Unconventional Deep Eutectic Solvents: Aqueous Salt Hydrates

Neoteric solvents for sustainable chemical processes are continuously sought nowadays. Deep eutectic solvents (DES), i.e., liquids that are mixtures of two components that have freezing points considerably below the melting points of these components, constitute a possible solution for this search. Mixtures of certain congruently melting salt hydrates with water have freezing points considerably below the melting points of ice and of the salt hydrate. They should be considered as DES for applications in which their properties could be useful, namely their being "green" and nonflammable ionic liquids at very low temperatures. The eutectic points and eutectic temperature distances of such salt hydrate DES, calculated from literature data, are tabulated, and some illustrative phase diagrams are shown. Physical properties of the aqueous salt hydrate DES that are relevant to their possible applications are reported, and they compare favorably with those of conventional DES. The Brunauer–Emmett–Teller modeling procedure is applied to the case of the aqueous potassium fluoride tetrahydrate phase diagram.