Simulations of 1-Butyl-3-methylimidazolium Tetrafluoroborate + Acetonitrile Mixtures: Force-Field Validation and Frictional Characteristics

To temper their prohibitively high viscosities, ionic liquids are commonly mixed with polar cosolvents to retain favorable physical properties and make them suitable for industrial applications. Molecular dynamics simulations of 1-butyl-3-methylimidazolium tetrafluoroborate ([Im₄₁][BF₄]) mixed with acetonitrile (CH₃CN) are conducted primarily to test the accuracy of a composite force field (FF) and also to provide some insights into the solvation and frictional characteristics of this mixture. The FF combines the united-atom model for imidazolium ionic liquids of Zhong et al. [J. Phys. Chem. B 2011, 115, 10027] with the acetonitrile model of Nikitin and Lyubartsev [J. Comput. Chem. 2007, 28, 2020]. Comparison of simulated properties such as mixture densities, viscosities, electrical conductivities, and component diffusion coefficients to experimental data at 298 K shows that this combined FF provides reasonable accuracy for both static and dynamic properties. Component rotational dynamics, as well as those of a dilute benzene solute, probed via new ²H NMR T₁ measurements, are also reasonably reproduced. Simulated coordination numbers reveal virtually random mixing between the [Im₄₁][BF₄] and CH₃CN components in this system. Comparison of translational diffusion coefficients and rotation times to simple hydrodynamic predictions indicates that the friction on most motions becomes increasingly decoupled from solution viscosity as the [Im₄₁][BF₄] concentration increases.