Swelling isotherms of surfactant-responsive polymer gels

Abstract

Recent progress in the study of surfactant-responsive polymer gels is reviewed. Polymer gels can be designed that drastically swell or shrink in response to small changes in the concentration of an ionic surfactant in an external bath containing the gel. The gel swelling isotherm, where the equilibrium gel volume is measured as a function of the external surfactant concentration, yields useful information on polymer—surfactant interactions. Associating and nonassociating polymer-surfactant pairs may be distinguished, and critical concentrations for surfactant binding can be determined. Generic swelling isotherms for different classes of systems have been established, which apply to chemically widely different gels (cellulose derivatives, vinyl polymers). Hydrophobicity and charge are the essential polymer parameters that determine the features of these isotherms. In order for a surfactant to associate to a non-ionic polymer, the polymer hydrophobicity has to exceed a certain threshold. The required threshold hydrophobicity varies with the alkyl chain length, the headgroup, and the counterion of the surfactant. Added salt has large effects on all swelling isotherms.