Investigations on the binding behavior of competitive ions to a cationic micelle by ¹H -NMR studies

Abstract

The binding nature of aliphatic carboxylate ions, viz. acetate, propionate and butyrate, to a cationic micellar surfactant, viz. cetylpyridinium chloride, has been investigated by ¹H-NMR spectroscopy. The carboxylate ions are expected to compete with the chloride ion for binding at micellar interface due to the specific ion effect. The counterions, Ac^-, Pr^- and Bu^-, bind to the cetylpyridinium micelle such that the distance of COO^- group from the head group of CPC is almost the same for all the three counterions. This implies that as the length of the alkyl chain of the aliphatic carboxylate counterion increases, it stretches deeper into the palisade layer or core of the micelle. Furthermore, the preference of one ion over another for binding at biological interfaces is broadly termed as specific ion effect and micelles are used as models to mimic the biological membrane. Therefore, effect of competitive ions on the micellization of ionic surfactants provides useful information to better understand the effect of specific ions.