Vapor pressure measurements and predictions for the binary systems containing ionic liquid [EMIM][BF4] and formic acid/acetic acid

The vapor pressures of the binary systems formic acid/acetic acid +1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) were measured using a modified equilibrium still at temperatures ranging from 323.15 to 358.15 K and IL contents ranging from 0.1 to 0.9 in mass fraction. The predictive thermodynamic models (i.e., UNIFAC-Lei and COSMO-RS models) were used to predict the vapor pressures with the ARDs of 4.54% and 22.24% for formic acid, respectively, and 6.38% and 16.43% for acetic acid, respectively. This indicates that the UNIFAC-Lei model can give a quantitative description with high accuracy, while the COSMO-RS model can only be used as a prior model to give a qualitative prediction. Moreover, the excess enthalpies and σ-profiles were calculated by the COSMO-RS model to achieve further understanding on the thermodynamic behavior at molecular scale. It was found that the hydrogen bonding interaction (HHB) plays a dominant role to the total excess enthalpy, and the HHB is mainly dependent on acid components. For IL, the contribution of anion is larger than that of cation as confirmed by the σ-profiles.