Rank-dependent orientational relaxation in an ionic liquid: an all-atom simulation study

We have employed all-atom molecular dynamics simulation to investigate the heterogeneity effects on reorientation correlation time in the ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), at 298 and 450 K. Our simulated density, radial distribution functions, viscosity, and self-diffusion coefficients compare well with the existing literature data. Even though the room temperature (298 K) simulations could not probe the entire decay of the reorientation correlation function (C-l(t)) due to the inherent slow diffusional timescale in this IL and limited computational resources available, the simulated C-l(t) at high temperature (450 K) captures the entire decay. The ratio between the simulated reorientation time constants, R = integral(infinity)(0) dtC(1)(t)/integral(infinity)(0) dtC(2)(t) = /, differs considerably from 3 at 450 K, indicating the presence of significant heterogeneity effects even at this high temperature. This observation corroborates well with the simulated non-Gaussian parameter at this temperature.