Simple Gel Swelling Experiments Distinguish between Associating and Nonassociating Polymer−Surfactant Pairs

Abstract

Simple experiments, based on volume changes of cross-linked polymer gels immersed in surfactant solutions, can be used to distinguish between associating and nonassociating nonionic polymer/ionic surfactant pairs. For an associating pair, the cooperative binding of the surfactant at the critical association concentration gives rise to a significant increase in the gel volume. Here, we study gels based on hydroxyethyl cellulose (HEC). Alkyl sulfate surfactants are shown to bind to HEC, but only for tail lengths longer than 8 carbons. No binding is found for cationic surfactants. All tested surfactants bind to hydrophobically modified HEC, but the extent of gel swelling (and, presumably, surfactant binding) is much larger for those surfactants that also bind to nonmodified HEC. We also study how the swelling equilibria are affected by added salt and demonstrate that valuable information may be gained from transient volume changes as the gels approach swelling equilibrium.