A PROTON NMR-STUDY ON THE STRUCTURE OF WATER AT INTERFACES OF CATIONIC MICELLES - EFFECTS OF THE NATURE OF THE SURFACTANT HEADGROUP

Fractionation factors, phi, were determined for water at interfaces of cationic micelles from solvent H-1 NMR chemical shifts in H2O-D2O mixtures. Values of phi are (cetyldimethyl(3-phenylpropyl)ammonium chloride) 1.08 +/- 0.02, (cetylquinuclidinium chloride) 1.07 +/- 0.02, (cetyltributylammonium bromide) 1.14 +/- 0.02, and (cetyltris(2-hydroxyethyl)ammonium chloride) ca. 1. The first two values are similar to that for cetyltrimethylammonium chloride (1.06 +/- 0.04). The dependence of phi on the structure of the surfactant headgroup is rationalized in terms of solvent-headgroup interactions, but for the tris(2-hydroxyethyl) surfactant we also have to consider specific interactions of Cl- with the 2-hydroxyethyl groups. The dependence of the chemical shift of H2O on the concentration of methyltris(2-hydroxyethyl)ammonium chloride (a nonaggregating model for the latter surfactant) shows that interaction of Cl- with 2-hydroxyethyl groups is a micelle-related phenomenon.