Equation of State for Characterizing the Density of Lithium, Sodium, and Potassium Molten Halides

In view of ever-increasing interest of the industry in multicomponent salt solutions, those engaged in physical chemistry are facing a problem of deriving equations of state that could correctly predict the density of liquid electrolytes in a wide range of temperatures and concentrations. Such an equation of state must take into account not only main contributors to the pressure but also significant second-order effects due to the electron shell polarizability in ions. In this study, an equation of state that considers the interaction of ionic charge with induced dipoles involving a thermodynamic perturbation theory (based on the model of charged hard spheres) has been applied to construct the temperature dependences of the density of molten lithium, sodium, and potassium halides. Using this equation of state, we have managed to fairly accurately describe main features in the variation of the melt density with temperature and composition. In passing from fluorides to chlorides, the densities of considered melts first slightly decrease near melting points and then rise as bromides and iodides are substituted for chloride anions. This is in complete agreement with experimental data. For all salts, the discrepancy between calculated and experimental dependences was no greater than ten percent. The best agreement was observed for bromide and chloride melts, which is qualitatively explained in the text. In general, it has been shown that calculation data are in good qualitative and quantitative agreement with published results in respect that calculations were performed using only tabulated values of ionic radii and polarizabilities.