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 octae thylene glycol monododecyl ether (C12E8) micelles with a lightly cross-link ed sodium polyacrylate gel is examined as a function of surfactant concentr ation and mixed-surfactant fraction. The interaction of the surfactants wit h 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 assess ment of the relative affinity of a polyelectrolyte gel for surfactants of v arying charge. As might be expected, the gel preferentially absorbs C(12)TA (+) under most conditions owing to electrostatic attraction to the opposite ly charged gel. For low initial C12E8 surfactant fractions and moderate C(1 2)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 ab sorption regimes are observed with increasing C12E8 surfactant fractions. A n absorption model is developed that combines the closed association model, describing ionic surfactant association with an oppositely charged polyele ctrolyte, with a mixed micellization model, describing nonionic surfactant association with polyelectrolyte-bound ionic micelles. The model semiquanti tatively captures many of the observed absorption trends and provides a bas is for interpreting the anomalous absorption behavior with changing surfact ant composition. In particular, the model captures the effects of mixed-sur factant micellization in bulk aqueous solution coupled with mixed-surfactan t aggregation within the gel network.