes comparisons with solubilization in free micelles, as well as an examination of the effects of gel chemistry and structure on solubilization. Clearly, the interaction of the surfactant with the polyelectrolyte chain will affect solubilization within that aggregate. Studies on the interaction of oppositely charged surfactants with free polyelectrolytes have been fairly extensive.3-9 An increase in concentration of polymer leads * Author to whom correspondence should be addressed. E-mail: [email protected]. † Current address: Department of Chemical Engineering, Northeastern University, Boston, MA 02115. (1) Komesvarakul, N.; Scamehorn, J. F.; Gecol, H. Sep. Sci. Technol. 2003, 38, 2465. (2) Dunn, R. O.; Scamehorn, J. F.; Christian, S. D. Sep. Sci. Technol. 1985, 20, 257. (3) Hansson, P.; Almgren, M. J. Phys. Chem. 1995, 99, 16684. (4) Kogej, K.; Evmenenko, G.; Theunissen, E.; Berghmans, H.; Reynaers, H. Langmuir 2001, 17, 3175. (5) Anthony, O.; Zana, R. Langmuir 1996, 12, 1967. (6) Kiefer, J. J.; Somasundaran, P.; Ananthapadmanabhan, K. P. Langmuir 1993, 9, 1187.

Figure 1. Schematic illustrations of micellar aggregates. (a) A simplified 2D illustration showing an anionic, polyelectrolyte chain wrapped around a micelle of cationic surfactants. The shaded circles represent the solubilized molecules. (b) An illustration of cross-linked polyelectrolyte chains folded around micelles. Note that the presence of micelles leads to gel volume reduction.

to an increase in the critical aggregation concentration (cac).3,10 However, for hydrophilic polyelectrolytes (as discussed here), the presence and concentration of polyelectrolyte have little effect on the aggregation number.3,4 Also, the concentration of surfactant has little to no effect on aggregation number.4,5 Surfactant interactions with polymer also depend on the chemistry of the polyelectrolyte. The hydrophobicity and the charge density of the polymer chain can affect the degree of binding, the cac, and the aggregation number.5,6,9 For the two polymers to be compared in this work, PAA and PMA, the difference between bound micellar structure has been found to be small,4 with a slight difference in cac and a difference in behavior at low degrees of neutralization.6,7 In our work here, the polymers are neutralized to at least 50% so the peculiar behavior of PMA at low degrees of neutralization should be avoided. Surfactants form micelles or aggregates in polyelectrolyte gels of opposite charge in much the same way as (7) Kogej, K.; Theunissen, E.; Reynaers, H. Langmuir 2002, 18, 8799. (8) Konop, A. J.; Colby, R. H. Langmuir 1999, 15, 58. (9) Sasaki, S.; Yamazoe, Y.; Maeda, H. Langmuir 1997, 13, 6135.

10.1021/la050155q CCC: $30.25 © 2005 American Chemical Society Published on Web 04/15/2005

Solubilization of Phenols in Anionic Polyectrolyte Gels

they do in solutions with free polyelectrolyte. Figure 1(b) gives an illustration of cross-linked, polyelectrolyte chains folded around micelles. The adsorbed surfactants concentrate in the gel and form micelle-like aggregates even where equilibrium bulk concentrations are well below the critical micelle concentration (cmc).11 The adsorption of surfactants in the gel causes a decrease in osmotic pressure and an interaction among the hydrophobic tails of the surfactants, leading to gel collapse. Researchers have studied these aggregates mainly for their interesting effects on gel swelling and phase behavior11-15 and to develop a better understanding of the nature of the surfactant-gel complex.10,12,13,15,16 Hansson10 made comparisons to show that the aggregates in gels behave in much the same way as they do for free polyelectrolyte solutions, provided that the cross-link density is not too large. The main difference is that polymer concentration for gels is dictated by the cross-link density and the degree of swelling, while concentration of free polymer in solution can be readily varied. Thus, as more surfactant adsorbs, the gel volume decreases and the concentration of polymer increases. The concentration of cross-links in the gel only affects surfactant adsorption by the effect it has on polymer concentration. Although there have been some solubilization studies for surfactant with free polyelectrolyte,17-21 there has been very little work with polyelectrolyte gels to confirm that behavior observed with free micelles and micelles with free polyelectrolyte is also observed for micelles within gels. Solubilization of organic compounds into micellar aggregates within gels and ion-exchange resins has been demonstrated,11,12,16,21-23 and solubilization isotherms have been constructed for a few ion-exchange resins.24,25 However, solubilization for hydrophilic, polyelectrolyte gels and the effects of gel chemistry have not been studied in any detail (other than our own preliminary work26). Solubilization of low to moderate polarity molecules in a micellar solution may be expressed as equilibrium between free solubilizate and solubilizate within the micelles, where the surfactant molecules in the micelles are in equilibrium with free surfactant molecules. Solubilization can be affected both by how much surfactant adsorbs (i.e., how much complexing occurs) and by the solute partitioning into each complex. Solubilization isotherms can be constructed showing how the solubilization constant (K) varies with the mole fraction of (10) Hansson, P. Langmuir 1998, 14, 2269. (11) Khokhlov, A.; Starodubtzev, S.; Vasilevskaya, V. V. In Responsive Gels: Volume Transitions I; Dusek, K., Ed.; 1993; p 123. (12) Khokhlov, A. R.; Kramarenko, E. Yu.; Makhaeva, E. E,; Starodubtzev, S. G. Macromolecules 1992, 25, 4779. (13) Shirahama, K.; Sato, S.; Niino, M.; Takisawa, N. Colloids Surf., A 1996, 112, 233. (14) Philippova, O. E.; Hourdet, D.; Audebert, R.; Khokhlov, A. R. Macromolecules 1996, 29, 2822. (15) Hansson, P. Langmuir 1998, 14, 4059. (16) Philippova, O. E.; Starodoubtzev, S. G. J. Polym. Sci. B 1993, 31, 1471. (17) Lee, B.-H.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1991, 7, 1332. (18) Hayakawa, K.; Satake, I.; Kwak, J. C. T. Colloid. Polym. Sci. 1994, 272, 876. (19) Hayakawa, K.; Fukutome, T.; Satake, I. Langmuir 1990, 6, 1495. (20) Uchiyama, H.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. AIChE J. 1994, 40, 1969. (21) Sudbeck, E. A.; Dubin, P. L.; Curran, M. E.; Skelton, J. J. Colloid Interface Sci. 1991, 142, 512. (22) Yim, C. T.; Brown, G. R. Langmuir 1994, 10, 4195. (23) Reid, K. R.; Kennedy, L. J.; Crick, E. W.; Conte, E. D. J. Chromatogr. A 2002, 975, 135. (24) Cheng, H.; Sabatini, D. A. Water Res. 2002, 36, 2062. (25) Yan, X.; Janout, V.; Regen, S. L. Macromolecules 2002, 35, 8243. (26) Anklam, M. R.; Wright, D. Proceedings of the ACS National Meeting in Boston, 2002.

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solubilizate in the aggregates (X), where K ) X/c and c is the concentration of free solubilizate in the bulk aqueous phase.17,20,27-29 The isotherms will not only give some indication as to how much solute is solubilized but also where or how the solute is solubilized within the aggregates (i.e., core versus palisade regions) from the slope of K versus X.27,28 In this study we investigate the solubilization properties of aggregates formed between a cationic surfactant, cetylpyridinium chloride (CPC), and the anionic, polyelectrolyte gels, PAA and PMA. We examine effects of gel structure (cross-link density), as well as chemistry (monomer type and degree of neutralization), and examine some effects of polymerization conditions and solution/gel ratios. Primarily, we were interested in whether the solubilization isotherms would be comparable to those for free micelles27 and whether small changes in gel and surfactant chemistry would have significant effects on solubilization. Experimental Section Chemicals. CPC from Sigma (99+%) was used as received. For the gels, N′,N-methylenebisacrylamide (BIS) and N,N,N′,N′tetramethylethylenediamine (TEMED) from Sigma and acrylic acid (AA), methacrylic acid (MAA), and ammonium persulfate from Aldrich were all used as received. For the solubilizates, phenol, 2-ethylphenol, and 4-methylphenol (p-cresol) (all from Aldrich) were used as received. Most comparisons of gel structure and chemistry were done using 2-ethylphenol. All chemicals were 99+% except ammonium persulfate at 98+%. Methods. A 25% monomer solution of either MAA or AA as the primary monomer and BIS as the cross-linking agent was polymerized using ammonium persulfate as the initiator and TEMED as the accelerator. Cross-linker was added at either 1% or 3% of the total monomer amount. The solution included enough sodium hydroxide to neutralize 50% of the primary monomer. pH measurements of AA solutions confirmed 50% neutrality for this method, as the pH values averaged 4.27 compared to a pKa of 4.2. Unless otherwise noted, solutions were not degassed prior to polymerization. However, for those few solutions that were degassed, the solution was degassed prior to the addition of ammonium persulfate and TEMED. Measured quantities of the solution were added to glass tubes (8 mm diameter) with a layer of mineral oil on top to prevent evaporation. The tubes were placed in a water bath at 65 °C for 18-24 h. After the gels formed, the test tubes were broken open and the gel was cut into disks which were weighed. The disks were then placed into deionized water and allowed to soak for 1 or 2 weeks with the water being replaced every day or two to purify the gels. For the 100% neutralized gels, the last wash contained enough sodium hydroxide to neutralize the remaining 50% of the acid groups in the gel. After the gels were washed, solution with known concentrations of surfactant and/or solubilizate was added to the gel at a ratio of 150 L/mol of monomer (unless otherwise noted). The gels in solution were then allowed to equilibrate for more than 4 weeks (found from preliminary experiments to be adequate time to reach steady, external concentrations). Solutions external to the gels generally had pH values in the range of 7.8-8.3, confirming phenols were not in a charged form. The pH tended to decrease with increasing phenol concentration. pH values internal to the gel were not determined. The concentrations of surfactant and solubilizate in the bulk solution were determined from UV spectroscopy (Perkin-Elmer Lambda 35) using measurements at multiple wavelengths (45) between 255 and 275 nm depending on the solute. Solutions were diluted as appropriate to ensure linearity of absorbance. Error bars shown on the figures are based on the estimated uncertainties for the concentration measurements. A simple (27) Lee, B.-H.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. J. Phys. Chem. 1991, 95, 360. (28) Lee, B.-H.; Christian, S. D.; Tucker, E. E.; Scamehorn, J. F. Langmuir 1990, 6, 230. (29) Kitiyanan, B.; O’Haver, J. H.; Harwell, J. H.; Osuwan, S. Langmuir 1996, 12, 2162.

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Figure 2. Typical plot of adsorbed amount of CPC in a gel versus the initial concentration of CPC in solution. The dashed curve shows ideal adsorbed amounts assuming 100% adsorption. The data are for the PAA gel with 1% cross-linker and 50% neutralization. material balance was then used to determine the amount of surfactant and phenol in the gel. Swell ratios (mass of gel/mass of polymer) were also measured to account for the water initially in the gel before the addition of surfactant solution.

Results and Discussion Adsorption of CPC in gels both in the absence and presence of solubilizate was measured in order to determine if any CPC remained in solution. Figure 2 shows a typical plot of β versus initial CPC concentration where β is the molar ratio of adsorbed CPC to repeat unit in the gel (not including cross-linker). The dashed line shows the relationship for complete adsorption of CPC. In every case, the concentration of surfactant left in solution was below the limit of precision for the spectrophotometer even for initial CPC concentrations well above those used in the solubilization studies. Thus, for the purpose of calculating the ratios of solute to surfactant in the gel, the surfactant can be considered as completely adsorbed. It should be noted that β values greater than 0.5 were observed even for the 50% neutralized gels (as seen in Figure 2). Thus, neutralization does not seem to limit the amount of CPC adsorbed. This is not that surprising because surfactant does not uniformly distribute itself throughout the gel.30 We observe that when gels collapse due to surfactant adsorption, the collapse is not uniform. Corners and small pieces collapse preferentially, and if the solubilizate is colored, it can be seen to partition into these regions and not to the bulk of the gel. This partitioning persists over long timesssurfactant/solute does not redistribute or diffuse. Most solubilization experiments in our studies were performed with CPC at 2.5 mM (initially), so that β would be less than 0.5 in all cases. Since surfactant aggregates within PAA and PMA gels are known to behave similarly to micelles in bulk solution,4 experiments were performed to compare solubilization within the aggregates to published data on micellar solubilization.27 Figure 3 shows isotherms for phenol, 4-methylphenol, and 2-ethylphenol in aggregates of CPC and PMA gels with 50% neutralization. These are compared with solubilization isotherms for phenol, 4-methylphenol, and 4-ethylphenol in bulk CPC micelles. K1/2 (rather than K) was plotted by Lee et al.27 because this gave a linear plot for their data. The trends in the isotherms are consistentsnegative slopes (although only slightly negative for phenol and 4-methyphenol in the gel aggregates) with the magnitude of the slopes increasing for more hydrophobic solutes. Also, the K values increase (30) Hansson, P.; Schneider, S.; Lindman, B. J. Phys. Chem. B 2002, 106, 9777.

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Figure 3. Solubilization isotherms for various phenols. Data points for solutes in PMA gels with 3% cross-linker and 50% neutralization and CPC initially at 2.5 mM: phenol (triangles), 4-methylphenol (squares), and 2-ethylphenol (diamonds). The solid lines are fitted curves taken from Lee et al.27 for free micelles. The solutes from bottom to top are phenol, 4-methylphenol, and 4-ethylphenol.

as solute hydrophobicity increases. The negative slopes are typical of relatively polar solutes and are indicative of solubilization into the palisade region of the micelle or aggregate.27,28 It is clear that the K values from the aggregates in the gels are somewhat larger than those for the free micelles; however, K values are sensitive to changes in the concentration of surfactant and the surfactant-to-solute ratio,31 so direct, quantitative comparisons between our data and the literature data are not appropriate. However, it should be noted that an increase in K values due to the presence of polyelectrolyte has been observed for chlorophenols.17 The plot of K1/2 versus X as used by Lee et al. does not have a theoretical basis and was used by the authors because of the apparent linear fit of the data plotted in this way. The solubilization isotherms for gel aggregates do not appear to be linear when plotted in this way, although most of the plots of K versus X can be fit by a quadratic expression as has been observed for surfactant/polyelectrolyte mixtures.17 We examined whether a Langmuir isotherm could accurately describe the solubilization behavior. As mentioned above, the negative slope for the 2-ethylphenol (2EP) solute in Figure 3 implies that the solute is located in the palisade region of the aggregates near the surface headgroups. For relatively small values of X (less than 0.5), one can imagine that this would give rise to a monolayer of sites between the surfactant headgroups. If the equation

q)

Sc KL + c

(1)

accurately describes the isotherm, then a plot of 1/q versus 1/c (or c/q versus c) should be linear. Here q is the adsorbed concentration ratio of solute in moles of solubilizate to moles of surfactant, c is the equilibrium solubilizate concentration in solution, and S and KL are constants with S being the asymptotic value of q at large c. The systems studied here do indeed give reasonably linear fits for 1/q versus 1/c, and Figure 4 shows one example of this. Values for S tend to be in the range of 1-3, which suggests that the description by a Langmuir isotherm may break down at higher concentrations. Langmuir behavior for phenols in free micelles has also been observed.27 The effect of solute hydrophobicity on solubilization or adsorption from aqueous solutions can be analyzed on the (31) Christian, S. D.; Smith, G. A.; Tucker, E. E. Langmuir 1985, 1, 564.

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Figure 4. Solubilization data for 2EP in CPC/PAA with 3% cross-linker and 50% neutralization plotted for a fit with a Langmuir adsorption isotherm. CPC initially at 2.5 mM.

basis of thermodynamic expressions.32 Solubilization is treated as a pseudoreaction such that

Figure 5. The effect of the number of carbons in the alkyl group attached to phenol on the low concentration limit solubilization. The q/c values are found from Langmuir isotherm fits to the data shown in Figure 3. The line is the linear fit through the points.

P + M T PM where P is the solute (a phenol in this case), M is the micelle, and PM is the micelle with solubilized solute. Then, the standard free energy change can be written as

∆G° ) -RT ln Keq

(2)

where the equilibrium constant, Keq, is given by the ratio of activities

Keq )

aPM aPaM

(3)

Assuming a negligible change in activity coefficients and constant aM with changing solute concentration, then Keq ∝ q/c where q/c is determined in the linear range of the isotherm (low concentration limit). The effect of hydrophobic interactions from methylene groups on the standard free energy change can be written as

∆G° ) ∆G°′ + nφC

(4)

where ∆G°′ is the standard free energy change of the molecule without the alkyl group under consideration (phenol), n is the number of carbons in the alkyl group added to this molecule (e.g., n changes from 1 to 2 when going from methylphenol to ethylphenol), and φC is the free energy change per methylene group. Since hydrophobic interactions are responsible for solubilization, eqs 2 and 4 can be combined with the proportionality relationship between Keq and q/c to give32

ln(q/c) ) ln(q/c)′ -

nφC RT

(5)

Langmuir isotherms were used to determine q/c for the data in Figure 2, and a plot of ln(q/c) versus n was produced and is shown in Figure 5. An average value for φC is determined to be -0.94RT. This is comparable to values found for phenol and methylphenol adsorbing from water onto a polycarboxylic ester resin driven by hydrophobic interactions (-1.05RT) and for phenyl alkanols adsorbing from water onto the same resin (-0.83RT).32 Note that, even though the location of the alkyl group on phenol changes for 4-methylphenol to 2-ethylphenol and one of the carbons added is actually from a methyl group and not a methylene group, φC does not appear to be affected, as can be seen from the constant slope in Figure 5. Similar (32) Maity, N.; Payne, G. F. Langmuir 1991, 7, 1247.

Figure 6. Solubilization isotherms showing effects of crosslinker amount and solution deaeration. 2EP in CPC/PAA with 50% neutralization and 3% cross-linker + no dearation (diamonds), 3% cross-linker + deaeration (triangles), and 1% crosslinker + deaeration (squares). Error bars shown for one set of data are approximately the same as those for other sets. CPC initially at 2.5 mM.

results to the former were seen by Mardis et al. where the location of a the methoxy group in methoxy phenol did not significantly affect ∆G° for adsorption from water.33 Figure 6 shows the effects of gel structure (the level of cross-linker and degassing prior to polymerization) for three sets of gels. Although the cross-linker level is known to significantly affect gel structure and swelling and deaeration can in some cases affect gel structure, these factors do not appear to significantly affect solubilization within the aggregates. This confirms observations in the literature that aggregates are not significantly affected by gel cross-linker levels over a large range10sthe distance between cross-links is large enough that the surfactant molecules interact with the polymer chains without “seeing” the cross-links. Figure 7 shows solubilization isotherms for PAA gels at 50% and 100% neutralization. Although there are some small differences between isotherms, there is no consistent effect of neutralization on measured solubilization. In Figure 7(a), larger K values are found for the more neutralized PAA gels, but the trend is reversed for PAA gels at a lower cross-linker level in Figure 7(b). The change in neutralization is not expected to affect solubilization since the amount of adsorbed surfactant remains the same and the aggregate structure is unlikely to change. Thus, the observed changes in K are most likely due to batch to batch variations. A small change in gel chemistry from PAA to PMA does not appear to lead an overall shift in K values, as shown in Figure 8, but there does appear to be slight differences (33) Mardis, K. L.; Glemza, A. J.; Brune, B. J.; Payne, G. F.; Gilson, M. K. J. Phys. Chem. B 1999, 103, 9879.

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Figure 7. Solubilization isotherms showing effects of neutralization. 2EP with CPC initially at 2.5 mM. (a) PAA gels with 50% neutralization (filled squares) and 100% neutralization (open squares). 3% cross-linker in gels. (b) PAA gels with 50% neutralization (filled squares) and 100% neutralization (open squares). 1% cross-linker in gels.

Figure 8. Solubilization isotherms showing effects of gel chemistry, CPC concentration, and a combination of CPC concentration decrease with solution volume increase. 2EP in gels with 50% neutralization and 3% cross-linker. PMA (open) and PAA (filled) with 1.5 mM CPC (squares) and 2.5 mM CPC (diamonds), and 3.25 mM CPC (triangle, PAA only). PMA with 0.833 mM CPC and 3X solution volume (open circles).

in slopes between isotherms with PMA versus PAA both at 2.5 mM and 1.5 mM CPC. Linear regression analyses for the four sets of data gave slopes of 1940 ( 230 and 1740 ( 420 M-1 for the PAA gels with 2.5 mM and 1.5 mM CPC, respectively, and 1360 ( 220 and 1370 ( 280 M-1 for the PMA gels with 2.5 mM and 1.5 mM CPC, respectively. Although, the data are not necessarily best fit with a line when plotting K versus X (the data are generally best fit with a quadratic expression and thus the large 95% uncertainties in slope) the slopes for 2.5 mM are significantly different and the trend (though not significant) is repeated at 1.5 mM. A decrease in magnitude of the slope (with a relatively constant average K) is generally observed when solute polarity decreases.27 In this case, it is due to the slight increase in hydrophobicity of the polymer chain near the surfactant headgroups in the aggregates. Evidently, however, the small difference in hydrophobicities between the gels is not enough to cause a significant difference between the average levels of solubilization (average K) in the aggregates across the range of Xsonly the slope appears to be affected. This corresponds with observations that aggregation number and micelle size (not including bound polymer) do not depend on the presence or amount of these polymers.3,4 Both polymers are quite hydrophilic overall, and the aggregates appear to be mostly insensitive to these small changes in gel chemistry. The other significant, consistent effects that were observed were not for variations in gel chemistry and structure but were for variations of surfactant level and solution amount. Figure 8 shows that when the surfactant concentration is varied from 1.5 to 3.25 mM for PAA and from 1.5 to 2.5 mM for PMA, there is a small but consistent increase in K. Thus, as the number of aggregates decreases

Figure 9. Plot of solubilized amount versus initial 2EP concentration in PMA gels with in gels with 50% neutralization and 3% cross-linker. 2.5 mM CPC with standard solution volume (squares) and 0.833 mM with 3X standard solution volume (diamonds).

(when the CPC concentration decreases), the same ratio of surfactant to solute in the aggregate (constant X) occurs at higher bulk solute concentrations (K decreases and K ) X/c). As mentioned earlier, the K value can change if the ratio of surfactant to solute changes.31 However, this change in K may be partly due to the fact that not all surfactant in the gel exists in aggregates, and these surfactant molecules will not contribute to solubilization. The nonaggregated, adsorbed surfactant represents a larger percentage of the total adsorbed surfactant for the lower concentrations. It is unclear, though, if the amount of nonaggregated, adsorbed surfactant would be enough to cause the shift in K values. In one experiment, we added CPC at one-third the concentration (0.83 mM) but with 3 times the ratio of solution to gel (SMA). This concentration is slightly lower than the cmc in the absence of solute (0.9 mM). The adsorption of surfactant was complete (within detectable limits), so the amount of adsorbed surfactant in the gel was the same as it was for our standard surfactant concentration and solution amount. However, Figure 8 shows that the solubilization isotherms are very different for the two systems. For the equivalent bulk 2EP concentration, the surfactant aggregates in the gels with a larger volume of solution hold a lower fraction of solute than the other gels. In addition to the general effect of K variation due to changes in solute-to-surfactant ratio,31 there may be effects due to the inhomogeneity of surfactant adsorption and to the initial surfactant concentrations being above or below the cmc. It was mentioned earlier that surfactant adsorption in the gels is not uniform. This behavior is more apparent at higher concentrations, and adsorption appears to be more uniform when the surfactant concentration is initially below the cmc. Thus, it may be that solutions initially above the cmc are able to form more aggregates and thus solubilize more solute, while

Solubilization of Phenols in Anionic Polyectrolyte Gels

solutions below the cmc give rise to more nonaggregated surfactant molecules within the gel. Whether or not this is the case, it is interesting that this does not necessarily translate into reduced solubilization effectiveness for this case. Figure 9 shows a plot of q versus the initial concentration of 2EP in solution. Within experimental error, there is no difference between the two systems. Thus, for the same initial concentration of solute, the gels end up essentially removing the same amount of surfactant and solute from solution. Conclusions In this work, we demonstrated that surfactant aggregates within polyelectrolyte gels are capable of solubilizing organic molecules from aqueous solution and the isotherms describing solubilization are quite similar to those found for free surfactant in solution. The effect of solute hydrophobicity on solubilization compared well with adsorption of phenols from aqueous solution driven by hydrophobic interactions. Solubilization was fairly insensitive to changes in gel chemistry and structure over the ranges in this study. The isotherms show a slight decrease in the magnitude of slope when the gel is changed from PAA to PMA, probably due to the small increase in hydrophobicity in the region around the surfactant headgroups. The fact that gel structure and chemistry

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have such a little effect on solubilization confirms studies in the literature on surfactant/polyelectrolyte interactions where there is little to no change in aggregate structure or size when changing from PAA to PMA, varying the cross-linker density, or changing the degree of neutralization above about 50%. Certainly more dramatic changes in cross-linker level or gel chemistry (e.g., using monomers with large alkyl groups) could give rise to changes in solubilization. Such changes are known to affect aggregate structure, and this would be an interesting subject for future work. The most significant factors appear to be the initial concentration of surfactant in solution and the ratio of surfactant solution to gel amount. A reduction in surfactant concentration (particularly when combined with an increase in solution volume) leads to a reduction in solubilization even for an equivalent amount of adsorbed surfactant. Acknowledgment. The authors acknowledge the donors of the Petroleum Research Fund, administered by the American Chemical Society, and the Joseph B. and Reba A. Weaver Undergraduate Research Award for support of this work. The authors also thank Jiao Guo, Brian Meents, Michael Zavatsky, and Nathan Kozman for preliminary and supporting work. LA050155Q