Polarizability Versus Mobility: Atomistic Force Field for Ionic Liquids

TL;DR

This study uses classical molecular dynamics to analyze Coulombic effects on ionic liquids and proposes a charge scaling method to improve the accuracy of force fields by accounting for electronic polarization effects.

Contribution

It introduces a charge scaling approach to enhance the realism of non-polarizable force fields for ionic liquids, addressing Coulombic interaction overestimations.

Findings

Charge scaling improves agreement with experimental densities.

The method reduces overestimation of vaporization heats.

Density scales linearly with the charge scaling factor.

Abstract

Based on classical molecular dynamics simulations, we discuss the impact of Coulombic interactions on a comprehensive set of properties of room temperature ionic liquids (RTILs) containing 1,3 dimethylimidazolium (MMIM+), N butylpyridinium (BPY+), and bis(trifluoromethane sulfonyl)imide (TFSI-) ions. Ionic transport is found to be noticeably hindered by the excessive Coulombic energy, originating from the neglect of electronic polarization in the condensed phase of these RTILs. Starting from the models, recently suggested by Lopes and Padua, we show that realistic ionic dynamics can be achieved by the uniform scaling of electrostatic charges on all interaction sites. The original model systematically overestimates density and heat of vaporization of RTILs. Since density linearly depends on charge scaling, it is possible to use it as a convenient beacon to promptly derive a correct scaling factor. Based on the simulations of [BPY][TFSI] and [MMIM][TFSI] over a wide temperature range, we conclude that suggested technique is feasible to greatly improve quality of the already existing non-polarizable FFs for RTILs.