Stability of Cholinium Chloride-Based Deep Eutectic Solvents with Carboxylic Acids: A Study on Ester Formation

Deep eutectic solvents (DES), especially those derived from cholinium chloride ([Ch]Cl), have been studied as alternative solvents for various chemical processes. Despite their widespread use, the understanding of the possible formation of impurities in these solvents, such as esters, is still incomplete. In this study, the thermal and long-term stability of [Ch]Cl-based DES were investigated. Specifically, three [Ch]Cl-based DES with carboxylic acids (acetic acid, formic acid, and lactic acid - and their mixtures with water were investigated. The results show that the DES formed with [Ch]Cl: formic acid has lower thermal and long-term stability, but the addition of water to DES significantly reduces the rate of ester formation. The DES [Ch]Cl: lactic acid, and [Ch]Cl: acetic acid were found to exhibit good long-term stability. However, [Ch]Cl: lactic acid has the disadvantage of containing a significant amount of lactide ester originally. It is noteworthy that the esterification in the preparation of the DES [Ch]Cl: acetic acid only took place at temperatures up to 40 °C and no further impurities were formed, making it the most suitable DES for processes at this temperature. In summary, the choice of acid appears to influence ester formation, with temperature accelerating the reaction and the addition of water playing a crucial role in slowing the rate of ester formation. In the long term, however, ester formation appears to be unavoidable, as esterification continues even after its production. Therefore, investigating [Ch]Cl: carboxylic acid combinations, with a focus on the ester content and its influence on the process, is essential.