Aqueous Biphasic System Containing Long Chain Anion-Functionalized Ionic Liquids for High-Performance Extraction

Ionic liquid-based aqueous biphasic systems (IL-based ABSs) offer a benign alternative process for conventional extraction systems with volatile organic solvents to separate biomass. Designing an IL-based ABS with excellent phase splitting ability, remarkable extraction efficiency, and good biocompatibility remains challenging. In this work, we report a series of novel ABSs using biocompatible ILs composed of long chain carboxylate anions and a cholinium cation that are all derived from biomass. This strategy introduced long alkyl chains into the anions, which not only significantly increased the hydrogen bond (HB) acceptor ability of the carboxylate anions through a remarkable electron-donating effect but also ensured good hydrophobicity for achieving better phase splitting. The developed IL-based ABS demonstrated a relatively broad biphasic area and extraordinary extraction efficiency for amino acids and bioactive compounds with distribution coefficients for tryptophan, phenylalanine, and caffeine of 58.5, 120 (120 was set as a maximum value for the partition coefficient because in some cases the concentration of the extracted material in the salt-rich phase was below the limit of detection), and 120, respectively, which were remarkably higher than those obtained in ABS with conventional ILs. This work shows that the long chain carboxylate anion is critical for the excellent extraction performance of the developed ABS and that their distribution coefficients increased with increasing anion alkyl chain lengths. In addition, liquid crystal structures were observed when the carbon number of the carboxylate anion of the ILs exceeded eight; thus, IL-based ABS with liquid crystal structures were reported for the first time.