High pressure-induced glass transition and stability of choline chloride/malonic acidic deep eutectic solvents with different molar ratios

Deep eutectic solvents (DESs) are simple, environmentally friendly synthetic neoteric solvents, and thus, the study of the phase transitions of DESs under extreme conditions is of great significance for the design of new green solvents. In this work, the pressure-induced phase transition of choline chloride/malonic acid (ChCl/Ma) DESs at 2:3, 1:1, and 3:2 M ratios was investigated by in situ Raman spectroscopy up to a pressure of 10.24 GPa. The results indicated that all the ChCl/Ma DESs, which had different molar ratios, underwent two phase transitions: a liquid-to-glass I transition and a glass I-to-glass II transition. The first phase transition occurred at 1.1 GPa, 2.01 GPa and 1.2 GPa, and the second phase transition occurred at 5.5 GPa, 6.80 GPa and 6.1 GPa, respectively, as the molar ratio of the ChCl/Ma DESs was increased from 2:3 to 3:2. The ChCl/Ma DES near the eutectic point of the 1:1 M ratio maintained high structural stability under high pressure. Our results suggested that the addition of excess hydrogen bond donors (HBDs)/acceptors (HBAs) leads to competition for hydrogen bonds near the eutectic point, which results in indecisive hydrogen bonding and decreases the stability of the structure of DESs under high pressure.(c) 2022 Elsevier B.V. All rights reserved.

Automated summary

In this study, the pressure-induced phase transitions of ChCl/Ma DESs with different molar ratios were investigated using in situ Raman spectroscopy. The results showed that DESs exhibited high structural stability under high pressure near the eutectic point. The findings provide valuable insights for the design of new green solvents.