Cosolvent polarity dependence of solution structure in [BMIM] [PF6] + acetonitrile/1, 4-dioxane/hexane binary mixtures: Insights from composition dependent Voronoi polyhedra analyses, iso-surfaces and radial distribution functions

We investigate in this paper, via molecular dynamics simulations, the composition dependent solution structures in the binary mixtures composed of the ionic liquid (IL), 1-butyl-3methyl-imidazolium hexafluorophosphate, ([BMIM] [PF6]), and three different co-solvents of comparable sizes but with differing polarities. These cosolvents are acetonitrile (dipolar), 1,4-dioxane (quadrupolar) and hexane (non-polar). Five different IL mole fractions for the binary mixtures (FIL = 0.90, 0.75, 0.50, 0.25, 0.10) along with the neat solvents have been studied. The cosolvent polarity and concentration dependencies have been assessed through the examination of the composition dependent spatial and radial distribution functions, coordination numbers and the Voronoi polyhedra analyses. The solution structure is found to be dominated by the cosolvent clustering at high IL concentrations, and segregation of the IL into polar and non-polar domains in dilute IL solutions, their extent being strongly dependent upon cosolvent polarity. The aggregation behavior of the IL in the non-polar and dipolar binary mixtures is explained in terms of the Debye screening length (DSL). Such an explanation, however, fails to explain the comparable solubility of 1,4-dioxane in this IL. Isodensity surfaces constructed from the simulated spatial distribution functions show a dramatic cosolvent polarity dependence for the binary solution structure and reflects that microscopic phase segregation is the most pronounced for IL + hexane binary mixtures. The Voronoi polyhedra (VP) analyses indicate broader void and neck distributions upon dilution of the IL by cosolvents, suggesting increased heterogeneity in the solution structure of the binary mixtures. The severe phase segregation observed for [BMIM] [PF6] + hexane mixtures at the low IL concentrations did not allow similar VP analyses for this binary mixture for all the mixture compositions considered.