On the Local Binding of Ionic Surfactants to Poly(N-isopropylacrylamide) Gels

The effects of sodium dodecyl sulfate (SDS) and sodium dodecylbenzenesulfonate(NaDBS) on the volume phase transition ofN-isopropylacrylamide (NIPA) gels were studied as a function of surfactant concentration and temperature. Both the transition temperature and the swelling ratio increased with increasing surfactant concentration due to the conversion of a nonionic into an ionic NIPA gel through the binding of SDS and NaDBS. This was qualitatively supported by the fact that an increase in the concentration of SDS or NaDBS facilitated the surfactant uptake by the gel. However, little difference was observed in the concentration dependence of the transition temperature between SDS and NaDBS, while SDS exhibited a stronger effect than NaDBS on the concentration changes of the swelling ratio and surfactant uptake. In order to elucidate this aspect, kinetic studies ofNaDBS uptake were performed using cylindrical gels with different diameters. We employed a dry gel free of water, in addition to the usual wet gel, because the dry gel is expected to immediately absorb the solution and to scatter the solute molecules all over the gel phase. An increase in the diameter (therefore, the available surface area) of the wet gels was found to facilitate the surfactant uptake over an experimental period, when its amount was expressed in moles per unit dry mass. However, such a size dependence was negligibly small in the dry gel. These results were interpreted by assuming that the surfactant molecules bind only to the polymer networks located near the gel surface but not to all the networks within the gel. Such an inhomogeneous local binding of surfactant was demonstrated by determining the distribution of NaDBS within thin disks of the surfactant-bound gels using microscopic spectroscopy.