Partition behavior and extraction mechanism of carboxylic acids in hydrophobic deep eutectic solvents of TOMAC and medium-chain fatty acids

In this study, hydrophobic deep eutectic solvents (HDESs) based on trioctylmethylammonium chloride (TOMAC) and medium-chain fatty acids were investigated for the extraction of carboxylic acids. The effects of molar ratio, extraction temperature, vortex time, pH, and carboxylic acid concentration on the partition behavior of carboxylic acids were evaluated. The natural logarithm of partition coefficients (lnK) for six kinds of mono and five kinds of binary carboxylic acids in the TOMAC-OctA system exhibits an excellent linear relationship to the ClogP of carboxylic acids. Quantum chemical calculations using density functional theory showed that one hydrogen bond is formed between lactic acid (LA) and TOMAC-OctA, while double hydrogen bonds between succinic acid (SA) and TOMAC-OctA. However, extraction of butyric acid (BA) is driven by van der Waals force. Furthermore, analysis of the interaction energy revealed that TOMAC-OctA possesses the highest affinity for the extraction of BA, followed by SA and LA.