Solvent properties of the 1-butyl-3-methylimidazolium hexafluorophosphate ionic liquid

Abstract

1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF₆) was synthesized and purified to be used as a ionic liquid solvent. Its physicochemical properties were studied. The ionic liquid/water (P _il/water) and ionic liquid/heptane (P _il/heptane) distribution coefficients of a set of 40 compounds with various functionalities, including organic acids, organic bases, amino acids, antioxidants, and neutral compounds, were measured using liquid chromatography. For ionizable compounds, the P _il/water values measured at pH 2, 5.1, and 10 were very different. These allowed the determination of both the molecular P^oil_/water values and the ion P⁻il_/water value for each compound. These coefficients were compared to the corresponding P _oct/water coefficients. Marked differences in the partitioning behavior of basic, acidic, and neutral compounds were observed. The relationship between P _il/water and P _oct/water is different from that reported previously. By using the linear free energy solvation approach and the descriptors found for 12 solutes, the BMIM-PF₆ solvent parameters were calculated for the ionic liquid/water and ionic liquid/heptane biphasic systems. The regression parameters show a low basicity of the BMIM-PF₆ solvent compared to octanol. The high cohesion of the ions in the ionic liquid phase is also indicated by the regression equations obtained. Ionized phenols (phenoxide ions) associate more strongly with BMIM-PF₆ than most other ionized molecules. Amino acids were not soluble in ionic liquid; however, it is possible to extract them partially by adding a crown ether to the ionic liquid phase and working at pH 1. The positive form of amino acids is complexed by the crown ether and the complex is extracted in the ionic liquid phase.