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

• The phase transition of ChCl/Ma DES was investigated under high pressure by Raman spectroscopy. • The ChCl/Ma DES underwent two phase transitions: a liquid-to-glass Ⅰ transition and a glass Ⅰ-to-glass Ⅱ transition. • The ChCl/Ma DES near the eutectic point of the 1:1 M ratio maintained high structural stability under high pressure. • The addition of excess hydrogen bond donors/acceptors leads to competition for hydrogen bonds near the eutectic point. 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.