Predicting Ionic Conductivity of Imidazolium-Based Ionic Liquid Mixtures Using Quantum-Mechanically Derived Partial Charges in the Condensed Phase

A considerable effort has been expended over the years to tune the properties of ionic liquids (ILs) by designing cations, anions, and pendant groups on the ions. A simple and effective approach to altering the properties of ILs is formulating IL-IL mixtures. However, the measurements and properties of such mixtures lag considerably behind those of pure ILs. From a molecular simulation point of view, binary IL mixtures have been investigated using charge distributions of pure ILs, which implicitly assumes that the ions of different polarizability do not influence the local electronic environment due to changing concentrations. To understand this effect, molecular dynamics (MD) simulations were conducted for a series of IL-IL mixtures containing the common cation 1-ethyl-3-methylimidazolium [C₂mim] varying the composition of various combinations of anions (tetrafluoroborate [BF₄] and dicyanamide [DCA], [BF₄] and bis(trifluoromethanesulfonyl)imide [NTF₂], [BF₄] and trifluoromethanesulfonate [TFO], and [TFO] and [NTF₂]). The effect of changing the electronic environment was evaluated by deriving partial charges using density functional theory (DFT) calculations in the condensed phase. It was observed that the overall charge on the cation and anion was a function of the cation-anion pairings for pure ILs. Moreover, the cation charge was found to vary linearly with anionic concentrations. Improved agreement of predicted density and ionic conductivity with experimental values was found for binary IL mixtures with this approach, in comparison to that when a fixed charge model is employed.