On the relevance of electrostatic interactions for the structural relaxation of ionic liquids: A molecular dynamics simulation study

We perform molecular dynamics simulations to investigate the structural (alpha) relaxation of models of 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim] [PF6]) room temperature ionic liquids. To determine the relevance of electrostatic interactions for the dynamical properties, we systematically scale the partial charges of the ions. Observing the structural relaxation of these charge-scaled ionic liquids from high temperature far above the melting point to low temperatures in the viscous regime, we find that the alpha-relaxation times tau(q) strongly depend on the strength of the electrostatic interactions. The glassy slowdown tau(g)(T) of all systems is successfully described by splitting the apparent activation energies E(T) into a temperature-independent contribution E-infinity, which characterizes the a relaxation of the high-temperature liquid, and a temperature-dependent contribution E-c(T), which describes the growing cooperativity and increases exponentially upon cooling. For all used partial charges, we find a common relation between the activation energy E-infinity and the glass transition temperature T-g, indicating a close link between simple-liquid and viscous-liquid dynamics. To ascertain the origin of the increase of the cooperative contribution E-c(T), we characterize the evolution of space-time characteristics of dynamical heterogeneities upon cooling. It turns out that the length and time scales of spatially heterogeneous dynamics are the same for all studied charge-scaled ionic liquids when plotted as function of tau(q). Moreover, we find a common relation between E-c, and the size of transient clusters of highly mobile ions.