Ion association of imidazolium ionic liquids in acetonitrile.

Molar conductivities, Λ, of dilute solutions of the ionic liquids (ILs) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF4]), 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]), 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF4]), and 1-hexyl-3-methylimidazolium bis-(trifluoromethanesulfonyl)amide ([hmim][NTf2]) in acetonitrile (AN) were determined as a function of temperature in the range 273.15-313.15 K. The data were analyzed with Barthel’s lcCM model to obtain limiting molar conductivities, Λ(∞)(T), and association constants, K(A)°(T) of these electrolytes. The temperature dependence of these parameters, as well as the extracted limiting cation conductivities, λ(i)(∞), were discussed. Additionally, dielectric spectra for [hmim][NTf2] + AN were analyzed in terms of ion association and ion solvation and compared with the inference from conductivity. It appears that in dilute solutions the imidazolium ring of the cations is solvated by ∼6 AN molecules that are slowed by a factor of ∼8-10 compared to the bulk-solvent dynamics. Ion association of imidazolium ILs to contact ion pairs is only moderate, similar to common 1:1 electrolytes in this solvent.