Interaction of hydrophobically modified poly(acrylic acid) hydrogels with ionic surfactants

The role of the hydrophobic effect in polyelectrolyte gel/ionic surfactant interactions was investigated. A series of hydrophobically modified poly(acrylic acid) gels with well-controlled hydrophobicity was synthesized by free-radical copolymerization of acrylic acid with n-alkyl acrylates (n = 8, 12, and 18) in the presence of a cross-linking agent. The interaction of the gels with ionic surfactants-N-cetylpyridinium chloride and sodium dodecylbenzenesulfonate-was studied. Four gel/surfactant combinations were considered: uncharged gel/cationic surfactant, uncharged gel/anionic surfactant, anionic gel/cationic surfactant, and anionic gel/anionic surfactant. It was shown that the gels absorb cationic surfactant ions via an ion-exchange reaction with network counterions. On the other hand, the absorption of anionic surfactant is governed primarily by hydrophobic interactions, and hence, due to conditions of electroneutrality, anionic surfactant penetrates the gel together with corresponding co-ions. Therefore, the uptake of cationic surfactant ions results in gel shrinkage, while the uptake of anionic surfactant induces gel swelling. In the anionic gel/anionic surfactant system, a significant interaction is observed only for the most hydrophobic gels when hydrophobic interactions overcome the electrostatic repulsion between similarly charged groups.