Simulations of Solvation and Solvation Dynamics in an Idealized Ionic Liquid Model

Equilibrium and nonequilibrium molecular dynamics simulations of solvation and solvation dynamics of a variety of solutes have been performed in the coarse-grained ionic liquid model ILM2 (Roy, D.; Maroncelli, M. J. Phys. Chem. B 2010, 114, 12629). Some comparisons are made between ionic and dipolar solvation using parallel simulations in CH3CN. Despite the fact that the multipolar character of electrostatic interactions and their spatial extent differ in the two solvents, solvation energies are equal to within about 10% in ILM2 and CH3CN. This near equality also holds with reduced accuracy in the case of reorganization energies. Solvation energies of spherical solutes in ILM2 and its variants can be correlated as a function of solute and solvent size using a Born-type expression with an effective cavity size. Solvation time correlation functions in ILM2 exhibit a subpicosecond inertial component followed by a broadly distributed component related to solvent viscosity, comparable to what has been observed in experiment. Direct comparison of simulation to experiment using the solute coumarin 153 (C153) shows general agreement on the time scales and character of the fast and slow components, but the amplitude of the fast component is overestimated by the simulations. Solute motion can significantly increase the speed of solvation, even in the case of large solutes such as C153. Good agreement is found between linear response estimates and the nonequilibrium dynamics associated with electronic excitation of C153. In contrast, perturbations involving changes of a full electron charge in atomic solutes lead to local heating which greatly hastens solvation compared to linear response predictions. The mechanism of charge solvation in atomic solutes is examined in some detail. It is found that ion translation dominates the inertial dynamics. The rotational contribution only becomes comparable to the translation contribution in the tail of the response. Adjustments of ion positions over distances of similar to 30% of their diameters are all that is required to relax the solvation energy in these systems.