Quantitative Relation between Ionic Diffusivity and Ionic Association in Ionic Liquid-Water Mixtures

Molecular dynamic simulations of aqueous mixtures of imidazolium ionic liquids (ILs) were performed to elucidate the dependence of the ionic diffusivity on the microscopic structures changed by water. Two distinct regimes of the average ionic diffusivity (D-ave) were identified with the increased water concentrations: the jam regime with slowly increased D-ave and the exponential regime with rapidly increased D-ave , which are found to be directly correlated to the ionic association. Further analysis leads to two general relationships independent of IL species between D-ave and the degree of ionic association: (i) a consistent linear relationship between D-ave and the inverse of ion-pair lifetimes (1/tau(IP)) in the two regimes and (ii) an exponential relationship between normalized diffusivities (D-ave) and short-ranged interactions between cations and anions (E-ions),with different interdependent strengths in the two regimes. These findings revealed and quantified the direct correlation between dynamic properties and ionic association in IL-water mixtures.