Molecular Dynamics Simulation of Imidazolium CnMIM-BF4 Ionic Liquids using a Coarse Grained Force-Field

TL;DR

This study develops coarse-grained force-fields for a family of imidazolium-based ionic liquids, enabling large-scale molecular dynamics simulations that accurately reproduce thermophysical properties and reveal nanostructuring phenomena at interfaces.

Contribution

The paper introduces new coarse-grained force-fields for [C_{2-8}MIM][BF4] ionic liquids, facilitating long-time scale simulations of their thermophysical and interfacial properties.

Findings

Simulated equation of state and diffusion coefficients match experimental data.

Long aliphatic chains induce nanostructuring in the ionic liquids.

Models reveal symmetry breaking and nanostructuring at liquid-vapor interfaces.

Abstract

Ionic Liquids feature thermophysical properties that are of interest in solvents, energy storage materials and tenable lubrication applications. Recently, a series of coarse grained (CG) models was developed to investigate 1-ethyl-3-methylimidazolium tetrafluoroborate [C2MIM][BF4] and 1-butyl-3-methylimidazolium hexafluorophosphate [C4MIM][PF6] ionic liquids. Building on these CG models we derive force-fields to investigate the [C_{2-8}MIM] [BF4] family of ionic liquids, as a starting step for systematic investigations of lubrication under nanoconfinement conditions. The simualted equation of state and diffusion coefficients are in good agreement with experimental data and with all-atom force fields. We use these model to analyze the nano nanostructuring of ILs characteristic of cations with longer aliphatic chains as well as the ILs liquidvapour interfacial structure. The CG nature of these models enables the simulation of very long time scales, which are needed to computer reliable results of dynamic and interfacial properties. For [C>4MIM] [BF4] the break in symmetry associated to the liquid-vapor interface induces nanostructuring in polar and non-polar domains in the direction perpendicular to the interface plane, hence mimicking the behavior observed in the bulk phases.