Association of Ionic Liquids in Solution: Conductivity Studies of [BMIM][Cl] and [BMIM][PF6] in Binary Mixtures of Acetonitrile + Methanol

The concentration dependence of molar conductance for 1-butyl-3-methylimidazolium chloride and 1-butyl-3-methylimidazolium hexafluorophosphate in binary mixtures of acetonitrile + methanol was investigated to explore the ion association behavior of imidazolium based ionic liquids. The limiting molar conductance $(Uplambda_{m}^{0})$ ( Λ m 0 ) , association constants (K _a ⁰ ) and the maximal distance between the oppositely charged ions in ion pairs (R _ij ) in the mixed solvent mixtures were evaluated following the framework of Barthel’s low-concentration chemical model. The investigated ILs display opposing trends in ion association behavior with change in solvent composition of acetonitrile + methanol binary mixtures. The results are discussed in light of ionic liquid and solvent specific ion?solvent and ion?ion interactions in the mixed solvent systems.