Swelling of poly(acrylamide) gels with pendant poly(ethylene oxide) chains in solutions of ionic surfactant and salt

The swelling in water and in ionic surfactant solutions of a polymer gel based on poly(acrylamide) (PAm) with pendent poly(ethylene oxide) (PEO) chains (0-2.6 mol %) was investigated. In pure water, the swelling increased linearly with the content of PEO side chains. The anionic surfactants sodium dodecyl sulfate (SDS) and sodium octylbenzenesulfonate (SOBS) both bound to the PEO side chains above a critical association concentration (cac). For SOBS, binding isotherms to the gels were obtained, and the cac values for the surfactant in PEO solutions were determined by NMR, both at varying concentrations of added NaCl. Both surfactants affected the swelling of the copolymer gels similarly. When the surfactant concentration in the swelling medium was increased at low concentrations of added NaCl, a substantial swelling occurred at the cac, and the volume continued to increase up to a concentration just above the critical micellization concentration (cmc) for the free surfactant in the swelling medium. At higher concentrations of surfactant the gels started to deswell. At high contents of added NaCl (ca. 0.5 M and above), the swelling isotherm changed: The gels instead began to shrink at the cac, indicating a crosslinking of the PEO chains by the surfactant micelles. The gel volume went through a minimum when the ratio of PEO chains to bound micelles was approximately 2. At higher concentrations, the gel continued to swell until it was saturated with surfactant. The amount of bound surfactant at saturation increased with large amounts of added salt. The experiments in the presence of salt clearly showed that the binding of surfactant to the gels continues even at surfactant concentrations exceeding the cmc in the swelling medium.