Influence of Butyl Side Chain Elimination, Tail Amine Functional Addition, and C2 Methylation on the Dynamics and Transport Properties of Imidazolium-Based [Tf2N–] Ionic Liquids from Molecular Dynamics Simulations

Abstract

Molecular dynamics simulations of four ionic liquids (ILs) based on the [Tf2N]−, bis(trifluoromethanesulfonyl)imide anion, and imidazolium cations with different alkyl side chains have been performed. These simulations investigate the influence of butyl side chain elimination, tail amine functional addition, and C2 methylation on the dynamics and transport properties of this family of ionic liquids at 400 K. In our earlier work (J. Chem. Eng. Data, 2014, 59, 2834–2849), a suite of thermodynamic quantities and microscopic structures of these ILs were studied by classical molecular dynamics simulations and ab initio calculations. In this work, the dynamics of the ILs are studied by calculating the mean-square displacement (MSD) and the velocity autocorrelation function (VACF) for selected atomic sites and the centers of mass of the ions. These results are used to calculate the self-diffusion and the ionic conductivity from both the Einstein and Green–Kubo formulas. The calculated ionic self-diffusion coefficients are used to estimate the cationic transference number and the Stokes–Einstein viscosity for the four ILs. In agreement with experiments, the general simulated trends in the MSD, self-diffusion, and ionic conductivity are [bmim][Tf2N] > [apmim][Tf2N] > [bmmim][Tf2N] > [mim][Tf2N]. These trends are the reverse of the trend in the viscosity of four selected ILs. As expected by applying a nonpolarizable force field, the simulation results tend to underestimate the self-diffusivity and conductivity, and overestimate the shear viscosity. The highest and the lowest degrees of ionic association are detected for [mim][Tf2N] and [bmim][Tf2N], respectively.