Molten fatty acid salts as model ionic liquids. I. Thermodynamic and transport parameters of some organic sodium salts

Abstract

Procedures are described for maintaining good chemical stability in molten alkali-metal carboxylates, up to about 350 °C. Valid physical measurements can be made and the fluids can be used up to about this temperature, above which spontaneous decomposition of the anions is difficult to repress. Molten sodium propionate has a useful liquid range of about 60 °C and sodium isobutyrate of about 90 °C. Sodium n -butyrate transforms into a ‘liquid crystal’ at about 250 °0 and into the isotropic liquid at 324 °C. For sodium isovalerate corresponding transition points are respectively 188 and 280 °C. Thermodynamic measurements are reported of volume and enthalpy changes at transition and melting points. Transport parameters measured include the viscosity and the electrical conductivity. The viscosity of these ionic melts undergoes a steep decrease at the transition from mesomorphic to isotropic liquids. Jumps in ionic conductivity are found both at the melting and clearing points. Even for the isotropic liquids, the ratio of viscosity to electrical conductivity is exceptionally high, compared with the other ionic melts. Mechanisms of melting for these ionic solids are discussed.