Hydration of NaCl on glassy, supercooled-liquid, and crystalline water.

Interactions of sodium chloride with amorphous and crystalline water films, leading to the possible formation of a dilute NaCl solution, were investigated using time-of-flight secondary ion mass spectrometry as a function of temperature. A monolayer of NaCl tends to remain on the surface or in subsurface sites of thick amorphous solid water films (200 monolayers); the Na+ ion is hydrated preferentially, whereas the Cl- ion is segregated at the surface. The hydration structure of NaCl is fundamentally unchanged for viscous liquid water that appears at temperatures higher than 136 K. The solubility of NaCl increases abruptly at 160 K because of the evolution of supercooled liquid water, which can hydrate the Cl- ion efficiently. However, the diffusion of the ions toward the bulk of supercooled liquid water is interrupted by crystallization; therefore, the dilute NaCl solution that is characterized by completely separated Na+-Cl- pairs may not be formed. When NaCl is deposited on the crystalline ice film, hydration of NaCl is enhanced above 160 K as well, indicating that a liquidlike phase coexists with crystals.