Corresponding states theory for the fusion of ionic crystals

Abstract

Empirical deviations from a general thermodynamic-reduced equation of state for the orthobaric melting temperatures of alkali metal halide crystals are interpreted. A corresponding states analysis based upon reduced variables for the interaction potentials between ions shows that two limiting laws are obeyed. When the anion/cation radius ratio approaches unity the characteristic corresponding reduced length is proportional to the sum of ionic radii, but when this ratio is large, e. g. Li salts, the radius of the anion becomes the appropriate reduction parameter. The large difference in reduced melting temperatures between common-cation conformal groups can be understood by means of a first order perturbation treatment for the contribution of ion-induced dipole and short-range anion-anion interactions to the free energy difference between the crystalline and liquid phases of a reference salt in which these potentials are considered to be absent. Potential energy contributions due to nonconformable perturbations are found to be relatively small; this explains the observed phenomenological conformality among all the molten alkali metal halides, and many other ionic liquids, for both thermodynamic properties and linear transport coefficients.