Noble Gas Aggregation and Dynamics in Molten Alkali Chloride Salts

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Abstract
Noble gas behavior in molten alkali chloride salts is important for understanding gas retention, transport, and removal in high-temperature molten salt environments. Although previous molecular simulations have examined single-particle properties such as noble gas solubility and diffusion, the thermodynamics of early-stage gas aggregation in molten salts remain less well understood. In this work, we develop an ab initio-informed classical molecular dynamics framework to quantify stepwise noble gas aggregation in molten LiCl and KCl. Salt-specific gas-ion interaction parameters are obtained by combining quantum mechanical gas-ion interaction scans with ab initio molecular dynamics structural benchmarks. The resulting classical models are then used to compute potentials of mean force for two-body, three-body, and four-body association geometries of argon and xenon. Long-timescale simulations are also used to compute diffusion coefficients, providing a complementary dynamical perspective on gas behavior in each salt. The potentials of mean force show that argon exhibits a consistent tendency toward local association in both salts, with stronger stabilization in LiCl. Xenon displays a more salt-dependent response: KCl supports increasingly favorable xenon association across the sampled aggregation geometries, whereas LiCl shows weaker association and larger intermediate barriers. Diffusion coefficients indicate that both gases are more mobile in KCl than in LiCl, with argon diffusing slightly faster than xenon within a given salt. Overall, these results show that noble gas aggregation in molten salts cannot be predicted from gas size or gas-gas attraction alone, but instead depends on the coupling between gas identity and the salt-specific local environment.

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