Presence or absence of counterion specificity in the interaction of alkylammonium surfactants with alkylacrylamide gels

Patterns in the interaction of cationic surfactants with nonionic polymer gels, which were inferred from a recent study from our laboratory, are confirmed by measurements of a series of alkylammonium surfactants with different counterions with a series of alkyl acrylamide gels of increasing hydrophobicity. Two swelling patterns were observed: Either the swelling continued above the surfactant critical micelle concentration (cmc) and the maximum swelling differed for different counterions and increased in the order of Br-<Cl-<Ac-, in agreement with the degree of dissociation of these counterions and with the Hofmeister series or the swelling stopped below (or close to) the surfactant cmc, implying saturation binding, and the maximum swelling was similar for each of the three counterions. Binding studies confirmed that the amount of surfactant bound depends on the counterion in cases where saturation binding is not reached but not in cases where saturation binding is reached. The swelling/binding patterns could be rationalized in terms of a shift from partial binding to saturation binding with increasing gel hydrophobicity and an electrostatic intermicellar repulsion effectively independent of specific counterion binding, owing to "condensation" at the highly charged micellar surface.