Phase behavior of aqueous solutions of ethaline deep eutectic solvent

Abstract

We have established a detailed phase diagram of a prototypical DES as a function of the hydration level. Two distinct thermal phase behaviors are observed depending on the water content with respect to a cross-over composition Wg’ = 30%. For W < Wg’, the formation of ice is not observed under the experimental conditions used in this study, and the solution falls in the category of glassforming systems. Fully vitreous states could also be obtained between 30% and 50%, but they are metastable with respect to water crystallization. For W > Wg’, ice crystallization occurs but the residual DES solution remains amorphous (liquid or glassy). In the latter case, whatever the initial water fraction, this transformation finally ends at the fixed composition Wg’ corresponding to 6 to 10 water molecules per choline ion for the two studied DESs. We infer that the residual liquid water molecules forming this maximally freeze-concentrated solution are strongly interacting with DES molecular units. This situation is also known as the “water-in-DES” case. Conversely, ice crystallization concerns free water molecules, provided that W > Wg’, also known as the “DES-in-water” case. This entire phase behavior is explained in the context of maximally freeze-concentrated solutions and attributed to the concomitant effects of ice freezing depression, glassforming ability of weakly hydrated DES (W < Wg’) and water structure distortion. This study also highlights the potential of DESs for their uses in freeze-drying processes and biopreservative applications.