Acetonitrile Drastically Boosts Conductivity of Ionic Liquids

TL;DR

This study demonstrates that adding acetonitrile to imidazolium-based ionic liquids can increase their ionic conductivity by over 50 times, significantly enhancing their potential as advanced electrolytes.

Contribution

It introduces a new methodology for force field generation and reveals the dramatic impact of acetonitrile on ionic conductivity and cluster stability in RTILs.

Findings

ACN increases ionic conductivity by more than 50 times.

Addition of ACN exponentially increases diffusion and decreases viscosity.

Large amounts of ACN stabilize ion pairs but disrupt larger ionic clusters.

Abstract

We apply a new methodology in the force field generation (PCCP 2011, 13, 7910) to study the binary mixtures of five imidazolium-based room-temperature ionic liquids (RTILs) with acetonitrile (ACN). The investigated RTILs are composed of tetrafluoroborate (BF4) anion and dialkylimidazolium cations, where one of the alkyl groups is methyl for all RTILs, and the other group is different for each RTILs, being ethyl (EMIM), butyl (BMIM), hexyl (HMIM), octyl (OMIM), and decyl (DMIM). Specific densities, radial distribution functions, ionic cluster distributions, heats of vaporization, diffusion constants, shear viscosities, ionic conductivities, and their correlations are discussed. Upon addition of ACN, the ionic conductivity of RTILs is found to increase by more than 50 times, that significantly exceeds an impact of most known solvents. Remarkably, the sharpest conductivity growth is found for the long-tailed imidazolium-based cations. This new fact motivates to revisit an application of these binary systems as advanced electrolytes. The ionic conductivity correlates generally with a composition of ionic clusters, simplifying its predictability. In turn, the addition of ACN exponentially increases diffusion and decreases viscosity of the imidazolium-based RTILs/ACN mixtures. Large amounts of acetonitrile stabilize ion pairs, but ruin greater ionic clusters.