Specific Interactions of Poly(4-vinyl phenol) Gel with Cationic and

behavior. In the anionic surfactant-P4VPh system, however, only solubilization of the polymer particle was observed, and the UV peak only showed a blu...
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Langmuir 2005, 21, 7153-7160

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Specific Interactions of Poly(4-vinyl phenol) Gel with Cationic and Anionic Surfactants Ling Xu, Eisuke Yokoyama, and Mitsuru Satoh* Department of Chemistry and Materials Science, Tokyo Institute of Technology, Ookayama 2-12-1, Meguro-ku, Tokyo 152-0033, Japan Received March 24, 2005. In Final Form: May 29, 2005 Binding behaviors of ionic surfactants (decyl- and dodecyltrimethylammonium bromide (C10TAB, C12TAB), sodium decane sulfonate (SDeSo), and sodium dodecyl sulfate (SDS)) to poly(4-vinyl phenol) (P4VPh) gel were investigated to elucidate a specific swelling behavior that has been found for P4VPh gel in aqueous solutions of tetraalkylammonium salts. With increasing cationic surfactant concentration, P4VPh gel significantly deswelled and then remarkably reswelled at a concentration somewhat below the respective cmc values. On the other hand, in the case of the anionic surfactants, the gel only showed a marked swelling at a concentration just below the respective cmc values. A similar charge-specific behavior of the surfactants was also found for the P4VPh dispersion system studied with a UV-vis spectroscopy; namely, in the cationic surfactant-P4VPh systems, the turbidity of the dispersion first increased with increasing the surfactant concentration and then decreased. This result suggests that aggregation of P4VPh particles first occurred and finally the particles were solubilized. A red shift followed by a blue shift observed for a π-π* absorption of phenol at around 278 nm was also consistent with the aggregation-solubilization behavior. In the anionic surfactant-P4VPh system, however, only solubilization of the polymer particle was observed, and the UV peak only showed a blue shift. All these results in the gel and the dispersion systems strongly suggest that the cation-π interaction is involved in the binding of the cationic surfactants to P4VPh.

Introduction The earliest studies on polymer-ionic surfactant interactions can be dated back to those on cationic proteins and anionic surfactants in the 1930s,1 while those for synthetic nonionic polymer dated back only to the 19501960’s.2 After that, however, various kinds of polymersurfactant systems have been investigated, which included systems consisting of all kinds of charge combinations, i.e., those of nonionic, cationic, and anionic polymers and surfactants. It has been found that in most of the combinations, a kind of polymer-surfactant complex is formed at a surfactant concentration that is typically below the critical micelle concentration (cmc) and called a critical association concentration (cac).1,3-6 Such a polymersurfactant complex may affect various physicochemical properties of the solution, such as surface activity, viscosity, and abilities for wetting, foaming, and solubilization.7 If one employs a polymer gel system instead of a solution, significant changes in the gel swelling would be found. In fact, there have been many papers reporting specific gel swelling in the presence of surfactants. For example, Sjo¨stro¨m and Piculell have reported a series of studies on swelling behaviors of hydroxyethyl cellulose (HEC) gel and a hydrophobically modified one (HM-HEC) in the presence of cationic (alkyltrimethylammonium bromides) * To whom correspondence should be addressed. Fax: 81-3-57342888. E-mail: [email protected]. (1) Wang, C.; Tam, K. C. Langmuir 2002, 18, 6484. (2) Molyneux, P. In Water; Franks, F., Ed.; Plenum Press: New York, 1975; Vol. 4, p 676. (3) Kevelam, J.; van Breemen, J. F. L.; Blokzijl, W.; Engberts, J. B. F. N. Langmuir 1996, 12, 4709. (4) Hoff, E.; Nystrom, B.; Lindman, B. Langmuir 2001, 17, 28. (5) Goddard, E. D. J. Colloid Interface Sci. 2002, 256, 228. (6) Rosen, O.; Sjo¨stro¨m, J.; Piculell, L. Langmuir 1998, 14, 5795. (7) Holmberg, K.; Jonsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, U.K., 2003; p 277.

and anionic (sodium dodecyl sulfate (SDS) and the analogues) surfactants.8-10 The authors found that an HMHEC gel showed a gradual deswelling with increasing cationic and anionic surfactant concentrations and then significantly reswelled with a further increase in the respective surfactant concentrations. The reswelling degrees were more significant for SDS than for cationic ones. The gradual deswelling was ascribed to a “micellar cross-linking” where a bound micelle is shared by two or more polymer chains. On the other hand, the reswelling was interpreted as being caused by an enhanced osmotic pressure due to counterions of the bound micelles. Similar significant deswelling-reswelling behaviors were also found for the HEC gel in the presence of added salt (NaCl). Cooperative binding of ionic surfactants on a nonionic polymer matrix around cac and the resultant remarkable swelling have also been found for poly(N-isopropylacrylamide) (PNIPA) gel,11-13 which is known to show a temperature-induced volume phase transition or collapse due to the hydrophobic interactions among the isopropyl groups. Sakai et al. reported11 that the phase transition temperature of PNIPA gel significantly increased upon addition of some ionic surfactants and the degree was strongly dependent on the surfactant species or variations in the chemical structure, e.g., at the headgroup. For example, anionic surfactants such as SDS most strikingly increased the transition temperature, whereas a phosphate-type surfactant (triethanolammonium dodecyl phosphate) and dodecyltrimethylammonium chloride only slightly affected the transition point. Murase et al. demonstrated12 that the degree of elevation of the collapse (8) Sjo¨stro¨m, J.; Piculell, L. Langmuir 2000, 16, 3836. (9) Sjo¨stro¨m, J.; Piculell, L. Langmuir 2001, 17, 4770. (10) Sjo¨stro¨m, J.; Piculell, L. Colloids Surf., A 2001, 183-185, 429. (11) Sakai, M.; Satoh, N.; Tsujii, K. Langmuir 1995, 11, 2493. (12) Murase, Y.; Onda, T.; Tsujii, K.; Tanaka, T. Macromolecules 1999, 32, 8589. (13) Miyagishi, S.; Takagi, M.; Kadono, S.; Ohta, A.; Asakawa, T. J. Colloid Interface Sci. 2003, 261, 191.

10.1021/la050777b CCC: $30.25 © 2005 American Chemical Society Published on Web 06/28/2005

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temperature was linearly dependent on the amounts of the surfactant binding, while its dependence on the surfactant chemical structure was left to be elucidated. In our previous research, we found that poly(4-vinyl phenol) (P4VPh) gel shows three unusual swelling behaviors in water and aqueous salt solutions.14,15 First, P4VPh gels, which were prepared by a chemical crosslinking, had very high water content, typically >90%, despite the insolubility of the parent polymer in water.14 Second, the gel hardly deswelled in many kinds of inorganic salt solutions including typical salting-out agents (e.g., KF and Na2SO4) even at their saturated concentrations.14 This “super-salt-resistivity” has been interpreted as being caused by stabilization of two kinds of hydrogen-bonding hydrations by ions; namely, one is a stabilization of the hydrogen-bonding hydration to the acidic phenol proton by anions, and the other is that of the π-hydrogen-bonding hydration to the phenol π system by cations. These mechanisms have been confirmed on the basis of IR spectroscopy and ab initio calculations.14,15 The third unusual swelling behavior of P4VPh gel was found in the presence of tetraalkylammonium chloride (TAACl) solutions,15 especially for the tetrabutylammonium chloride (TBACl) system, where P4VPh gel sharply deswelled in the lower concentration region (∼1 M) and reswelled in the higher concentration region (>2 M). The initial deswelling of the gel was explained as being caused by a “cross-linking” by a TBA+ intervening two phenol rings. With increasing TBACl concentration, TBA+ was multiply bound onto one monomer residue, leading to gel swelling through detachment of the physically cross-linked or aggregated polymer chains. As a possible interaction responsible for the TBA+ binding to P4VPh in the higher TBACl concentration region, the authors considered cation-π and van der Waals interactions, while a hydrophobic one was considered for the lower concentration region. This speculation has been partly confirmed by IR and ab initio calculations.15 These specific swelling behaviors found for the P4VPh gel-TBA+ system reminded us of the similar deswellingreswelling one for the HEC gels-ionic surfactant systems. Therefore, it seems worthwhile to study P4VPh gel-ionic surfactant systems in order to clarify the deswellingreswelling mechanism and to identify interactions involved with the TBA+ binding. Thus, in the present study, we report on the swelling behaviors of P4VPh gel in aqueous solutions of cationic and anionic surfactants. The headgroups of the cationic surfactants employed here, trimethylalkylammonium cations, are potentially amenable to the cation-π interaction, while anionic surfactants with long alkyl chains may be used as a reference that interacts with P4VPh mainly via hydrophobic interaction. To further investigate interactions involved in the surfactant binding to P4VPh, dispersion systems consisting of P4VPh microparticles and the relevant surfactants were also utilized and monitored with UVvis spectroscopy. Experimental Section Materials. Poly(4-vinylphenol) (MW ) 22 000) was purchased from Polyscience Inc. Ethylene glycol diglycidyl ether (EGDGE, 50% solution) as a cross-linking agent was purchased from Aldrich Chemical Co., Ltd. Cationic and anionic surfactants used in the present study were decyltrimethylammonium bromide (C10TAB) (99% pure, ACROS Organics), dodecyltrimethylammonium (14) Muta, H.; Taniguchi, T.; Watando, H.; Yamanaka, A.; Takeda, S.; Ishida, K.; Kawauchi, S.; Satoh, M. Langmuir 2002, 18, 9629. (15) Xu, L.; Yokoyama, E.; Watando, H.; Okuda-Fukui, R.; Kawauchi, S.; Satoh, M. Langmuir 2004, 20, 7064.

Xu et al. bromide (C12TAB) (99% pure, Aldrich Chem.), sodium 1-decanesulfonate (SDeSo) (98% pure, Tokyo Kasei, Japan), and sodium n-dodecyl sulfate (SDS) (99.1% pure, Kanto Chem., Japan). Deionized and then distilled water was used for all the experiments. Measurements for Gel System. Gel Preparation. P4VPh was dissolved in aqueous 1 M NaOH to adjust the concentration to 22 wt %. P4VPh gel was prepared by adding a desired amount of EGDGE (7mol % of the polymer phenolhydroxyl group) into the polymer solution. The cross-linking or gel-forming reaction occurred in a glass capillary tube (Drummond Scientific Co. Ltd., 25 µL, i ) 0.690 mm) at 25 ( 0.1° C for 24 h. The scheme of gelation was reported in our previous work.14 Rod-type P4VPh gel samples thus prepared were cut and immersed in distilled water until the equilibrium swelling was established. After this treatment, the water content of the gel samples was 97.3%, the diameter was ∼860 µm, and the length was ∼3 mm. Measurement of Swelling Degree. The water-swollen gel samples prepared as above were immersed in 1 mL aqueous surfactant solutions, the concentration of which was set between 1 and 100 mM. Each gel diameter was periodically measured using a microscope (Diaphot 200, Nikon Co., Ltd.) until an equilibrium swelling was confirmed, typically after a long-term immersion of more than 1 week. The swelling degree of the rodtype gel was defined as d/d0, where d and d0 are gel diameters swollen in aqueous surfactant solutions and distilled water, respectively. Besides the measurement of the equilibrium swelling degree, changes in gel diameter in C12TAB or SDS solution of several selected concentrations were observed as a function of time within a limited immersion term (2 weeks or 2 months). All the measurements were performed at room temperature (∼25 °C). Measurements for Dispersion System. Preparation of Aqueous P4VPh Dispersion. An amoount of 0.12 g of P4VPh was dissolved in 6 mL of 0.1 M NaOH. Then 84 mL of distilled water and 10 mL of 0.1 M HCl were added under continuous stirring. With this treatment, P4VPh precipitated as microparticles dispersed in water. The stock polymer dispersion of ∼10 mM was stabilized by sonication using an ultrasonic cleaner (35 W, VC-1, AS ONE Co. Ltd., Japan) for 5 min. Before UV-vis measurements, the desired amount of the stock dispersion was diluted to 5 mM. Sample dispersions for the spectroscopy were prepared by mixing 1 mL of the 5 mM P4VPh dispersion, the desired amounts of 100 mM surfactant solution, and distilled water. The total volume of the sample dispersion was adjusted to 10 mL, where P4VPh was finally diluted to 0.5 mM. The dispersion thus prepared was sonicated for 5 min with a 35 W ultrasonic wave and kept in a water bath at 25.0 ( 0.1 °C. The size of P4VPh microparticle in the 0.5 mM dispersion in the absence of surfactants was estimated by dynamic light scattering (DLS) (ELS-8000, Ootsuka Electrical Co. Ltd., Japan) as ∼2000 nm. UV-Visible Spectroscopy. To investigate interactions of P4VPh and surfactants, a characteristic absorption of the phenol residue at around 278 nm was monitored as a function of the surfactant concentration with a U-3210 spectrophotometer (Hitachi, Japan). The absorbency at 600 nm, A600, was utilized as a measure of the turbidity of the dispersion, which may change through interaction of the polymer with the pertinent surfactants. Each measurement was made three times to get an average. All the measurements were performed at 25 ( 0.1 °C.

Results and Discussion Swelling Behaviors. Swelling degrees of P4VPh gel are shown as a function of surfactant concentration for the cationic surfactants (C12TAB and C10TAB) in Figure 1 and for anionic ones (SDS and SDeSo) in Figure 2. In the cationic surfactant solutions, the swelling degree decreased in lower concentration regions and then started to increase at 10 mM for C12TAB and at 40 mM for C10TAB. The minima appeared at surfactant concentrations that are rather lower than the respective cmc values: 16

Interactions of Poly(4-vinyl phenol) Gel

Figure 1. Dependence of swelling ratio (d/d0) on the cationic surfactant concentration for P4VPh gel. Arrows show the cmc values.

Figure 2. Dependence of swelling ratio (d/d0) on the anionic surfactant concentration for P4VPh gel. Arrows show the cmc values.

and 65 mM, respectively.16 These critical concentrations are often referred as “critical association concentration”, cac, and it is assumed that at the above cac a polymerinduced micellization occurs on the relevant polymer substrate; namely, in a surfactant concentration region much lower than the cmc, unimers are uncooperatively bound to the polymer to increase the local concentration. Then, with increasing bulk concentration to cac, the polymer-bound surfactant molecules start to reorganize to form micelles below the cmc because of the extra interaction of the surfactants with the polymer. In fact, many polymer gel-ionic surfactant systems show significant swellings above the respective cac values, which has been ascribed to enhanced osmotic pressure due to counterions of surfactant micelles bound to polymer. The sharp swelling observed for the present P4VPh-cationic surfactant systems may also be safely explained in terms of the same mechanism. On the other hand, the initial deswelling observed for the cationic surfactant systems is rather specific. Although the present P4VPh gel is certainly negatively charged in water, the dissociation degree of the phenol OH must be negligibly low and the relevant gel may be safely treated as a neutral one. In fact, even in the presence of 0.01 M HCl, an essentially same deswelling-reswelling behavior was observed for the two kinds of cationic surfactant systems (data not shown). As described in Introduction, we have already found a similar deswelling-reswelling behavior for the P4VPhTBACl system and proposed a mechanism for the unique (16) Klevence, H. B. J. Am. Oil Chem. Soc. 1953, 30, 74.

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swelling behavior, namely, a deswelling due to “crosslinking” by a TBA+ cation intervening two phenol rings, followed by a swelling through detachment of the physically cross-linked or aggregated polymer chains by multiple binding of TBA+ cations onto one monomer residue. If the “multiple binding” was replaced by micelle formation on the polymer substrate, this mechanism may also be acceptable for the present surfactant systems. In fact, the cation-π interaction, which was supposed as one of the attractive interactions between TBA+ and P4VPh, must also be operative in the present cationic surfactant systems because the trimethylmonoalkylammonium headgroup is a typical cation for the interaction. On the other hand, however, the cation-π interaction was not supposed for the tetramethylammonium chloride (TMACl) system in our previous study because no specific reswelling was observed. This contrasting swelling behavior between TMACl and the cationic surfactant systems strongly suggests that in order for the cation-π interaction to be effective in water, some assistance by other interactions, probably the hydrophobic one, must be available to pay the dehydration “penalty”. These speculations will be further discussed in a later section on the dispersion systems. Swelling behavior in the anionic surfactant systems was fairly different from that of the cationic surfactant systems as shown in Figure 2. No significant deswelling occurred with increasing surfactant concentration, and only marked swellings were observed. On the basis of these contrasting swelling behaviors for the cationic and anionic surfactant systems, the binding scheme for the latter must be different from that for the former. Here, we invoke a fluorescence study by Karukstis et al.,17 who investigated sites of an aromatic probe (6-propionyl-2-(dimethylamino)naphthalene, Prodan) bound to C12TAB and SDS micelles. According to their study, the probe molecules are bound on the C12TAB micelle surface, except for a minor binding in the micelle core, not only via nonspecific dipolar interaction but also by the cation-π interaction with the headgroup. On the other hand, almost all of the probe molecules are bound on SDS micelles via nonspecific dipolar interaction with the headgroup. Thus, the following binding scheme may be considered for the present P4VPhcationic and -anionic surfactant systems. In C12TAB, C10TAB, when below the cac, unimer molecules of the cationic surfactants are bound to P4VPh via cation-π and hydrophobic interactions and cross-link the polymer chains through the hydrophobic interaction between the alkyl chains. Above the cac, the cationic surfactant micelles are bound to the polymer to swell the gel substrate because of the enhanced osmotic pressure due to the counterions of the bound micelles. Here, a possibility may not be excluded that the polymer segments are partly incorporated within the micelle core, besides main binding on the micelle surface. This model is schematically illustrated in Figure 3. In SDS, SDeSo, when below the cac, free unimer molecules of the anionic surfactants are bound to P4VPh via the hydrophobic interaction between the polymer’s hydrophobic moieties and the alkyl tail as well as through the nonspecific dipolar interaction with the headgroup. This unimer binding seems to be rather weak compared with that of the cationic surfactants, since no appreciable changes in the swelling degree were observed until the surfactant concentrations were near the respective cmc values. Above the cac, which appeared just below the cmc, (17) Karukstis, K. K.; Frazier, A. A.; Loftus, C. T.; Tuan, A. S. J. Phys. Chem. B 1998, 102, 8163.

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Xu et al. Table 1. Some Critical Values for the Surfactant-P4VPh Gel Systems surfactant cac (mM) cmc (mM) cac/cmc (d/d0)min C12TAB C10TAB SDS SDeSo

Figure 3. Schematic illustration of the interaction between the cationic surfactant and P4VPh: (a) unimer; (b) micelle.

Figure 4. Schematic illustration of the interaction between the anionic surfactant and P4VPh: (a) unimer; (b) micelle.

micelles are formed and the alkyl tails become unavailable for interaction with the polymer. Then, the micelles are bound with the polymer chains at the surface to induce the gel swelling. This binding scheme is illustrated in Figure 4. It is noted here that this model (Figure 4b), together with that of Figure 3b, may be comparable with the socalled necklace model,18-21 which is often invoked to visualize surfactant-polymer complexes. In the necklace model, polymer segments reside mainly at the micelle surface and partly penetrate into the region of the surfactant head to shield the micelle’s hydrophobic core from the surrounding aqueous medium. This shielding effect has been considered as a main driving force for the surfactant-nonionic polymer complex formation. Thus, this hydrophobic interaction between the polymer segments and alkyl groups near the headgroup may also contribute to the present P4VPh-surfactant, especially cationic micelle complex formation as well as the nonspecific dipolar interaction supposed by Karukstis et al. for the aromatic probe systems. To compare the effects of the cationic and anionic surfactants to the P4VPh gel swelling, some critical values, i.e., cac, the ratio of cac to cmc, maximum and minimum (18) Shirahama, K.; Ide, N. J. Colloid Interface Sci. 1976, 54, 450. (19) Cabane, B. J. Phys. Chem. 1977, 81, 1639. (20) Nagarajan, R. Colloids Surf. 1985, 13, 1. (21) Nikas, Y. J.; Blankschtein, D. Langmuir 1994, 10, 3512.

10 40 6 35

16 65 8.3 40

0.63 0.62 0.72 0.88

0.61 0.47 1.00 0.95

(d/d0)max 1.82(16) 1.27(100) 1.79(60) 1.44(100)

(if any) swelling degrees, are summarized in Table 1. The ratios cac/cmc for the cationic surfactants (C12TAB, C10TAB) are definitely lower than those for the anionic ones (SDS, SDeSo), demonstrating that the pertinent cationic surfactants more strongly interact with P4VPh than the anionic ones. This is rather unique for common cationic surfactants because it is known that binding of cationic surfactants to nonionic polymers is generally weaker than that of anionic ones. For example, it has been reported that relatively hydrophobic polymers, i.e., poly(propylene oxide) (PPO) and poly(vinyl methyl ether) (PVME) interact with SDS more strongly than with cetyltrimethylammonium bromide.22 As referred to in the Introduction, the same can be said also for a typical hydrophobic polymer, PNIPA. Since P4VPh is more hydrophobic than the polymers PPO, PVME, and PNIPA, as judged from the former’s sparing solubility in water, the interaction with cationic surfactants would be enhanced compared with those for the latter three polymers. However, the present experimental results, indicating that the interaction of the cationic surfactants with P4VPh is definitely stronger than that of the anionic ones, must be uncommon or probably the first finding. As a matter of fact, it has been predicted that cationic surfactants may form a complex with nonionic polymers such as anionic surfactants, if the cationic headgroup attractively interacts with the pertinent polymer.23 The present finding seems to be a realization of the prediction, since the cation-π interaction can be assigned to the necessary attractive force between surfactant head and polymer. The minimum and maximum swelling degrees, (d/d0)min and (d/d0)max, for C12TAB are both higher than those for C10TAB. This suggests that the enhancement in the osmotic pressure due to the micelle formation of C12TAB on the polymer substrate, which overwhelms the deswelling caused by cross-linking-binding of unimer surfactant molecules, is more effective than the case of C10TAB. This marked increment in the osmotic pressure for the C12TAB system may be simply ascribed to the longer alkyl chain, or stronger hydrophobic association of the C12 alkyl chains, which must lead to formation of a larger number of micelles on the polymer substrate. On the other hand, one may consider that the almost same (d/d0)max values, 1.82 and 1.79 for C12TAB and SDS systems, contradict with the supposed stronger binding of cationic surfactants to P4VPh through the cation-π interaction. However, the coincidence of the maximum swelling degrees may be only apparent because the gel swelling in the cationic surfactant system occurred after the significant deswelling; namely, the (d/d0)max values for the cationic surfactant systems must result from higher amounts of cationic surfactants bound to the gel polymer compared with those of the anionic ones. Thus, the whole swelling behavior seems to be consistent with the supposed scheme; namely, the cation-π interaction exerts an extra interaction with the hydrophobic one in the cationic surfactant binding to P4VPh. (22) Brackman, J. C.; Engberts, J. B. F. N. Chem. Soc. Rev. 1993, 22, 85. (23) Nikas, Y. J.; Blankschtein, D. Langmuir 1994, 10, 3512.

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Table 2. Total Amounts of the Ionic Surfactants in P4VPh Gel (Surfactant/Polymer Monomer Residue) concn (mM) 6 8 12 25 100

C12TAB 0.15 0.21 0.75

SDS

C10TAB

SDeSo

0.69

0.25

0.022 0.11 0.25

To estimate equilibrium amounts of the surfactants in the swollen gel, we performed elemental analysis for dried gel samples that had been equilibrated with aqueous solutions of the cationic and the anionic surfactants. The results are shown in Table 2. It is noted here that there is given a total amount including surfactants that were partitioned into the gel by the Donnan distribution24 instead of those for specifically bound surfactant molecules. Since the surfactant molecules bound to the polymer substrate should serve as a fixed charge, the concentration of the ionic surfactants absorbed in the inner bulk solution of the swollen gel phase would not be obtained without a quantitative evaluation of the Donnan potential evolved at the gel-solution interface. However, the potential should depend on the binding form of the surfactant, i.e., binding as unimer and/or micelle, and the relative amount, which in fact is hard to know a priori. Thus, we employ the total amount as a measure of surfactant binding to P4VPh to make a qualitative comparison among the surfactants. As seen from Table 2, the cationic surfactants are more strongly bound to P4VPh than the anionic ones. For example, P4VPh gel at 8 mM C12TAB significantly deswelled (d/d0 ≈ 0.8), while it swelled for the corresponding SDS system (d/d0 ≈ 1.3). Nevertheless, more amounts of C12TAB were absorbed into the gel than SDS. This demonstrates that the binding interaction of the cationic surfactant with P4VPh is significantly stronger than the anionic one. The same may be said from a comparison at 25 mM and also from a comparison between C10TAB and SDeSo (100 mM). It may be worthwhile comparing the present estimations with those of PNIPA gel, a typical hydrophobic polymer, reported by Murase et al.12 According to their study, amounts of SDS and dodecylammonium chloride bound to the PNIPA gel were about 0.4 and 0.2 near their respective cmc values (8 and 15 mM), showing a stronger interaction with the anionic surfactant than with the cationic one. Since the authors estimated the adsorbed surfactant molecules per NIPA monomer by simply subtracting the amounts of surfactant in the aqueous phase inside the gel, the actual binding amounts must be larger than the above values. Thus, we can safely conclude that the binding interaction of SDS with the present P4VPh gel is definitely weaker than that for PNIPA on the basis of the estimated binding amounts of SDS at 8 mM, i.e., 0.11 and 0.4, respectively. This must be ascribed to the stronger hydrophobic interaction for the PNIPA gel than the P4VPh gel. In other words, the hydrophobic hydration around the P4VPh polymer chain is more stabilized than that of PNIPA, being consistent with our previous finding that P4VPh gel in water is not as thermoresponsive as PNIPA and only slightly deswelled with increasing temperature from 10 to 40 °C. Swelling Time-Course. Figure 5 illustrates the swelling time-course of P4VPh gel in aqueous solutions of C12TAB and SDS with different concentrations. In the case of the cationic surfactant system, the swelling degree (24) Shibayama, M.; Uesaka, M.; Inamoto, S.; Mihara, H.; Nomura, S. Macromolecules 1996, 29, 885.

Figure 5. Time-courses of swelling ratio (d/d0) since immersion of water-swollen P4VPh gels into C12TAB (a) and SDS (b) aqueous solutions.

monotonically decreased with time when immersed in solutions of the lower concentrations (5 and 10 mM), while in the higher concentrations (12, 15, and 20 mM) the gel samples showed significant swelling after passing relatively shallow minima. This specific deswelling-reswelling behavior is comparable with the concentrationdependent one shown in Figure 1 and may correspond to a transient change in the surfactant concentration in the gel sample toward the equilibrium value.8 A similar swelling profile was also found for TBACl systems of higher concentrations (2.1 and 2.5 M) in our previous study.15 However, the deswelling-reswelling speed for the C12TAB system is rather different from that for the TBACl system. For example, the reswelling for TBACl systems occurred very slowly (about 100 h up to the equilibrium swelling) compared with the rapid deswelling (about 7-9 min up to the minimum swelling).15 On the other hand, the deswelling process for C12TAB systems took about 20-50 min, while the reswelling occurred in about 8-14 h. The difference in the deswelling and reswelling speed may be ascribed to the difference in the solute concentration; namely, the TBACl concentration was so high (∼2.5 M) that water in the gel phase was absorbed because of the large osmotic pressure, and then the gel further deswelled because of cross-linking binding of TBA+ cations between the phenol rings. In the present case, however, such a substantial “water suction” is not to be expected for the low surfactant concentration of ∼20 mM. Hence, a main cause for the observed deswelling must be the hydrophobic interaction among the alkyl chains of the cationic surfactant, the headgroup of which was bound to the phenol rings (physical cross-linking, Figure 3a). The reswelling for the cationic surfactant system is, as previously stated, ascribed to the micelle binding to the polymer or to the enhanced osmotic pressure due to the

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counterions of the bound micelles. In the TBACl system too, such an enhancement in the osmotic pressure can occur because of the multiple binding of the cation to the polymer in the reswollen state. However, the high TBACl concentration (∼2.5 M) in the outer solution would mute the effect. In this context, the slow reswelling observed for the 12 mM C12TAB system suggests that enhancement of the osmotic pressure is not so effective to induce a rapid reswelling. This may be because the concentration of micelles bound to the polymer is not so high compared with the free surfactant concentration in the relevant systems with a surfactant concentration near the cac. In the anionic surfactant systems, only a slow swelling was observed irrespective of the surfactant concentration; even in a 20 mM SDS solution, which is much more concentrated than the cac (6 mM) and cmc (8.3 mM), it took more than 10 days to reach the equilibrium swelling. This suggests that the micelle formation of SDS (and SDeSo) involving with the polymer substrate is not as cooperative as that for the cationic surfactants. All these kinetic behaviors observed for the cationic and anionic surfactant systems, in fact, seem to be consistent with the estimated binding amounts of surfactants in Table 2. Finally, we note an apparent difference between C12TAB and SDS in the timing where the first swelling or deswelling becomes appreciably noted. For example, at 20 mM in the cationic surfactant system, the first sign of the deswelling was noted at 5 min while the swelling at 20 mM for the anionic one became appreciable only after ∼30 min. The surfactant concentration, 20 mM, is higher than the respective cmc values, and there exist micelles as well as unimers in the surfactant solutions. Since diffusion of a micelle is much slower than that of the unimer, say by several times,25 the observed difference in the timing for (de)swelling may be ascribed to the difference in the diffusion speed of the solutes that are responsible for the deswelling or the swelling; namely, the deswelling may be ascribed to the unimer binding and their hydrophobic association as depicted in Figure 3a. On the other hand, the swelling for the anionic surfactant system must be caused by the micelle binding as shown in Figure 4b. On the other hand, the SDS unimer should also diffuse as fast as C12TAB and be bound to the polymer segment. The unimer binding of SDS, however, cannot induce deswelling or swelling because of the difference in the binding scheme (Figure 4a compared with Figure 3a) and the inherent weakness of the binding. Thus, one should note that the observed swelling timecourses are not so simple, as understood in terms of the diffusion-limited binding scheme only. P4VPh Dispersion Systems. To further obtain information on the specific interactions of the ionic surfactants with P4VPh, we attempted an investigation in solution system. However, because of the sparing solubility of the polymer in water at neutral pH, we inevitably treated a dispersion system instead of a clear solution. The P4VPh dispersion, which was prepared as described in the Experimental Section, turned out to be amenable to UV-vis spectroscopy. Thus, we were able to obtain the absorption spectra as a function of surfactant concentration. We especially noted a peak position of the π-π* absorption of the phenol ring that appears at ∼280 nm as well as the absorption at 600 nm as a measure of turbidity of the dispersion. Besides the surfactant systems, the P4VPh-TBACl dispersion system was investigated as a (25) Holmberg, K.; Jonsson, B.; Kronberg, B.; Lindman, B. Surfactants and Polymers in Aqueous Solution; John Wiley & Sons: Chichester, U.K., 2003; p 56.

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Figure 6. Dependence of the turbidity (A600) and the π-π* absorption peak of the phenol ring (λpeak), for P4VPh dispersion, on the cationic surfactant concentration.

Figure 7. Dependence of the turbidity (A600) and the π-π* absorption peak of the phenol ring (λpeak), for P4VPh dispersion, on the anionic surfactant concentration.

reference because the cation-π interaction of the polymer and TBA+ has already been suggested on the basis of IR spectroscopy.15 The results are shown in Figures 6-8. Both the peak position, λpeak, and the turbidity, A600, significantly changed with increasing the respective surfactant or salt concentrations. First, we inspect the results on the cationic surfactants and TBACl systems. A600 values for these systems show definite maxima and finally fall to 0 or nearly 0. This result means that the P4VPh particles were aggregated and then were solubilized with increasing the surfactant (salt) concentrations, which was consistent with observations with the naked eye. These aggregation-solubilization behaviors seem to correspond to the deswellingreswelling observed for the relevant gel systems. Thus, the aggregation of the polymer particles may be caused

Interactions of Poly(4-vinyl phenol) Gel

Langmuir, Vol. 21, No. 16, 2005 7159 Table 3. Critical Surfactant Concentrations (SC), A600, and λmax Values in P4VPh Dispersion Systems

C10TAB C12TAB TBAClb SDeSo SDS

SCa (mM) for A600max, the ratio to cmc

SCa (mM) for λpeakmax

25, 0.38 10, 0.63 1.0 20, 0.50 3, 0.36

20 5 0.75 10 3

A600max 0.542 0.136 0.256

λpeakmax (nm)

λpeak (nm) at SC ) 90 mM

287.4 280.3 281.6 278.8 278.8

279.6 279.9 280.9 278.0 278.0

a For SDeSo and SDS systems, surfactant concentrations (SC) where A600 and λmax started a significant decrease are shown. b For TBACl, the SC unit is M and λpeak at 0.75 M is shown in the third column.

Figure 8. Dependence of the turbidity (A600) and the π-π* absorption peak of the phenol ring (λpeak), for P4VPh dispersion, on TBACl concentration.

by a mechanism similar to that for the gel-cationic surfactant system; the respective polymer chains in Figure 3a may be replaced by those on the surface of the polymer particle. Of course, a similar chain contraction as in the gel system may occur in a polymer particle besides the polymer particle aggregation. As an illustration for the solubilized state, Figure 3b may be used. The degree of aggregation, judged by A600 values at the maximum, A600max, increases in the following order: C12TAB < TBACl < C10TAB. Although the position of TBACl cannot be directly compared with the others because of the much higher concentration at the maximum, the difference between the two cationic surfactants is remarkable; the A600max value, ∼0.14, for C12TAB is much lower than ∼0.50 for C10TAB. Although C12TAB should interact more strongly with P4VPh because of the higher hydrophobicity than C10TAB, at the same time the micelle formation takes place at the lower concentration. Thus, it seems that in the C12TAB system the solubilization started before the aggregation grew to a large scale. These speculations are all consistent with the behaviors observed for λpeak; namely, the π-π* absorption of the phenol ring at ∼280 nm for the cationic surfactants and TBACl systems showed a red shift and the degree was remarkable, in order of C12TAB < TBACl < C10TAB, being the same as that for the aggregation. Since the cation-π interaction has been assigned to the binding of TBA+ to P4VPh, the observed red shift may be safely ascribed to that interaction. In fact, it has been known that π-π* absorption of 4-methylphenol or p-cresol, which may be employed as a monomer analogue of P4VPh, shows an appreciable red shift in nonpolar media. For example, the λpeak values in water, methanol, and cyclohexane are 277, 280, and 286 nm, respectively.26 Since the π-π* absorption is for an inhibited transition to a nonpolar exited state, in hydrogen-bonding solvents such as water the absorption shows a blue shift. Upon cation-π interaction, methyl or butyl groups on the cationic surfactants and TBA+ would (26) Phillips, J. P., Feuer, H., Laughton, P. M., Thyagarajan, B. S., Eds. Organic Electronic Spectral Data; John Wiley: New York, 1969; Vol. XI, p 76.

serve as a nonpolar medium to the aromatic system, thus showing relatively large λpeak values. On the other hand, A600 and λpeak behaviors for anionic surfactant systems were both in contrast to those for the cationic surfactants. As seen in Figure 7, only a solubilization and a blue shift were observed. This result seems to be parallel with that for the gel system, in which only swelling was observed. The significant decrement in A600 above 3 and 20 mM for SDS and SDeSo, respectively, suggests that the polymer chains that were trapped in a microparticle dissociated because of the binding of SDS or SDeSo molecules to a P4VPh chain, as illustrated in Figure 4a. The corresponding blue shift is ascribed to an exposure of the phenol rings to water, as stated above. To compare the effects of the cationic and anionic surfactants to the P4VPh dispersion system, as for the gel system, we note some critical values, e.g., surfactant concentrations (SC) at which the maximum A600 and λpeak (A600max and λpeakmax, respectively) appeared for C10TAB, C12TAB, and TBACl systems and those where A600 and λpeak started a significant decrease for SDeSo and SDS systems. These values and some others were listed in Table 3. First, one may note that the SC values for A600max (mM) and the ratios to cmc, except for the C12TAB system, are significantly lower than those cac and cac/cmc values in Table 1. Further, the ratios to cmc for the anionic surfactants are comparable to or even smaller than those for the cationic ones. These results strongly suggest that the solubilization or dissolution of P4VPh occurred before the surfactant micelles were formed on the polymer chains. Since the polymer chains in the dispersion were not as tightly cross-linked as in the gel phase, binding of unimer surfactant molecules may be enough to solubilize the hydrophobic polymer. In the case of the cationic surfactants, the unimer binding to P4VPh through the cation-π interaction as well as the hydrophobic one would cause aggregation of the polymer particles and physical crosslinking of the polymer chains. Thus, the relatively lower ability for the solubilization compared with the anionic surfactants, as judged on the basis of the ratio of SC for A600max to cmc, may be ascribed to the difference in the binding mode as illustrated in Figures 3a and 4a. The behavior of λpeak as a function of SC is qualitatively parallel to that of A600. However, the SC values for λpeakmax except for that of the SDS system are somewhat lower than those for A600max. This may be because λpeak monitors changes in the local environment of the phenol ring while A600 reflects the aggregation state of P4VPh particles; namely, the binding of unimer surfactant molecules to the polymer chains would loosen the local aggregation of the polymer chains in a microparticle to allow exposure of the polymer residues to water medium before total solubilization occurs. The significantly large λpeakmax value, 287.4 nm, for the C10TAB system which is comparable to the λ peak of

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p-cresol in cyclohexane, 286 nm, suggests that the phenol rings almost lost contact with the aqueous medium. This seems consistent with the concomitant remarkable aggregation of the polymer microparticles. A reason that the λpeakmax for the TBACl system is much lower than that for C10TAB irrespective of the former’s much higher concentration must be the less significant degree of aggregation of the polymer particles, as suggested by comparing their A600max values. The λpeak values at SC ) 90mM, standing for those of the dissolved polymer, are larger for the cationic surfactants than for the anionic ones. This is also consistent with the cation-π interaction of the unimer molecules with the phenol rings (Figure 3b), thus reducing exposure of the residue to water. The same must also be applicable to the TBACl system. The slightly larger value may be safely ascribed to the much higher concentration. Concluding Remarks Complex formation between cationic surfactants and polymers containing the π-electron systems or aromatic ring(s) has been investigated by many researchers. As a typical polymer system with an aromatic ring, poly(styrene) (PSS) latex and sulfonated PSS have often been employed to examine interactions with or binding of cationic surfactants in aqueous media. Although PSS itself is of course nonionic, the latex inevitably contains small amounts of sulfate group at the chain ends. Thus, in the pertinent studies, electrostatic binding of cationic surfactants to the PSS contributed to the total binding force in addition to the hydrophobic interaction. For example, Zhao and Brown reported a dynamic light scattering (DLS) study on PSS latex in the presence of C12TAB and discussed the initial electrostatic binding of the cationic surfactant and subsequent hydrophobic interactions with the polymer to cause polymer chain aggregation.27 Xu and Smart proposed two adsorption models for C12TAB on curved (27) Zhao, J.; Brown, W. Langmuir 1995, 11, 2944.

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and flat surfaces of PSS latex, where the cationic surfactant molecules are bound to the surface via electrostatic and hydrophobic interactions.28 Meng et al. found a reversible sol-gel transition for partially sulfonated PSS and C12TAB system in the presence of excess amounts of the surfactant to the polymer charge, which may be comparable with the present aggregation-dissolution behavior of the P4VPh particles.29 All these studies, however, have not explicitly referred to the cation-π interaction as a driving force of those complex formations. It seems to be interesting and necessary to clarify if the cation-π interaction contributes to interactions between cationic surfactants and aromatic polymers such as PSS in aqueous media. On the other hand, the present experimental results obtained for P4VPh gel and dispersion systems strongly suggest that the cationic surfactants interact with the polymer via cation-π interaction as well as hydrophobic interaction. Otherwise, the stronger binding of the cationic surfactants compared with the anionic ones and their contrasting behaviors for the gel swelling and the UVvis results in the dispersion system would not be reasonably interpreted. Of course, all has not yet been clarified. For example, unimer molecules, both the cationic and anionic ones, must be bound to the polymer even in the lowest concentration region studied (