Modelling phase equilibria of pure ionic liquids from a new equation of state

Abstract

A perturbed hard-trimer (PHT) equation of state (EOS) has been employed to model densities and some derived thermodynamic properties of 39 ionic liquids (ILs) considering a trimer expression obtained from the statistical associating fluid theory as the reference physical model. The van der Waals dispersion forces were applied as perturbation term. The proposed model was tested using ILs containing imidazolium, pyrrolidinium, pyridinium, phosphonium and piperidinium cations. Two parameters appeared in the PHT EOS which are temperature-dependent, reflecting the dispersive energy parameters among trimers, e and the hard-core diameter, σ, were determined based on the molecular scaling parameters. The performance of the proposed PHT EOS has been evaluated by predicting the volumetric and first and second derivatives thermodynamic properties in the pressure and temperature ranges within 0.1–200 MPa and 273–472.6 K, respectively. From 6331 data points examined, the average absolute deviation (AAD) of the correlated (at 0.1 MPa) and predicted (at high pressures) densities from the experimental ones was found to be 0.18 %. Furthermore, the isothermal compressibilities and thermal expansion coefficients as well as the heat capacities were estimated through the PHT EOS with uncertainties of the order of ±11.09, ±11.76 and ±3.34 %, respectively. Further, the vapour pressures of ILs are also predicted by the proposed model. The trend of the predicted vapour pressure is in accord with those reported in literature.