A molecular simulation study of Cs-Cl and Cs-F ion pairs in hydrothermal fluids

Magmatic-hydrothermal processes play an important role in the transport, enrichment, and mineralization of cesium. In this study, classical molecular dynamics simulations were performed to investigate the properties of Cs-Cl and Cs-F ion pairs in hydrothermal fluids. The association constants (log10KA(m)) under a wide range of temperature (i.e. 298–1273 K) and fluid density (i.e. 0.1–1.0 g/cm3) were derived from the potential of mean force (PMF) curves. The results indicate that Cs-Cl and Cs-F ion pairs have similar stabilities. This is different from other alkali metal cations (e.g., Li+, Na+, and K+), which prefer binding with F over Cl. The stabilities of Cs-Cl and Cs-F ion pairs increase with increasing temperature (except for the fluid density ≤ 0.1 g/cm3) or decreasing fluid density, which is similar to other alkali halide ion pairs. Comparisons among the stabilities of Cs-Cl/F and other alkali halide ion pairs indicate that the Li–F ion pair has the highest stability in hydrothermal fluids.