Solvation Structure and Dynamics of Alkali Metal Halides in an Ionic Liquid from Classical Molecular Dynamics Simulations

The structure and dynamics of the solvation of several alkali metal halides in an ionic liquid (IL), 1-butyl-3-methylimidazolium trifluoromethanesulfonate ([BMIm][OTf]), were investigated by classical molecular dynamics simulations. Various properties such as density, radial distribution functions, coordination numbers, spatial distribution functions, mean-square displacements, self-diffusion coefficients, and velocity-velocity autocorrelation functions were calculated to understand the solvation environment of alkali metal halide salts in IL at various salt concentrations. We observe that the halide anions are coordinated in two different ways with [BMIm](+) in all of the mixtures. However, the alkali metal cations interact more with the anion of the ionic liquid as we go from fluorides to iodides. When a common anion was used for the salt and the ionic liquid, we observe significant coordination of Na+ with the anion of the ionic liquid in two different ways, which was not observed in the case of lithium salt. We also find that the Li+ and Na+ ions are involved in the formation of a aggregate-like, stable kinetic entity with anions in their first solvation shells. These aggregate-like entities were seen to be relatively stable, and we noted a rattling motion of salt ions inside them.