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 (C 12 E 8 ) 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 1 H NMR, to determine the relative fraction of unabsorbed C 12 E 8 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 C 12 E 8 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 C 12 E 8 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.