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

Abstract

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, $$ R = \frac{{\int_{0}^{\infty } {{\text{d}}tC_{1} (t )} }}{{\int_{0}^{\infty } {{\text{d}}tC_{2} (t )} }} = \frac{{\left\langle {\tau_{1} } \right\rangle }}{{\left\langle {\tau_{2} } \right\rangle }} $$ , 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.