Thermodynamic modeling of CO2 solubility in ionic liquid with heterosegmented statistical associating fluid theory
Heterosegmented statistical associating fluid theory is used to represent the CO2 solubility in ionic liquids. As in our previous work, ionic liquid molecule is divided into several groups representing the alkyls, cation head, and anion. The cation of ionic liquid is modeled as a chain molecule that consists of one spherical segment representing the cation head and groups of segments of different types representing different substituents (alkyls). The anion of ionic liquid is modeled as a spherical segment of different type. To account for the electrostatic/polar interaction between the cation and anion, the spherical segments representing cation head and anion each have one association site, which can only cross associate. Carbon dioxide is modeled as a molecule with three association sites, two sites of type O and one site of type C, where sites of the same type do not associate with each other. The parameters of CO2 are obtained from the fitting of the density and the saturation vapor pressure of CO2. For the CO2-ionic liquid systems, cross association between site of type C in CO2 and another association site in anion is allowed to occur to account for the Lewis acid–base interaction. The parameters for cross association interactions and the binary interaction parameters used to adjust the dispersive interactions between unlike segments are obtained from the fitting of the available CO2 solubility in ionic liquids. The model is found to well represent the CO2 solubility in the imidazolium ionic liquids from 283 to 415 K and up to 200 bar.