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

摘要

The molecular dynamics (MD) simulation study of solvation structure and freeenergetics in 1-ethyl-3-methylimidazolium chloride and1-ethyl-3-methylimidazolium hexafluorophosphate using a probe solute in thepreceding article [Y. Shim, M. Y. Choi and H. J. Kim, cond-mat/0501305] isextended to investigate dynamic properties of these liquids. Solventfluctuation dynamics near equilibrium are studied via MD and associatedtime-depedent friction is analyzed via the generalized Langevin equation.Nonequilibrium solvent relaxation following an instantaneous change in thesolute charge distribution and accompanying solvent structure reorganizationare also investigated. Both equilibrium and nonequilibrium solvation dynamicsare characterized by at least two vastly different time scales--a subpicosecondinertial regime followed by a slow diffusive regime. Solvent regionscontributing to the subpicosecond nonequilibrium relaxation are found to varysignificantly with initial solvation configurations, especially near thesolute. If the solvent density near the solute is sufficiently high at theoutset of the relaxation, subpicosecond dynamics are mainly governed by themotions of a few ions close to the solute. By contrast, in the case of a lowlocal density, solvent ions located not only close to but also relatively farfrom the solute participate in the subpicosecond relaxation. Despite thisdifference, linear response holds reasonably well in both ionic liquids.