Interactions of Cationic/Nonionic Surfactant Mixtures with an Anionic Hydrogel: Absorption Equilibrium and Thermodynamic Modeling

Abstract

Association of mixed dodecyl trimethylammonium bromide (C12TAB) and octaethylene glycol monododecyl ether (C12E8) micelles with a lightly cross-linked sodium polyacrylate gel is examined as a function of surfactant concentration and mixed-surfactant fraction. The interaction of the surfactants with the gel is quantified by absorption isotherms measured using a surfactant-specific electrode, to determine the unabsorbed C12TA+ concentration, and by 1H NMR, to determine the relative fraction of unabsorbed C12E8 to C12TA+. These experiments provide, for the first time, a detailed assessment of the relative affinity of a polyelectrolyte gel for surfactants of varying charge. As might be expected, the gel preferentially absorbs C12TA+ under most conditions owing to electrostatic attraction to the oppositely charged gel. For low initial C12E8 surfactant fractions and moderate C12TA+ concentrations, however, the situation is reversed and the nonionic surfactant is preferentially absorbed. Furthermore, although pu...