To Freeze or Not to Freeze: Questioning the Origin of the Deep Eutectic in the Zinc Chloride Hydrate System

Abstract

Zinc chloride is one of the most soluble salts in aqueous solutions, with an ambient temperature solubility of over 430 g per 100 g of water (ZnCl2·R H2O, R = 1.8). It also exhibits a deep eutectic, at R = 7.25 and -60 °C between pure water and the most water-rich crystalline hydrate, R = 4.5 (see Figure). We previously demonstrated that the molecular ions of the crystalline R = 3 hydrate persist into the melt (Tm = 5 °C) resulting in the ionic liquid [Zn(OH2)6][ZnCl4]. Results from an extensive series of Raman spectroscopy and synchrotron diffraction measurements will be shown to demonstrate that the [ZnCl4]2- anion persists to high dilution. These data further provide evidence for a complex hydration structure beyond the first neighbor water of the hexaaquo zinc cation. Notably the complex speciation/hydration structure directly correlates with the concentration-dependent differential scanning calorimetry of this system and will be shown to be the origin of the system’s deep eutectic. Evaluating the concentration dependence of the temperature and heat of the liquidus reveals that this salt solution exists as an ionic liquid up to a concentration of about 98 mole percent water, beyond which it follows more traditional electrolyte descriptions. The principles derived from this structural and spectroscopic analysis can be applied to more generally explain the freezing point depression in diverse systems. In particular, this analysis demonstrates how the phase diagrams and liquidus heat and temperature can reveal the nature of speciation within aqueous salt solutions. Figure 1