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.