Exploring the Structure and Dynamics of Ionic Liquids Using Molecular Dynamics

Experimental studies of solvation dynamics in imidazolium-based ionic liquids (ILs) have revealed complex kinetics over a broad range of time scales from femtoseconds to tens of nanoseconds. Microsecond-length molecular dynamics (MD) simulations of coumarin 153 (C153) in a series of imidazolium-based ILs were performed to reveal the molecular-level mechanism for solvation dynamics over the full range of time scales accessed in the experiments. An analysis of the structure of the IL in the vicinity of the probe molecule revealed preferential solvation by the cations. Despite this observation, decomposition of the solvation response into components from the anions and cations and also from translational and rotational motions show that translations of the anions are the dominant contributor to solvation dynamics. The kinetics for the translation of the anions into and out of the first solvation shell of the dye were found to mimic the kinetic profile of the solvation dynamics response. This mechanism for solvation dynamics contrasts dramatically with conventional polar liquids in which solvent rotations are generally responsible for the response.

The structure and dynamics of water as measured experimentally in ILs have revealed local ion rearrangements that occur an order of magnitude faster than complete randomization of the liquid structure. Simulations of an isolated water molecule embedded in 1-butyl-3-methyl imidazolium hexafluorophosphate, [bmim][PF6], were performed to shed insight into the nature of these coupled water-ion dynamics. The theoretical calculations of the spectral diffusion dynamics and the infrared absorption spectra of the OD stretch of isolated HOD in [bmim][PF6] agree well with experiment. The infrared (IR) absorption lineshape of the OD stretch is narrow and blue-shifted in the IL compared to the OD stretch of HOD in H2O. Decomposition of the OD frequency time correlation function revealed the translation of the anions dominate the spectral diffusion dynamics.