Monte Carlo Simulations of Pure and Mixed Gas Solubilities of CO₂ and CH₄ in Nonideal Ionic Liquid–Ionic Liquid Mixtures

The knowledge of mixed gas solubility in ionic liquids at high pressures, which becomes relevant in applications such as tertiary oil recovery and landfill gas utilization, is critical for the design of gas separation technologies. In this study, we examine ionic liquid mixtures for tuning the solubility of carbon dioxide (CO₂) and methane (CH₄). Using Gibbs ensemble Monte Carlo simulations, absorption isotherms of pure CO₂ and CH₄ are computed in the binary ionic liquid mixtures containing the common cation 1-n-butyl-3-methylimidazolium [C₄mim]⁺ and the anions chloride Cl^– and bis­(trifluoromethanesulfonyl)­imide [NTf₂]⁻ using five molar compositions at 353 K and the pressures ranging from 1 to 100 bar. Additionally, mixture gas solubilities of CO₂ and CH₄ with the starting gas phase mole ratios of 5:95 and 15:85 at a total pressure of 100 bar are determined and compared with the ideal predictions obtained from the solubilities calculated in pure ionic liquids. Results show that the mixture solubilities deviate from the ideal mixing rule at high pressures with improved solubilities in ionic liquid mixtures having up to 10% of [C₄mim]­[NTf₂] in [C₄mim]­Cl. Furthermore, for the entire ionic liquid mixture composition range, the CO₂/CH₄ solubility selectivities exhibited a nonlinear behavior. Interestingly, simulated CO₂/CH₄ solubility selectivities at the mole ratios of 05:95 and 15:85 do not differ significantly from the ideal solubility selectivities suggesting independent gas absorption, except for 05:95 at 10% of [C₄mim]­[NTf₂] in [C₄mim]­Cl. Thus, there is a potential for improving CO₂/CH₄ selectivity in pure [C₄mim]Cl with the addition of a small amount of [C₄mim]­[NTf₂] ionic liquid.