Thermodynamic Perturbation Theory for Taking into Account Dipole–Dipole Interactions to Describe the Densities of Molten Alkali Metal Halides

Given the growing industrial interest in the use of multicomponent molten salts, the development of an equation of state capable of correctly predicting the densities of liquid electrolytes over wide temperature and concentration ranges is one of the challenging problems of physical chemistry. Such an equation should take into account not only the main contributions to pressure, but also the most significant second-order effects caused by the polarizability of ionic electron shells and the mutual orientation of induced dipoles in space. In this work, a new version of the equation of state is proposed to take into account both the charge–dipole and dipole–dipole corrections to the pressure of molten salts using a thermodynamic perturbation theory based on the charged hard sphere model. This equation of state is applied to calculate the packing coefficients of molten alkali metal halides at their melting points with allowance for the new dipole–dipole correction. A combined consideration of the charge–dipole and dipole–dipole contributions to the pressure is found to increase the packing coefficients of the melts by up to 25%. The contribution of dipole–dipole interactions is shown to be an order of magnitude smaller than the charge–dipole contribution. An analysis of the influence of these second-order effects on the packing coefficient in a raw of 20 alkali metal halides demonstrates that taking into account both the charge–dipole and dipole–dipole interactions leads to a more significant increase in the densities of the melts containing larger and more polarizable cations and anions.