Degradation of Deep-Eutectic Solvents Based on Choline Chloride and Carboxylic Acids

Mixtures of carboxylic acids and choline chloride are one of the most commonly used families of deep-eutectic solvents. The thermal and long-term stability of carboxylic acid–choline chloride (ChCl) deep-eutectic solvents was investigated. This family of DESs was found to degrade due to an esterification reaction, mainly between the carboxylic acid and the alcohol moiety of ChCl. The esterification reaction occurs even at room temperature over extended periods of time and is promoted at elevated temperatures. The esterification reaction takes place independently of the preparation method used. Moreover, the deep-eutectic solvent malonic acid–ChCl (xChCl = 0.50) was found to decompose into acetic acid and carbon dioxide when prepared via the heating method, or when heated after preparation at room temperature. Therefore, the applicability of carboxylic acid–ChCl-based solvents is compromised.