Molecular Dynamics Simulation Study of Room-Temperature Ionic Liquids: Dynamic Heterogeneity and Solvation Dynamics

Abstract

In this thesis, we study dynamics of room-temperature ionic liquids using computational method. At first, we investigate the dynamic propensity in a coarse-grained model of a room-temperature ionic liquid. Dynamic propensity is defined as the average of squared displacements for each ion during a given time interval over the isoconfigurational ensemble. As the temperature is lowered, distributions of the dynamic propensity develop fat tails at high values, indicating the presence of dynamic heterogeneity in the system. The increase in the heterogeneity for the cation is more evident than that for the anion, and a high propensity exhibits a large variance in the isoconfigurational ensemble, implying that dynamic propensity is related to ions motions at a large length scale, rather than a direct measure of the individual ion dynamics. In addition, large non-Gaussian parameters observed for small dynamic propensities reveal intermittent dynamical behaviors of ions. In order to reveal the origin of the dynamic heterogeneity in a room-temperature ionic liquid, a possible correlation between the mobility and dynamic propensity is further probed. It is observed that spatial distributions of the dynamic propensity coincide with those of the mobility. The results suggest a possible connection between the structure and heterogeneous dynamics on large length scales. In the second part of this thesis, we study how the excitation energy affects the dynamic properties and the emission spectrum of solvation in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI+PF – ) by perform- ing the molecular dynamics simulations on its coarse-grained model. We use very narrowly distributed excitation energies unlike former studies where excitation energy was expressed in an averaged way from a distribution. The isoconfigurational ensemble method enables this sampling with a reliable statistics. Using that, we calculate the Stokes shift function, S(t), to show that its relaxation becomes slower with the lower excitation energy, converging to the equilibrium correlation function, C(t), of the solvation energy fluctuation, implying that the red-shifted system approaches the linear response regime. We also analyze variances of solvation energy between nonequilibrium trajectories to verify that the effect of initial config- uration is profound at short time, ∼1 ps, and diminishes with time passing by whereas that of velocity dominates at long time. We also calculate the emission spectrum showing the red-edge effect, consistently with previous studies.