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 W g ' = 30%. For W < W g ', 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 > W g ', 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 W g ' 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 > W g ', 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 Journal Pre-proof J o u r n a l P r e-p r o o f 2 (W < W g ') and water structure distortion. This study also highlights the potential of DESs for their uses in freeze-drying processes and biopreservative applications.

Is ethaline a deep eutectic solvent?

“…2 displays the heating regime of representative DSC traces, recorded according to the procedure described in the ESI. † Similar to the observations of Jani et al, 21 for all studied mixture compositions crystallization did not occur during cooling. However, all samples, including neat ethaline, crystallized during the subsequent isothermal holding step.…”

“…1). Recent publications of Alfurayj et al 20 and Jani et al 21 revealed a strong impact of moderate amounts of water on various physical properties of ethaline (x ChCl = 0.333), including heat flow. We observed that in ethaline containing less than 3000 ppm of water ChCl crystals form at room temperature, see Fig.…”

Do Deep Eutectic Solvents Form Uniform Mixtures Beyond Molecular Microheterogeneities?

“…For practical reasons, aqueous solutions of DESs have been formulated and their physical properties, including excess properties, viscosity, acoustic, structural and optical properties have been studied thoroughly. [9][10][11][12][13][14][15][16][17][18][19][20][21] These works have also raised the question of the resilience of the specific local arrangement of DES during dilution with water and its evolution towards an aqueous solution of its constituents. 9-11, 19, 22 The structure of DESs has attracted a lot of interest during the last four years.…”

Group and group‐interaction contribution method for estimating the melting temperatures of deep eutectic solvents

“…11 The rational design and selection of DESs for a particular process require general knowledge and consideration of the properties of DESs, thus increasing attention is being paid to experimentally and analytically understand their properties, such as melting temperatures, density, surface tension, speed of sound, and conductivity. [12][13][14][15] Melting temperature, as one of the most important physical properties, is useful in chemical identification, 16 purification, 17 and drug research. 18 In addition, the properties, experimentally difficult to determine, for example, enthalpy of fusion and activity coefficient, 19 can also be derived from melting temperature data.…”