  Knowledge of the vapor-liquid equilibrium (VLE) properties of molten salts is
important in the design of thermal energy storage systems for solar power and
nuclear energy production applications. The high temperatures involved make
their experimental determination problematic, and the development of both
macroscopic thermodynamic correlations and predictive molecular-based
methodologies are complicated by the requirement to appropriately incorporate
the chemically reacting vapor-phase species. We derive a general
thermodynamic-based atomistic simulation framework for molten salt VLE
prediction and show its application to NaCl. Its input quantities are
temperature-dependent ideal-gas free energy data for the vapor phase reactions,
and density and residual chemical potential data for the liquid. If these are
not available experimentally, the former may be predicted using standard
electronic structure software, and the latter by means of classical atomistic
simulation methodology. The framework predicts the temperature dependence of
vapor pressure, coexisting phase densities, vapor phase composition, and
vaporization enthalpy. It also predicts the concentrations of vapor phase
species present in minor amounts (such as the free ions), quantities that are
extremely difficult to measure experimentally. We furthermore use the VLE
results to obtain approximations to the complete VLE binodal dome and the
critical properties. We verify the framework for molten NaCl, for which
experimentally based density and chemical potential data are available in the
literature. We then apply it to the analysis of NaCl simulation data for two
commonly used atomistic force fields. The framework can be readily extended to
molten salt mixtures and to ionic liquids.
