Calculation of the Densities of Molten Alkaline Earth Metal Halides with Allowance for the Interionic Charge–Dipole Interactions in the Melts

The aim of this work is to develop and verify an equation of state to calculate the densities of molten electrolytes under various conditions using a minimum set of parameters (ionic radii and polarizabilities) describing the pair interactions of ions in the melts. To increase the accuracy of calculations, this equation of state takes into account not only the basic repulsive and Coulomb contributions to pressure, but also the second-order corrections caused by interionic charge–dipole interactions. The developed approach is based on the application of thermodynamic perturbation theory, which considers the interactions between ion charges and induced dipoles using a reference system of charged hard spheres of arbitrary diameters and charges, since it has an exact solution within the mean spherical approximation. This version of the equation of state is applied to calculate the densities of the entire subclass of molten alkaline earth metal halides at their melting points and to analyze the influence of the charge–dipole correction on the density. Discrepancies of no more than a few percent are achieved for a number of salts using the proposed approach; however, these discrepancies increase in systems with larger ions. The charge–dipole interactions are found to increase the densities of the melts by up to 20%, which improves the agreement with experiment. This contribution increases when going to melts containing larger and more polarizable cations and anions.