Hydration of ions and salt crystallization in liquid phase coexistent with ice at temperature below eutectic point

Abstract

X-ray absorption fine structure (XAFS) clearly indicates the existence of hydrated Br⁻ in a frozen aqueous RbBr solution at the temperatures lower than the eutectic point (t_eu) of the system. The relative population of hydrated Br⁻ decreases with increasing concentration of RbBr in an original solution, and Br⁻ consisting of the RbBr crystal lattice becomes dominant at the higher concentration of RbBr. This implies the existence of the liquid phase at the subeutectic temperatures. Continuous XAFS measurements over ca 6 h detect the progress of the RbBr crystallization. The analysis based on the populations of hydrated Br⁻ and Br⁻ in the RbBr crystal suggests that the ions on the surface of the salt crystal are partially hydrated. These strongly imply that the metastable liquid phase is in contact with salt crystals at the temperature below t_eu. Since impurities such as a salt are accumulated along the grain boundaries of ice crystals, the metastable liquid phase should thus be present together with the accumulated salt crystals.