Understanding the Water-in-Salt to Salt-in-Water Characteristics across the Zinc Chloride : Water Phase Diagram.

Using a series of time- and temperature-resolved synchrotron diffraction experiments, the relationship between multiple polymorphs of ZnCl and its respective hydrates is established. The δ-phase is found to be the pure anhydrous phase, while the α, β, and γ phases result from partial hydration. Diffraction, gravimetric, and calorimetric measurements across the entire ZnCl· HO, 0 > > ∞ composition range using ultrapure, doubly sublimed ZnCl establish the ZnCl : HO phase diagram. The results are consistent with the existence of crystalline hydrates at = 1.33, 3, and 4.5 and identify a mechanistic pathway for hydration. All water is not removed from hydrated ZnCl until the system is heated above its melting point. While hydration/dehydration is reversible in concentrated solutions, dehydration from dilute aqueous solutions can result in loss of HCl, the source of hydroxide impurities commonly found in commercial ZnCl preparations. The strong interaction between ZnCl and water exerts a significant impact on the solvent water such that the system exhibits a deep eutectic at a composition of about = 7 (87.5 mol %) and a eutectic temperature below -60 °C.