A Molecular Dynamics Computer Simulation Study of Room-Temperature Ionic Liquids: II. Equilibrium and Nonequilibrium Solvation Dynamics

TL;DR

This study uses molecular dynamics simulations to analyze the equilibrium and nonequilibrium solvation dynamics in room-temperature ionic liquids, revealing multiple time scales and the influence of local density on relaxation processes.

Contribution

It extends previous work to investigate the dynamic properties of ionic liquids, highlighting the role of local density and initial configurations in solvation relaxation times.

Findings

Two distinct time scales in solvation dynamics: subpicosecond and diffusive.

Local density near the solute significantly affects subpicosecond relaxation mechanisms.

Linear response approximation remains valid despite complex relaxation behaviors.

Abstract

The molecular dynamics (MD) simulation study of solvation structure and free energetics in 1-ethyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium hexafluorophosphate using a probe solute in the preceding article [Y. Shim, M. Y. Choi and H. J. Kim, cond-mat/0501305] is extended to investigate dynamic properties of these liquids. Solvent fluctuation dynamics near equilibrium are studied via MD and associated time-depedent friction is analyzed via the generalized Langevin equation. Nonequilibrium solvent relaxation following an instantaneous change in the solute charge distribution and accompanying solvent structure reorganization are also investigated. Both equilibrium and nonequilibrium solvation dynamics are characterized by at least two vastly different time scales--a subpicosecond inertial regime followed by a slow diffusive regime. Solvent regions contributing to the subpicosecond nonequilibrium relaxation are found to vary significantly with initial solvation configurations, especially near the solute. If the solvent density near the solute is sufficiently high at the outset of the relaxation, subpicosecond dynamics are mainly governed by the motions of a few ions close to the solute. By contrast, in the case of a low local density, solvent ions located not only close to but also relatively far from the solute participate in the subpicosecond relaxation. Despite this difference, linear response holds reasonably well in both ionic liquids.