Nonpolar solvation dynamics for a nonpolar solute in room temperature ionic liquid: a nonequilibrium molecular dynamics simulation study

Abstract

Nonpolar solvation dynamics of a nonpolar diatomic solute in a room temperature ionic liquid (RTIL) has been followed via nonequilibrium molecular dynamics (MD) simulation. Frank-Condon type excitation of the solute, previously in equilibrium in RTIL solvent, has been modelled by abruptly changing the Lennard-Jones (LJ) diameter of the solute atoms and thereby disrupting the equilibrium situation. The rearrangement of the RTIL solvent molecules, which has been seen to be mostly contributed by the solute’s first solvation shell, around the excited solute results overall spectral narrowing and biphasic decay of the solvation energy; a dominant and very rapid process having sub-100 fs relaxation time, followed by a slower one relaxing at a timescale of $$\sim $$ 5 ps. A mode-coupling theory based calculation is also used to obtain the nonpolar solvation relaxation function for a model nonpolar solute dissolved in model RTIL solvent. The theoretical relaxation decay is not in very good agreement with the simulated nonequilibrium solvation response function; the theory predicts the short time relaxation component slower and the longtime component faster than those of the simulated nonequilibrium relaxation. We have also checked the validity of the linear response theory (LRT) for nonpolar solvation in RTIL by looking at the equilibrium solvation energy correlation in the RTIL solvent in presence of the ground state (GS) and the excited state (ES) solute. Apparent breakdown of the LRT in the present case elucidates the probable disagreement between the theoretical and simulated nonequilibrium nonpolar solvation response functions. SYNOPSIS Nonpolar solvation dynamics of a nondipolar solute probe in an imidazolium ionic liquid has been studied using classical molecular dynamics simulation method. The equilibrium and non-equilibrium simulated solvation response functions have been compared with experimentally measured (using 3PEPS) and theoretically predicted (using mode coupling based theory) solvation response function. The study reveals that the experimental solvation timescales can originate from the purely nonpolar interaction between excited solute and ionic liquid solvent molecules.