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

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