Thermophysical Properties of [EMIM][BF4] and [EMIM][EtSO4] Mixtures with Molecular Solvents from a Charge-Transfer-Informed Force Field

Thermophysical properties of the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) and 1-ethyl-3-methylimidazolium ethyl sulfate ([EMIM][EtSO4]), and of their binary mixtures with acetonitrile and ethanol, were investigated by molecular dynamics simulations using a classical force field informed by quantum chemical calculations. Optimizations at the MP2/6–311G(d, p) level identified six stable conformers for the [EMIM][BF4] ion pair and nine for [EMIM][EtSO4], revealing a charge transfer of about 0.21–0.25e from anion to cation upon ion pairing. Boltzmann-weighted RESP charges derived from these conformers replaced empirical charge scaling. Densities of the pure liquids were reproduced with root-mean-square percentage errors of 0.25% for [EMIM][BF4] (283–323 K) and 0.23% for [EMIM][EtSO4] (293–353 K), and the vaporization enthalpies (133.04 and 163.83 kJ·mol–1) agree closely with experiment. Self-diffusion coefficients of both ions fall within the experimental uncertainty, a marked improvement over earlier fixed-charge models. Mixture densities were reproduced with errors below 0.36% over the entire composition range, and the derived excess molar volumes agree quantitatively with experiment for the aprotic cosolvent. Radial distribution functions reproduce coordination numbers and hydrogen-bonding patterns consistent with diffraction data. The protocol is computationally inexpensive and may be reapplied to other ionic liquids.