Simultaneous conductance and volume measurements on molten salts at high pressure

Abstract

A simple method is described which allows the simultaneous measurement of the conductance and volume of ionically conducting liquids at high pressure. Precise and extensive (90 to 220°C, 10 to 2000 bar) pressure, temperature, volume and conductance data are presented for the salts R₄N⁺BF (where R is n-butyl to n-heptyl). None of the existing theories which predict the temperature and volume dependence of transport in liquids is consistent with the results. A simple model is developed which quantifies the notion of structure in liquids in terms of a distribution of coordination number, as suggested by Bernal.¹ The postulate that transport is related to the incidence of uncoordinated molecules, N₀, and the calculation of N₀ in terms of the partition function for the model allow general relations to be established between the temperature and volume dependence of transport and the thermodynamic properties of the liquid, which account in a comprehensive qualitative way for the trends observed in real liquids. A further development of the model accounts quantitatively for the temperature and volume dependence of the molar conductivity of the series of molten salts. These results support the basic physical postulates of the model, that the temperature and volume dependence of transport in liquids are related to the frequency of the local fluctuations in structure as indicated by the incidence of uncoordinated molecules.