Heterogeneous dynamics in [BMIM][PF6] + Cosolvent binary Mixtures: Does It depend upon cosolvent Polarity?

Abstract

It is known that dilution of ionic liquid with appropriate cosolvents modifies a number of physicochemical properties that include surface tension, viscosity, ion diffusivity and many others. The cosolvents, particularly when present in low concentrations, also undergo substantial changes in their inherent dynamics. The work reported here explores, via molecular dynamics simulations, the composition dependence of modifications on motional features of both the neutral cosolvent molecules and the charged molecular ions constituting the ionic liquid, 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM] [PF6]). The cosolvents have been carefully chosen so that the polarities differ widely; they are acetonitrile (dipolar), 1,4-dioxane (quadrupolar) and n-hexane (non-polar). Mixture compositions studied are, FIL = 1.00, 0.90, 0.75, 0.50, 0.25, 0.10 and 0, FIL being the IL mole fraction. Signature of heterogeneous relaxations have been searched via examining several dynamical markers, such as, four-point density-time correlation functions, overlap functions, self-intermediate scattering functions, and other motional features arising from the deviations of displacements from Gaussian distributions. The presence of cosolvents affects the diffusivity of both the ions and the cosolvents, with dipolar acetonitrile causing the fastest ion diffusion in the mixture. Interestingly, all the dynamical heterogeneity features studied here have been found to respond significantly to both the cosolvent polarity and the mixture composition.