Revisiting Imidazolium Based Ionic Liquids: Effect of the Conformation Bias of the [NTf$_{2}$] Anion Studied By Molecular Dynamics Simulations

TL;DR

This study improves molecular dynamics models of imidazolium-based ionic liquids by reparametrizing the [NTf2] anion, resulting in better agreement with experimental data on structure, thermodynamics, and dynamics.

Contribution

The paper introduces a new force field parameter set for the [NTf2] anion that accurately captures its conformations and enhances simulation accuracy for ionic liquids.

Findings

Enhanced force field reproduces experimental densities and viscosities.

Better representation of anion conformations ({trans} and {gauche}).

Improved simulation agreement for diffusion and vaporization enthalpies.

Abstract

We study ionic liquids composed 1-alkyl-3-methylimidazolium cations and bis(trifluoromethyl-sulfonyl)imide anions ([C$_n$MIm][NTf$_2$]) with varying chain-length $n\!=\!2, 4, 6, 8$ by using molecular dynamics simulations. We show that a reparametrization of the dihedral potentials as well as charges of the [NTf$_2$] anion leads to an improvment of the force field model introduced by K\"oddermann {\em et al.} [ChemPhysChem, \textbf{8}, 2464 (2007)] (KPL-force field). A crucial advantage of the new parameter set is that the minimum energy conformations of the anion ({\em trans} and {\em gauche}), as deduced from {\em ab initio} calculations and {\sc Raman} experiments, are now both well represented by our model. In addition, the results for [C$_n$MIm][NTf$_2$] show that this modification leads to an even better agreement between experiment and molecular dynamics simulation as demonstrated for densities, diffusion coefficients, vaporization enthalpies, reorientational correlation times, and viscosities. Even though we focused on a better representation of the anion conformation, also the alkyl chain-length dependence of the cation behaves closer to the experiment. We strongly encourage to use the new NGKPL force field for the [NTf$_2$] anion instead of the earlier KPL parameter set for computer simulations aiming to describe the thermodynamics, dynamics and also structure of imidazolium based ionic liquids.