Water Films: The Motor of Phase Transitions in Salt Mixtures

The interaction of water vapor with soluble electrolytes has been comprehensively studied due to their ubiquity in nature and industry. Enormous experimental works have elucidated the mechanisms of water molecule adsorption, layer growth, and ion solvation in the water film on a NaCl crystal surface. In this study, we employ a new method to investigate the moisture uptake behavior on the NaCl crystal surface under varying humidity levels by using environmental scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (ESEM-EDX). Additionally, water vapor sorption, ESEM, and Raman microscopy are used to examine the role of water films in facilitating phase transitions and chemical reactions of salt mixtures. Our results indicate that the water uptake on a NaCl crystal surface initiates early and increases with rising humidity. A sudden increase in the condensation rate above 60% RH suggests ion solvation in the water film. In ternary mixtures, the water film occurs by water vapor adsorption or capillary condensation at the interface between two crystals in an early stage, where ions from both sides dissolve into the interfacial water film to form a nanoscale brine film. This process initiates the mutual deliquescence of a NaCl-KCl mixture at a relative humidity lower than the deliquescence humidity of either of the two single salts. For the solid-solid reaction NaNO₃ + KCl → NaCl + KNO₃, the process involves the dissolution of reactants and the precipitation of products, where the interfacial water film acts as a liquid bridge, promoting the ion diffusion and exchange between the two reactants.