Abstract: Ionic liquid-based aqueous two-phase system (IL-ATPS), a type of mild and efficient liquid-liquid extraction system, has attracted wide-ranging attention. However, the practical application of imidazolium- and pyridinium-based ionic liquids are severely restricted because of their high toxicity. Deep eutectic solvents (DES), which inherit the advantages of IL and exhibit biodegradability, have been used as substitutes for IL to construct ATPS. In this study, two biocompatible DES were synthesized using L-carnitine and two common organic acids (L-lactic acid and DL-malic acid). The structures of the two DESs were confirmed by FTIR and 1H NMR. DES-based ATPS (DES-ATPS) was constructed using L-carnitine-based DES and polyethylene glycol (PEG). The liquid-liquid phase behavior of the DES-ATPS was investigated, and the main factors influencing its phase-forming ability were examined. The results indicate that the DES-ATPS exhibits typical low critical solution temperature (LCST) phase transition behavior. The increase in the average molecular weight of PEG, the decreased hydrophilicity of the hydrogen bond donor in the DES, and the increased molar ratio of the hydrogen bond acceptor all contribute to enlarging the hydrophilic/hydrophobic difference between the DES and PEG, thereby promoting the formation of the ATPS. These align with the principle that the greater the difference in hydrophilicity/hydrophobicity between components forming ATPS, the stronger the phase-forming ability. Vanillin was selected as the model extraction compound to systematically investigate the extraction performance and potential of the DES-ATPS. The experimental results demonstrate that under the optimized conditions, the system achieves a notably high extraction efficiency, with the recovery rate of vanillin reaching up to 93.6%, which verifies the possibility of its application in extraction systems.

Key words: Deep eutectic solvents, Aqueous two-phase system, Phase behavior, Extraction; Vanillin