Temperature- and pressure-induced phase transitions of choline chloride-urea deep eutectic solvent

FT-IR spectroscopy has been used to decipher structural rearrangements in a choline chloride-urea (ChCl-Urea) deep eutectic solvent, as a function of temperature (−60–20 °C) at atmospheric pressure and as a function of pressure (0.1 MPa to 5.2 GPa) at room temperature. Under atmospheric pressure, the ChCl-Urea deep eutectic solvent (DES) underwent glass-to-crystal and crystal-to-liquid phase transitions during the heating process. However, at room temperature, with pressure increasing, the ChCl-Urea DES experienced liquid-liquid and liquid-crystal phase transitions at approximately 1.6 and 2.6 GPa. In addition, the partial breakage of intermolecular hydrogen bonds between ChCl and urea was observed during the isothermal crystallization process; however, intermolecular hydrogen bonding between two urea molecules was enhanced during the pressure-induced crystallization process. Consequently, urea could act as a “spacer” in the regular ChCl packing of ChCl-Urea DES cocrystals, and could regulate the intermolecular hydrogen bondings according to the competition among the complexed cations and anions in different crystallization environments.