2H and14N NMR Studies of Amphiphilic Liquid Crystals: Effect of Solubilisation, Electrolyte and Temperature on Water Orientation

Abstract

Deuteron NMR on hexagonal and lamellar amphiphile -D₂O mesophases was employed to study the partial orientation of the water molecules and how the degree of orientation is influenced by solubilisation of organic compounds, by addition of simple electrolytes and by temperature changes. Solubilisation effects follow roughly the polarity of the solubilisate, a more polar solubilisate producing a greater reduction in the degree of water orientation, but important differences exist between -CO⁻ ₂ and -OSO⁻ ₃ end-groups of the surfactant. These results are discussed in terms of changes in amphiphile hydration and altered packing conditions in the aggregates. The effect of electrolyte addition depends markedly on both counterion and co-ion and is discussed on the basis of ion hydration. For the tetramethylammonium octanoate -D₂O system the degree of water orientation increases with increasing water content and this is suggested to arise from a particular counter-ion binding mechanism in this case. The counter-ion binding in this system as well as the ammonium octanoate -D₂O system was further studied by means of ¹⁴N quadrupole splittings.