Slow solvation in ionic liquids: connections to non-Gaussian moves and multi-point correlations

This paper explores an interconnection between timescales of dynamic heterogeneity (DH) in a neat ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]), and slow solvation of a dipolar solute, coumarin 153 (C153) in it at 298 K and 450 K. Molecular dynamics simulations employing realistic interaction potentials for both the IL and the solute have been performed. DH timescales have been obtained from non-Gaussian and new non-Gaussian (NNG) parameters, and four-point dynamic susceptibilities (χ4(k, t)) and overlap functions (Q(t)). Simulated ion displacement distributions exhibit pronounced deviations from Gaussian behaviour and develop bimodality in the timescale of structural relaxation, τ(α), indicating ion hopping at long-time. DH timescales from χ4(k, t) and Q(t) have been found to be longer than τ(NNG) although τ(α) ≈ τ(NNG). Maximum cation jump length detected here corresponds to ~50% of the ion diameter and agrees well with experimental estimates. DH length-scale (ξ) extracted from χ4(k, t) spans about an ion diameter and shows correct temperature dependence. Our simulated solvation response functions for C153 in [Bmim][PF6] are tri-exponentials with fast time constants in good agreement with the available experimental and/or simulation data. The slow solvation rate at 298 K, however, is ~4 times slower than that found in experiments, although the same at 450 K corroborates well with simulation data at similar temperature from different sources. Importantly, our simulated slow solvation rates at these temperatures strongly correlate to longer DH timescales, suggesting DH as a source for the slow solvation at long-time in IL. Moreover, ion jumps at long-time suggests viscosity decoupling of long-time solvation rate in ILs.