Proton NMR Studies on the Structure of Water at Interfaces of Aqueous Micelles. Part 4: Effects of Cationic and Zwitterionic Headgroups

Fractionation factors, φ M , of interfacial water at surfaces of cationic and zwitterionic surfactants were determined from the dependence of the 1 H chemical shift of water on the isotopic composition of the solvent (H 2 O plus D 2 O). Values of φ M are greater than unity, showing that interfacial water is more structured than bulk water, whose fractionation factor = 1, by definition. The structuring increases with increasing bulk of the headgroup, but is unaffected by the length of the cationic surfactant hydrophobic tail. The effect of zwitterionic sulfobetaine surfactants on the structure of their water of hydration is less than that of cationic surfactants that have otherwise similar headgroups. Values of φ M for cationic micelles are: cetyldimethylphenylammonium chloride, 1.08; cetylpyridinium chloride, 1.06; and dodecyltrimethylammonium bromide, 1.06. Values of φ M for sulfobetaine micelles are: 3‐N‐tetradecyl‐N,N‐di‐n‐butylammonio‐1‐propane sulfonate, 1.07, and 3‐N‐tetradecyl‐N,N‐diethylammonio‐1‐propane sulfonate, 1.03.