Thermodynamic behavior of [OMIM]-based ionic liquid and H2O systems: Experimental, model prediction and mechanism insights

Abstract

The vapor pressures of the binary systems (water + [OMIM][BF4], water + [OMIM][Tf2N], and water + [OMIM][PF6]) were measured by a modified equilibrium still at temperatures ranging from 323.15 to 358.15 K and water mole fraction ranging from 0.1 to 0.9 for water + [OMIM][BF4], from 0.1 to 0.4 for water and [OMIM][Tf2N] and from 0.05 to 0.45 for water + [OMIM][PF6] binary systems, respectively. The experimental results indicate that with the addition of ionic liquids (ILs) the vapor pressure of binary systems decreases. In addition, the popular and widely used UNIFAC-Lei model was extended to the corresponding systems to predict the vapor pressures with the average relative deviations (ARDs) of 4.99%, 4.71% and 5.15% for water + [OMIM][BF4]/[OMIM][Tf2N]/[OMIM][PF6] systems, respectively. Moreover, the binding energy, excess enthalpies and σ-profiles were investigated to analyze the thermodynamic behavior at the molecular level. It was found that the interaction between [OMIM][BF4] and water is significantly higher than those between [OMIM][Tf2N]/[OMIM][PF6] and water as shown by the σ-profiles and the binding energy, and the hydrogen bonding interaction is the dominant driving force contributing to the overall excess enthalpy.