Comparison of Single-Ion Molecular Dynamics in Common Solvents

Abstract

Laying a basis for molecularly specific theory for the mobilities of ions in solutions of practical interest, we report a broad survey of velocity autocorrelation functions (VACFs) of Li${}^{+}$ and PF${}_6{}^{-}$ ions in water, ethylene carbonate, propylene carbonate, and acetonitrile solutions. We extract the memory function, $\gamma (t)$, which characterizes the random forces governing the mobilities of ions. We provide comparisons, controlling for electrolyte concentration and ion-pairing, for van~der~Waals attractive interactions and solvent molecular characteristics. For the heavier ion (PF${}_6{}^{-}$), velocity relaxations are all similar: negative tail relaxations for the VACF and a clear second relaxation for $\gamma \left(t\right)$, observed previously also for other molecular ions and with \emph{n}-pentanol as solvent. For the light Li${}^{+}$ ion, short time-scale oscillatory behavior masks simple, longer time-scale relaxation of $\gamma \left(t\right)$. But the corresponding analysis of the \emph{solventberg} Li${}^{+}{\left({\mathrm{H}}_2\mathrm{O}\right)}_4$ does conform to the standard picture set by all the PF${}_6{}^{-}$ results.

10 pages, 8 figures, minor revisions to improve presentation. Now in press \emph{J. Chem. Phys.}