Mass Transport in Ionic Liquids

Abstract

Mass Transport In Ionic Liquids by Robin John Speedy The measurement and interpretation of the volume and temperature dependence of the electrical conductance of some ionic liquids are presented. The conductance (۸) and volume (V) of the salts R4N+BF-4, where R=n-butyl to n-heptyl, were measured in the temperature range 90°c to 230°c, and at pressures up to 2000 bar. A method was developed to measure ۸ and V simultaneously. The results are more precise (۸ (p,T) ±0.1% ^ (V,T) ±0.15%, and V(p,T) ±0.03%)than previous data for ionic or molecular liquids. In addition, the melting point of the salts, and their n-octyl analogue, was measured as a function of pressure, and a new method was developed for preparing quaternary ammonium salts efficiently from the primary amine. A model is presented which quantifies the notion of structure in liquids, in terms of a distribution of coordination. The postulate that transport is related to the incidence of uncoordinated molecules, No, and the calculation of No 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. These relations account comprehensively for the complicated temperature and volume dependence of Ev = -R(d ln۸/d1/T)v observed for many liquids, including the present salts, and for the conjugate parameter Δp = R(d ln۸/dV)T and its variation. Further development of the theory leads to an expression for ۸, as an explicit function of volume and temperature, which accounts for the conductance measurements. The model is more plausible than most other simple models of liquids, and it gives greater insight into the structure and properties of liquids than the formal radial distribution function theories.