The Origin of Ion-Pairing and Redissociation of Ionic Liquid.

We address the possible occurrence of a minimum extent of dissociation (α) of ionic liquid (IL) in IL-solvent mixtures. This phenomenon, known as the redissociation of IL, is responsible for many interesting composition-dependent properties in such mixtures. A thermodynamic model is developed to provide a semiquantitative prediction on the change of α with solvent concentration. It is found that the occurrence of minimum α coincides with the occurrence of a maximum in the mean activity coefficient of dissociated ions, indicating better solvation of free, dissociated ions both with decreasing and increasing solvent concentration. The favorable solvation of free ions is found to change from long-range ion-solvent dielectric polarization to ion-ion-pair dielectric polarization with decreasing solvent concentration. Therefore, the composition dependence of the IL solution dielectric constants, determined from that of the ion-pair and the solvent, is found to be the most important factor for the presence of redissociation in IL solutions.