Aggregation Behavior of Surfactants in Polymer Gel Networks

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Articles Aggregation Behavior of Surfactants in Polymer Gel Networks Yasuyuki Murase,† Kaoru Tsujii,*,†,‡ and Toyoichi Tanaka§ †Tokyo Research Center, Kao Corporation, 2-1-3 Bunka, Sumida-ku, Tokyo 131-8501, Japan, and ‡Department of Physics, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139

Received November 10, 1999. In Final Form: May 8, 2000 Aggregation behavior of anionic sodium dodecyl sulfate and a nonionic surfactant, polyoxyethylene (p ) 10) nonylphenyl ether, in the inside and outside of polymer hydrogels has been studied through solubilization techniques of an oil-soluble dye, measurements of surfactant concentration in the aqueous phase of the gel interior, and binding isotherms of the surfactants onto polymer chains of the gel. The surfactant aggregation is strongly affected by the presence of polymer gel networks. Interestingly, it is found that the nonionic surfactant cannot form micelles inside the gel at concentrations greater than even the critical micelle concentration in the outer bulk solution. The anionic surfactant molecules adsorb onto the polymer chains of a relatively hydrophobic N-isopropylacrylamide gel but not onto the hydrophilic acrylamide (AAm) gel. In the AAm gel, the anionic surfactant forms micelles in the aqueous solution phase inside the gel, avoiding the exclusive space of the polymer chains.

Introduction Polymer gels undergo volume-phase transition in response to changes of the surrounding solution conditions.1-5 They are used as drug delivery systems6 and actuators or chemomechanical devices.7-9 In particular, the controlled release of small molecules from a gel has attracted much attention and has been extensively studied.10 The N-isopropylacrylamide (NIPA) gel is wellknown as a thermoresponsive gel,11,12 and it shows a phase transition at about 34 °C. This phase transition temperature increases dramatically upon the addition of small amounts of ionic surfactants.13-18 It is interesting to note that the change of the transition temperature depends †

Kao Corporation. Present address: The DEEPSTAR Group, Japan Marine Science and Technology Center, 2-15 Natsushima-cho, Yokosuka 237-0061, Japan. § Massachusetts Institute of Technology. ‡

(1) Tanaka, T. Phys. Rev. Lett. 1978, 40, 820. (2) Tanaka, T.; Fillmore, D.; Sun, S. T.; Nishio, I.; Swislow, G.; Shan, A. Phys. Rev. Lett. 1980, 45, 1636. (3) Hirotsu, S.; Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1987, 87, 1392. (4) Annaka, M.; Tanaka, T. Nature 1992, 355, 430. (5) Shibayama, M.; Tanaka, T. Adv. Polym. Sci. 1993, 109, 1. (6) Okano, T. Adv. Polym. Sci. 1993, 110, 179. (7) Tanaka, T. Science 1982, 218, 467. (8) Osada, Y.; Okuzaki, H.; Hori, H. Nature 1992, 355, 242. (9) Osada, Y.; Matsuda, A. Nature 1995, 376, 219. (10) Derossi, D.; Kajiwara, K.; Osada, Y.; Yamauchi, A. In Polymer Gels; Plenum Press: New York, 1991. (11) Hirokawa, Y.; Tanaka, T. J. Chem. Phys. 1984, 81, 6379. (12) Schild, H. G. Prog. Polym. Sci. 1992, 17, 163. (13) Zhang, Y.; Tanaka, T.; Shibayama, M. Nature 1992, 360, 142. (14) Wada, N.; Kajima, Y.; Yagi, Y.; Inomata, H.; Saito, S. Langmuir 1993, 9, 46. (15) Inomata, H.; Goto, S.; Saito, S. Langmuir 1992, 8, 1030. (16) Kokufuta, E.; Zhang, Y.; Tanaka, T.; Mamada, A. Macromolecules 1993, 26, 1053. (17) Zhang, Y.; Tanaka, T. Polym. Mater. Sci. Eng. 1992, 66, 369. (18) Sakai, M.; Satoh, N.; Tsujii, K.; Zhang, Y.; Tanaka, T. Langmuir 1995, 11, 2493.

highly on the chemical structure of the surfactants, i.e., hydrophobic chain length and hydrophilic headgroup.17,18 For example, the effectiveness of the hydrophilic group on the elevation of phase transition temperature is roughly in the order of anionic > cationic > nonionic. Even among the anionic surfactants, however, a sulfate-type surfactant elevates the transition temperature by more than 60 °C, and a phosphate-type surfactant elevates it by only 2-3 °C.18 To explain why the small difference in surfactant chemical structure influences so greatly the phase transition temperature of the NIPA gel, we observed the isotherms for binding of the surfactants onto the NIPA gel.19 From this process, we found a strange phenomenon, i.e., a nonionic surfactant shows apparently negative adsorption. Then we extended the binding study of surfactants to acrylamide (AAm) gel and found some interesting aggregation phenomena of the surfactants inside the gel networks. Experimental Section Materials. Used for the preparation of gel samples were NIPA (Eastman Kodak Co.) and AAm (Wako Pure Chemical Industries Ltd.) as the monomers, N,N′-methylenebis(acrylamide) (Wako) as the cross-linker, and N,N,N′,N′′-tetramethylethylenediamine (Wako) as the accelerator of the polymerization reaction. The surfactants used in this work were sodium dodecyl sulfate (R12SO4Na; Sigma Chemical Co.) and polyoxyethylene (p ) 10) nonylphenyl ether (R9φ(EO)10; Kao Corp.). The Polyoxyethylene chain length of the nonionic surfactant is not monodispersed and is in a certain distribution. An oil-soluble dye, Yellow AB (1-phenyl-azo-2-naphthylamine), was purchased from Tokyo Chemical Industry Co. Ltd. This dye is solubilized into aggregates of surfactant molecules.20 All samples were used without further purification. Deionized and distilled water was purchased from Wako and used in preparing the gel samples. (19) Murase, Y.; Onda, T.; Tsujii, K.; Tanaka, T. Macromolecules 1999, 32, 8589. (20) McBain, J. W.; Merrill, R. C., Jr.; Vinograd, J. R. J. Am. Chem. Soc. 1941, 63, 670.

10.1021/la991480e CCC: $19.00 © 2000 American Chemical Society Published on Web 07/08/2000

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Figure 1. Isotherm for R9φ(EO)10 binding onto NIPA gel at 25 °C. Data calculated under the assumption that the concentration of the surfactant in the aqueous phase inside the gel is equal to that in the outer bulk phase (9) and after correction of the concentration inside the gel (4).

Figure 2. Isotherm for R12SO4Na binding onto AAm gel at 25 °C. Data are calculated under the assumption that the concentration of the surfactant in the aqueous phase inside the gel is equal to that in the outer bulk phase. Sample Preparation. NIPA and AAm gels were prepared by radical polymerization. Mixtures of 39.6 g of NIPA or 25 g of AAm (both at 700 mM) monomer, 0.65 g of N,N′-methylenebis(acrylamide), and 1.2 mL of N,N,N′,N′′-tetramethylethylenediamine were dissolved in pure water to be 500 mL of the solution. Nitrogen gas was bubbled into the solution to purge oxygen for at least 30 min prior to polymerization. Aqueous solutions of ammonium persulfate (4 wt %) were bubbled by N2 gas, and parts of the solution (5 mL) were added to the monomer solution. The polymerization reaction was performed under an N2 gas atmosphere in a thin capillary (inner diameter of 800 µm) or in a beaker dipped in an ice/water bath. Gel samples thus obtained were washed thoroughly with enough pure water. The cylindrical gel 800 µm in diameter was employed for the solubilization measurements. The bulk gel prepared in a beaker was cut into cubes several centimeters in diameter. These cut gels were used for the determination of surfactant concentration inside the gel and of binding isotherms. Binding Isotherms. Cube gels of NIPA and AAm (∼15 g) cut from the bulk sample were put in a glass tube with a screw seal containing a surfactant solution. Many such tubes with different surfactant concentrations were allowed to stand at 25 °C ((1 °C) for 2 months. Equilibrium was maintained for 2 months, since no change in binding amount was observed for longer equilibra-

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Figure 3. Concentration of R12SO4Na in the NIPA gel interior plotted against its concentration in the outer bulk phase.

Figure 4. Concentration of R9φ(EO)10 in the NIPA gel interior plotted against its concentration in the outer bulk phase. tion times. The binding amount of surfactant onto the gel was calculated from eq 1 in determining the surfactant concentration change in the solution outside the gel

A)

W0C0 - C1(W0 + WW) WP/MW

(1)

where A is the binding amount expressed as the number of surfactant molecules per monomer unit of NIPA or AAm, C0 and C1 are the surfactant concentrations (mol/kg) of the solution outside the gel at initial and final (equilibrated) states, respectively, W0 is the mass (g) of the initial surfactant solution, WW and WP are the masses of water and polymer, respectively, in the gel at initial state (before immersed in the surfactant solution), and MW is the molecular weight of the NIPA or AAm monomer. In eq 1, the surfactant concentration in the aqueous phase of the gel interior is assumed to be equal to that of the solution outside the gel. The surfactant concentration of the solutions was determined by the high-performance liquid chromatography (HPLC) technique. A refractive index (RI) detector was employed for R12SO4Na, and a UV (275 nm) detector was used for R9φ(EO)10. The elution solvent was an 85:15 (v/v) mixture of methanol/ aqueous 0.4 M NaCl. The Cica-MERCK Hibar Lichrosorb RP-18 (5 µm) column was used. Kokufuta and his collaborators reported on the inhomogeneous distribution of an anionic surfactant, sodium dodecylbenzene

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Figure 5. Concentration of R12SO4Na in the AAm gel interior plotted against its concentration in the outer bulk phase. sulfonate, in a NIPA gel.21-23 If this is the case also in our system, our data of the binding amount should be the average of those in the whole gel body. But the inhomogeneity in the surfactant distribution is small and does not affect our results too much. Determination of Surfactant Concentrations in the Gel Interior. The cube gels of ∼15 g equilibrated with surfactant solutions were put in an injector and pushed under the pressure of about 2.5 × 104 Pa. This pressure was high enough to squeeze out the solution inside the gel, since the gel employed in this work was nonelectrolytic. The outlet of the injector was plugged with a small piece of cotton to avoid the leak of the gel. The aqueous solution inside the gel sample was squeezed out at room temperature at about 25 ˚C. One milliliter of the solution was squeezed out at early stage and thrown away, and the second milliliter was analyzed by HPLC in determining the surfactant concentration . It took about 15 min to collect the 2-mL solution. Solubilization of Yellow AB by Surfactant Aggregates. Cylindrical gels (1-2 g) 800 µm in diameter were put into a glass tube with a screw seal containing a surfactant solution (30 mL). The surfactant concentrations of the solutions were 4.0, 10, and 20 mM for R12SO4Na and 0.01, 1, and 10 mM for R9φ(EO)10. The samples were allowed to stand for 2 days at 25 °C to attain the binding equilibrium of the surfactant onto the gels. Small amounts of fine crystalline powders of Yellow AB were added to the surfactant solutions containing the gel samples, and the solutions were allowed to be solubilized. The water-insoluble dye is solubilized only into hydrophobic organic moieties, i.e., any surfactant aggregates in the present case. Equilibrium of the solubilization was obtained for 1 week, since no color change was observed in the gel samples or the outer surfactant solutions. The gels and the solution solubilizing Yellow AB were transferred to a shallow dish and were photographed.

Results Binding Isotherms. Figure 1 shows an isotherm for R9φ(EO)10 binding onto NIPA gel. As pointed out in the Introduction, the adsorption amount is apparently negative. A similarly strange result was also observed in the isotherm of R12SO4Na onto an AAm gel, as shown in Figure 2. The isotherm for R12SO4Na binding onto NIPA gel was presented in the previous paper19 and was normally in a sigmoidal shape at 25 °C. We calculated the binding amount in terms of eq 1, assuming that the surfactant (21) Kokufuta, E.; Nakaizumi, S.; Ito, S.; Tanaka, T. Macromolecules 1995, 28, 1704. (22) Kokufuta, E.; Suzuki, H.; Sakamoto, D. Langmuir 1997, 13, 2627. (23) Suzuki, H.; Kokufuta, E. Colloids Surf., A 1999, 147, 233.

Figure 6. Photographs of NIPA gels (two long cylindrical materials) and outer solutions solubilizing an oil-soluble dye, Yellow AB. The concentrations of R12SO4Na are 4 mM (a), 10 mM (b), and 20 mM (c).

concentration in the aqueous phase inside the gel was equal to that in the bulk solution outside the gel. This assumption may be doubtful. Then, we measured experimentally the concentration of surfactant inside the gel. Surfactant Concentrations in the Gel Interior. Figures 3-5 show the surfactant concentrations inside the gel plotted against the concentrations in the bulk solution outside the gel. Three completely different types of curves were obtained, depending upon the combinations of the surfactant and the gel. In the combination of R12SO4Na and NIPA gel (Figure 3), the surfactant concentrations both inside and outside the gel are identical below the critical micelle concentration (CMC; 8.2 mM) of the surfactant. Above the CMC, however, the surfactant concentration inside the gel deviates to a lower value and becomes parallel again to the diagonal line. This deviation is much larger in the

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Figure 7. Photographs of AAm gels and outer solutions solubilizing Yellow AB. The concentrations of R12SO4Na are 4 mM (a), 10 mM (b), and 20 mM (c).

R9φ(EO)10 and NIPA gel system, as seen from Figure 4. In this case, the surfactant concentration inside the gel seems to almost level off from the CMC (0.08 mM) of the agent. If this is actually the case, the result means that the nonionic surfactant, R9φ(EO)10, cannot form any micelles inside the NIPA gel. In the third case of R12SO4Na and AAm gels, both surfactant concentrations inside and outside the gel are same in all ranges of concentration tested. The binding amount calculated from eq 1 should be corrected when the surfactant concentration inside the gel is different from that outside the gel. The correction is not so large in the system of R12SO4Na and NIPA, and the data shown in the previous paper19 are approximately valid. The corrected binding isotherm for the system of R9φ(EO)10 and NIPA is shown in Figure 1 with the open triangles. The binding amount of the nonionic surfactant becomes zero, and the strange negative binding is cor-

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Figure 8. Photographs of NIPA gels and outer solutions solubilizing Yellow AB. The concentrations of R9φ(EO)10 are 0.01 mM (a), 1.0 mM (b), and 10 mM (c).

rected. The problem of the apparent negative binding of R12SO4Na onto AAm gel remained unsolved, since the surfactant concentrations were the same both inside and outside the gel (Figure 5). This problem will be discussed later. Solubilization of Yellow AB. Figures 6-9 are color photographs of the gels and the solutions equilibrated with the gels after the oil-soluble dye, Yellow AB, is solubilized. The part of yellowish color contains some aggregates of surfactant molecules. One can see from Figure 6 that R12SO4Na forms some aggregates in the NIPA gel interior before its micelle forms in the bulk outer solution. The binding of surfactant onto polymer chains in forming aggregates at concentrations less than the surfactant CMC also occurs in the case of linear polymer,24 and the result of Figure 6 is reasonably understood. The same surfactant, on the other hand, forms aggregates (24) Goddard, E. D. Colloids Surf. 1986, 19, 255.

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Figure 10. Model of the aqueous phases inside and outside the gel. Region I: the aqueous phase in the vicinity of polymer chains; the solution in this region is assumed not to be squeezed out even under pressure. Region II: the aqueous phase in the interchains of the gel networks; the solution in this region is assumed to come out under pressure. Region III: the bulk aqueous phase outside the gel.

Figure 9. Photographs of AAm gels and outer solutions solubilizing Yellow AB. The concentrations of R9φ(EO)10 are 0.01 mM (a), 1.0 mM (b), and 10 mM (c).

inside and outside the gel simultaneously, i.e., at CMC, when the AAm gel is employed (Figure 7). In this case, the surfactant behaves as if the gel networks were not present. The most surprising results in the solubilization experiments are shown in Figures 8 and 9. No aggregate of the nonionic surfactant, R9φ(EO)10, was observed in NIPA or AAm gel, even when the dye was solubilized in the outer bulk solutions at concentrations greater than CMC. These results correspond well to the data shown in Figure 4, indicating that the concentration of the surfactant inside the gel is kept nearly at CMC even above CMC in the outer bulk solution. Discussion Figure 10 shows a model interpreting all the experimental data obtained in this work. Region I is the aqueous phase in the vicinity of polymer chain. The water in this region is assumed not to be squeezed out even when pressure is applied. Adsorption of some surfactants onto

the polymer chains takes place in this region. The center of any surfactant micelles cannot enter this region due to the exclusive space of the polymer chains. Region II is the aqueous phase present in the interchains of gel networks. The water in region II is assumed to come out under pressure. Micelle formation of surfactants is strongly affected by the polymer chain networks in this aqueous phase. Region III is the aqueous phase outside the gel. The water in this region behaves as normal bulk water. Let us discuss the behavior of the surfactant in each system from the working hypothesis. R12SO4Na and NIPA Gel System. The surfactant molecules adsorb onto hydrophobic NIPA chains in region I to form some aggregates at concentrations lower than even the CMC of the agent (Figure 6). This is the same behavior as that of linear polymer solutions.24 The surfactant molecules binding in region I do not come out even under pressure. Micelle formation in region II of the surfactant also takes place in this system but in a different way from that in bulk aqueous phase. The surfactant concentration in the solution squeezed out under pressure was lower than that in the outer bulk solution. This may suggest that the number of micelles per unit volume is less than that in bulk phase resulting from electrostatic repulsion between micelle and the adsorbed layer of the agent on the polymer chain,25 or it suggests that the micellar molecular weight is smaller. In any case, we can say that the micelle formation in region II is strongly affected by the gel networks. R12SO4Na and AAm Gel System. Binding of the surfactant onto the polymer chain does not occur in this system (Figure 2). This may be reasonable, since the interaction between R12SO4Na and the AAm polymer with no hydrophobic part is known to be weak.26 Micelles form (25) Fava, A.; Eyring, H. J. Phys. Chem. 1956, 60, 890.

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in region II, as shown in Figure 7. Apparent negative binding in Figure 2 is interpreted as showing that the micelles are excluded from the vicinity of the polymer chains (region I). The distance from the polymer chain of the excluded space was estimated to be 2-4 nm from the amount of apparent negative adsorption, and the estimate agreed well with the radius of the micelle. R9O(EO)10 and NIPA or AAm Gel System. The nonionic surfactant shows neither binding nor micelle formation in both NIPA and AAm gels (Figures 1, 4, 8, and 9). These results are quite novel and interesting. The gel networks prevent the nonionic surfactant molecules from forming their micelles. The micelles of nonionic surfactants are generally larger than those of ionic ones (26) Goddard, E. D. In Interactions of Surfactants with Polymers and Proteins; Goddard, E. D., Ananthapadmanabhan, K. P., Eds.; CRC Press: Boca Raton, FL, 1993; p 158.

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so that a micelle of the R9φ(EO)10 might not be contained in the mesh size of the gel networks. In other words, the micelle is not stable when it includes the polymer chain(s) in its interior. Another possible explanation is the segregation mechanism. In general, two types of nonionic polymer do not mix with each other in their solutions because of the small gain of mixing entropy. Samesegregation phenomena take place when one polymer is exchanged to a nonionic surfactant micelle.27,28 Polymer concentration is considerably high inside the gel, and this segregation mechanism could also work in our system. LA991480E (27) Piculell, L.; Bergfeldt, K.; Gerdes, S. J. Phys. Chem. 1996, 100, 3675. (28) Bergfeldt, K.; Piculell, L.; Linse, P. J. Phys. Chem. 1996, 100, 3680.