Modeling pVT Properties and Phase Equilibria for Systems Containing Ionic Liquids Using a New Lattice-Fluid Equation of State

The cations or anions of ionic liquids (ILs) usually have long alkyl chains or chainlike structures. Therefore, ILs can be reasonably considered as fluids containing neutral chainlike molecules. Lattice-based molecular thermodynamic models generally used for polymer systems can be applied to describe the thermodynamic properties and phase behavior of IL systems. In our previous work, a new lattice-fluid equation of state (LF EoS) was developed and successfully applied to normal fluid systems (Xu et al., Fluid Phase Equilib. 2008, 265, 112.). In this work, this LF EoS is further extended to model the pVT properties and phase equilibria of IL systems. The molecular parameters of ILs in this EoS were determined by correlating the experimental pVT data of pure ILs. It is shown that the pVT behavior of IL mixtures can be fairly well predicted by these parameters. The vapor-liquid equilibria (VLE) of binary IL-solvent systems were calculated by using an adjustable binary parameter, kappa(12). For liquid-liquid equilibria (LLE) of binary IL systems, a parameter C, describing the effect of the mixture composition oil the chain-length parameter r is further used, and satisfactory correlation is obtained. The upper critical solution temperature (UCST) can be predicted successfully. Moreover, the EoS reproduces the solubility data for carbon dioxide (CO2) in various ILs covering a wide range of pressures (0-100 MPa), and it describes the global behavior of trifluoromethane (CHF3) and IL mixtures. The results reveal that the LF EoS is well-suited for the calculation or prediction of the thermodynamic properties of systems containing ILs.