Specific phenomena in carboxylic acids extraction by selected types of hydrophobic ionic liquids

An overview on specific phenomena in extraction of carboxylic acids with hydrophobic ionic liquids (ILs) based on results of new measurements with selected phosphonium, ammonium and imidazolium ILs and published data is presented. Formation of IL - acid hydrated complexes with multiple molecules of organic acid per one IL ion pair was observed. The distribution coefficient of carboxylic acids and water content in ILs strongly decreases with the increasing acid concentration. Dependence of water content in the solvent passes through a minimum at loading of IL with butyric acid of about 3. Two extraction mechanisms are involved: competitive extraction of acid and water with the release of water from the solvent and co-extraction of water with acid depending on the IL concentration. A strong synergistic effect was observed between the cation and anion of ILs enhancing their extractive properties compared to IL precursors. A new extraction model suggests the formation of water bridges and polar nano-channels which is in agreement with the molecular modelling results. ILs are nano-segregated liquids with a structure sensitive to the content of molecular compounds. Water and carboxylic acids accumulate in polar domains and dodecane in non-polar domains modifying the IL structure and decreasing the solvent phase viscosity. The hypothesis of hopping mechanism in polar channels for acid molecules transport between acid chains at IL binding sites is suggested.