Long-Chain Fatty Acid-Based Phosphonium Ionic Liquids with Strong Hydrogen-Bond Basicity and Good Lipophilicity: Synthesis, Characterization, and Application in Extraction

The development of ionic liquids (ILs) with strong hydrogen-bond basicity is of great importance for the application of ILs in liquid-liquid extractions, but a long-standing problem is that the increase in hydrogen-bond basicity of ILs usually comes along with undesired changes in other properties such as reduced lipophilicity, raised melting point, and difficulty of forming biphasic systems with water. Herein, we provided a promising solution to this problem by synthesizing a class of functional phosphonium ILs with the use of biocompatible saturated/unsaturated long-chain fatty acids (carbon number up to 20) as anion precursors, and we also carried out systematic investigations on their physicochemical properties. These long-chain fatty acid ILs (LCFA-ILs) featured very strong hydrogen-bond basicity, up to the top level of all reported solvents, along with good lipophilicity and a wide liquid range. Moreover, the viscosity of LCFA-ILs exhibited only a slight increase with increasing chain length, and the hydrophilicity/hydrophobicity could be tuned by a change in the side chain of the phosphonium cation. Excellent extraction efficiency was achieved by applying LCFA-ILs to both aqueous and nonaqueous extractions. The distribution coefficients of specific solutes reached unprecedented values up to 3000-5000, which were 170-650 times higher than those by common ILs and molecular solvents.