Interactions of cationic/nonionic surfactant mixtures with an anionic hydrogel: Absorption equilibrium and thermodynamic modeling

Association of mixed dodecyl trimethylammonium bromide (C(12)TAB) 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 C(12)TA(+) concentration, and by H-1 NMR, to determine the relative fraction of unabsorbed C12E8 to C(12)TA(+). 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 C(12)TA(+) under most conditions owing to electrostatic attraction to the oppositely charged gel. For low initial C12E8 surfactant fractions and moderate C(12)TA(+) concentrations, however, the situation is reversed and the nonionic surfactant is preferentially absorbed. Furthermore, although pure C(12)TA(+) exhibits only one cooperative absorption regime, multiple cooperative absorption regimes are observed with increasing C12E8 surfactant fractions. An absorption model is developed that combines the closed association model, describing ionic surfactant association with an oppositely charged polyelectrolyte, with a mixed micellization model, describing nonionic surfactant association with polyelectrolyte-bound ionic micelles. The model semiquantitatively captures many of the observed absorption trends and provides a basis for interpreting the anomalous absorption behavior with changing surfactant composition. In particular, the model captures the effects of mixed-surfactant micellization in bulk aqueous solution coupled with mixed-surfactant aggregation within the gel network.