Nuclear magnetic resonance studies of liquid crystal phase structure and self-diffusion coefficients in the system lithium perfluoro-octanoate + water

Abstract

In the lithium perfluoro-octanoate + water system, the reversed hexagonal structure assigned previously to the highest surfactant concentration liquid crystalline phase is inconsistent with ¹⁹F n.m.r. observations, and an alternative lamellar structure (viscous neat) is proposed. Self-diffusion coefficients are reported for water, lithium ions and perfluoro-octanoate ions in lamellar (neat) and viscous neat phases, as a function of temperature. The difference between diffusion along the water layers, and diffusion across the perfluoro-octanoate bilayers was small for both water and lithium ions, although the Li⁺ diffusion coefficient was more than an order of magnitude less than that in lithium chloride solutions, and only a factor of three larger than that of perfluoro-octanoate ions. No discontinuity in the diffusion coefficients was observed at the viscous neat/lamellar transition. This and other evidence is consistent with the transition being second order.