Role of Dynamic Heterogeneities in Ionic Liquids: Insights from All-Atom and Coarse-Grained Molecular Dynamics Simulation Studies.

Abstract

We performed molecular dynamics simulations for the room-temperature ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6 ]). By employing all-atom (AA) and coarse-grained (CG) models, we compared the characteristic times of various dynamical modes, from vibration to diffusion, and the importance of dynamical heterogeneities at different levels of chemical resolution and over broad temperature ranges. It was shown that coarse graining leads to a substantial speedup in molecular dynamics, whereas it weakly affects the strength of dynamical heterogeneities. Despite the general speedup, several relationships between dynamical modes on different timescales were preserved. In particular, the heterogeneity timescales of the AA and CG models collapse onto the same curve as a function of the structural (α) relaxation time τα . Moreover, vibrational amplitudes and relaxation times τα are related at both levels of chemical resolution. We argue that the robustness of these relationships assists the development of dynamically consistent coarse-graining strategies and justifies the use of simplified models for a theoretical understanding of viscous liquids.