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 ZnCl₂ 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 ZnCl₂·R H₂O, 0 > R > ∞ composition range using ultrapure, doubly sublimed ZnCl₂ establish the ZnCl₂ : H₂O phase diagram. The results are consistent with the existence of crystalline hydrates at R = 1.33, 3, and 4.5 and identify a mechanistic pathway for hydration. All water is not removed from hydrated ZnCl₂ 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 ZnCl₂ preparations. The strong interaction between ZnCl₂ and water exerts a significant impact on the solvent water such that the system exhibits a deep eutectic at a composition of about R = 7 (87.5 mol %) and a eutectic temperature below -60 °C.