Simple gel swelling experiments distinguish between associating and nonassociating polymer-surfactant pairs

Simple experiments, based on volume changes of cross-linked polymer gels im mersed in surfactant solutions, can be used to distinguish between associat ing and nonassociating nonionic polymer/ionic surfactant pairs. For an asso ciating pair, the cooperative binding of the surfactant at the critical ass ociation concentration gives rise to a significant increase in the gel volu me. Here, we study gels based on hydroxyethyl cellulose (HEC). Alkyl sulfat e surfactants are shown to bind to HEC, but only for tail lengths longer th an 8 carbons. No binding is found for cationic surfactants. All tested surf actants bind to hydrophobically modified HEC, but the extent of gel swellin g (and, presumably, surfactant binding) is much larger fur those surfactant s that also bind to nonmodified HEC. We also study how the swelling equilib ria are affected by added salt and demonstrate that valuable information ma y be gained from transient volume changes as the gels approach swelling equ ilibrium.