3 years ago

Comparison of Single-Ion Molecular Dynamics in Common Solvents.

L. R. Pratt, A. Muralidharan, S. B. Rempe, M. I. Chaudhari

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. With this model, distinct ion-pairing is noted for LiPF$_6$/ethylene carbonate at 1M concentration. 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 solventberg Li$^+\left(\mathrm{H}_2\mathrm{O}\right)_4$ does conform to the standard picture set by all the PF$_6{}^-$ results.

Publisher URL: http://arxiv.org/abs/1801.07116

DOI: arXiv:1801.07116v1

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