Molecular dynamics study of the dynamics near the glass transition in ionic liquids

Molecular dynamics (MD) simulations have been performed to study the dynamics near the glass transition regime of molecular ions in ionic liquids. The glass transition temperature in the simulated 1-ethyl-3-methyl imidazolium nitrate (EMIM-NO(3)) system was determined by plotting density against temperatures. The dynamics at several temperatures in the liquid, supercooled liquid, and glassy states have been characterized by the diffusion coefficients, fractal dimension analysis of the trajectories, and the van-Hove functions. The diffusion coefficient approximately obeys the Vogel-Fulcher-Tammann (VFT) relation. However, two power laws or two exponentials are also good descriptions of the data. The fractal dimension of the random walks is a measure of the complexity of the trajectory, which is attributed to the geometrical correlations among successive motions. Rapid increase of the fractal dimension of the random walks on decreasing temperature is found for both cations and anions. Temperature dependence of the fractal dimension of the random walks for the long range (accelerated) motion is larger than that for short range (localized) motion. This reasonably explains the change in the slopes found in the temperature dependence of the diffusion coefficients. At around the glass transition temperature, long range motion is essentially absent during the observed times, up to several nano seconds. This feature is also confirmed by the van-Hove functions. Such slowing down of the dynamics in the fragile ionic liquids is characterized by the changes from long range motion to short range motion instead of sudden changes at around T(0) in the VFT relation.