Assessment of the potential models of acetone/CO2 and ethanol/CO2 mixtures by computer simulation and thermodynamic integration in liquid and supercritical states

Abstract

Binary mixtures of CO₂ with ethanol and with acetone are studied by computer simulation, including extensive free energy calculations done by the method of thermodynamic integration, at 313 K, i.e., above the critical point of CO₂ in the entire composition range. The calculations are repeated with three different models of acetone and ethanol, and two models of CO₂. Comparisons of the molar volume of the different systems as well as of the change of their molar volume accompanying the mixing of the two components with experimental data reveal that, among the model pairs tested, the best results are obtained if both components are described by the Transferable Potentials for Phase Equilibria (TraPPE) force field. Around the ethanol/acetone mole fraction of 0.05 all ethanol/CO₂ and almost all acetone/CO₂ model pairs considered predict the existence of a sharp maximum of the molar volume. Due to the lack of experimental data in this composition range, however, these predictions cannot be verified/falsified yet. Most of the model pairs considered also predict limited miscibility of these compounds, as seen from the positive values of the free energy change accompanying their mixing, and the miscibility gap is located at the same composition range as the aforementioned molar volume maximum.