Correlation between Dynamic Heterogeneity and Local Structure in a Room-Temperature Ionic Liquid: A Molecular Dynamics Study of [bmim][PF6]

The complex dynamics of a room-temperature ionic liquid, 1-n-butyl-3-methylimidazolium hexafluorophosphate ((bmim)-[PF6]), is studied using equilibrium classical molecular dynamics simulations in the temperature range of 250-450 K. The activation energies for the self-diffusion of ions are around 30-34 kJ mol(-1), with that of the anion a little higher than that for the cation. The electrical conductivity of the liquid is calculated and good agreement with experiments is obtained. Structural relaxation is studied through the decay of coherent (total density-density correlation) and incoherent (self part of density-density correlation) intermediate scattering functions over a range of temperatures and wave vectors relevant to the system. The relaxation data are used to identify and characterize two processes, alpha and beta. The dependence of the two relaxation times on temperature and wave vector is obtained. The dynamical heterogeneity of the ions determined through the non-Gaussian parameter indicates the motion of the cation to be more heterogeneous than that of the anion. The faster ones among the cations are coordinated to faster anions, while slower cations are surrounded predominantly by slower anions. Thus, the dynamical heterogeneity in this ionic liquid is shown to have structural signatures.