Aqueous solution of [bmim][PF6]: ion and solvent effects on structure and dynamics.

A dilute aqueous solution of the salt 1-n-butyl,3-methylimidazolium hexafluorophosphate ([bmim][PF(6)]) has been studied using molecular dynamics simulations to investigate the effect of ions on water and vice versa. The anion is found to diffuse faster than the cation in the solution, in contrast to observations in the pure ionic liquid. Distributions of pair energies have been employed to identify ion association, and around 13% of the ions were found to exist as pairs. The mean potential energy of water molecules present in the coordination shell of an anion is less than that of water molecules coordinated to a cation. The former kind also exhibit two distinct orientational preferences with respect to the anion. The larger diffusion coefficient of the anion is related to the faster dynamics of water molecules in its hydration layer, as evidenced from the relaxation of their residence time correlation function.