Capturing the solubility Behavior of CO2 in ionic liquids by a simple model

The goal of this work was to check the ability of the soft-SAFT equation to capture the solubility of carbon dioxide (CO2) in ionic liquids (ILs) with a simple model for the IL. Imidazolium-based ILs were modeled as Lennard-Jones chains with an associating site mimicking the specific interactions between the cation and the anion as a pair. The CO2 was modeled as a Lennard-Jones chain with explicit quadrupolar interactions, with parameters taken from a previous work (Dias, A. M. A.; et al. Ind. Eng. Chem. Res. 2006, 45, 2341); no specific interaction between the ILs and the CO2 molecules was necessary to reproduce the solubility of Co-2 in these ILs, even at high pressures. The parameters for the pure components were estimated with data from literature for two different families of ILs: the 1-alkyl-3-methylimidazolium tetrafluoroborate [C-n-mim][BF4] and the 1-alkyl-3-methylimidazolium hexafluorophosphate [C-n-mim][PF6]. The study was performed in a systematic manner: First, the equation was used to model the pure components, obtaining a correlation of the molecular parameters of the ILs as a function of the molecular weight; then, the phase behavior of ILs + CO2 systems was studied over a wide range of temperatures and pressures. The simple model proposed here is able to capture the sharp rise in pressure as the solubility increases, without assuming specific interactions between the CO2 and the IL. The model quantitatively describes the solubility of CO2 in [C-n-mim][BF4], while a binary parameter, independent of temperature, is needed for the high pressure behavior as well as for the [C-n-mim][PF6] family with CO2. This work illustrates that relatively simple models are able to capture the solubility behavior of CO2 in ILs, provided that accurate models are available for the pure components as well as an accurate equation of state to model the behavior of complex systems.