Thermophysical Properties of Homologous Tetracyanoborate-Based Ionic Liquids Using Experiments and Molecular Dynamics Simulations.

Abstract

Thermophysical properties of low-viscosity ionic liquids (ILs) based on the tetracyanoborate ([B(CN)4]-) anion carrying a homologous series of 1-alkyl-3-methylimidazolium ([AMIM]+) cations [EMIM]+ (ethyl), [BMIM]+ (butyl), [HMIM]+ (hexyl), [OMIM]+ (octyl), and [DMIM]+ (decyl) were investigated by experimental methods and molecular dynamics (MD) simulations at atmospheric pressure and various temperatures. Spectroscopic methods based on nuclear magnetic resonance and surface light scattering were applied to measure the ion self-diffusion coefficients and dynamic viscosity, respectively. In terms of MD simulations, a nonpolarizable molecular model for [EMIM][B(CN)4] developed by optimization to experimental data was transferred to the other homologous ILs. For the appropriate description of the inter- and intramolecular interactions, precise and approximate force fields (FFs) were tested regarding their transferability within the homologous IL series, aiming at reducing the computational effort in molecular simulations. It is shown that at comparable simulated and experimental densities, the calculated and measured data for viscosity and self-diffusion coefficients of the ILs agree well mostly within combined uncertainties, but deviate stronger for longer-chained ILs using an overly coarse FF model. For the [B(CN)4]--based ILs studied, a comparison with literature data, the influence of varying alkyl chain length in the cation on their structural and thermophysical properties, and a correlation between self-diffusivity and viscosity are discussed.