Comparison of Single-Ion Molecular Dynamics in Common Solvents

TL;DR

This study investigates ion mobility in various solvents by analyzing velocity autocorrelation functions and memory functions for Li+ and PF6- ions, revealing differences based on ion mass and solvent characteristics.

Contribution

It provides a comprehensive comparison of ion velocity autocorrelations and memory functions across different solvents and ions, advancing molecularly specific theories of ion mobility.

Findings

PF6- ions show similar relaxation behaviors across solvents.

Li+ ions exhibit short-time oscillations masking longer relaxation.

Solventberg analysis of Li+ confirms standard mobility models.

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.