Capturing Thermodynamic Behavior of Ionic Liquid Systems: Correlations with the SWCF-VR Equation

An equation of state for square-well chain fluids with variable well-width range (SWCF-VR EoS) [Li et al. <i>Fluid Phase Equilib.</i> <b>2009</b>, <i>276</i>, 57] was applied to ionic liquid (IL) systems. ILs were treated as the square-well chain with hydrogen bonding. The corresponding association parameters were given according to our previous work [He et al. <i>Fluid Phase Equilib.</i> <b>2011</b>, <i>302</i>, 139]. The nonassociation parameters were obtained by correlating the experimental liquid densities. Excellent agreements were observed between experimental and theoretical results for pure ILs, and the molecular parameters were linearly correlated with the molecular masses of the [C_<i>n</i>mim]­[NTf₂] members (<i>n</i> = 2, 3, ..., 8, 10). It is found that the other thermodynamic properties such as the vapor pressure and the enthalpy of vaporization, etc., can be reasonably predicted by using the obtained molecular parameters. The phase behavior of the binary systems containing ILs was well-represented with a simple mixing rule. For the vapor–liquid equilibria (VLE) of a system of volatile fluid + IL at low pressures, a temperature-independent binary interaction parameter was adopted. Satisfactory results were achieved for both the self- and cross-associating systems. The influence of temperature on the binary interaction parameters was taken into account in the correlation for the gas–liquid equilibria (GLE) of CO₂ + IL mixtures and liquid–liquid equilibria (LLE) of IL-containing systems. For CO₂ + IL mixtures, the multipolar interactions between like and unlike molecules, and the cross-association between CO₂ and IL molecules were neglected to reduce the computational complexity, and the correlated results agree well with the experimental ones over a wide range of temperatures and pressures. The LLE of alkanol + IL systems were acceptably reproduced with moderate deviations between the experimental and calculated mass fractions. In the water-rich phase of water + IL with LLE, the neglect of electrostatic interaction caused correlated results to deviate from experimental ones greatly