Molecular Origins of the Apparent Ideal CO₂ Solubilities in Binary Ionic Liquid Mixtures

Molecular dynamics (MD) simulations were conducted to investigate the variation of Henry's constant of CO₂ in two binary ionic liquid mixtures. One of the mixtures is formed by pairing the cation 1- n-butyl-3-methylimidazolium [C₄mim]⁺ with chloride Cl^- and methylsulfate [MeSO₄]^-, whereas the other binary ionic liquid mixture contains [C₄mim]⁺ in combination with the anions Cl^- and bis(trifluoromethanesulfonyl)imide [NTf₂]^-. In order to provide a microscopic understanding of the behavior of the Henry's constant with the anion composition, MD simulations of ionic liquid mixtures with and without CO₂ saturation were performed at 353 K and 10 bar. Our calculations indicate that the Henry's constant for CO₂ follows a highly nonlinear, although expected based on ideal solubility, trend with respect to the increasing concentration of Cl^- in [C₄mim]Cl _x[NTf₂]_{1- x}, whereas the Henry's constant is almost independent of the anion composition in the [C₄mim]Cl _x[MeSO₄]_{1- x} system. Structural analyses presented in terms of radial, spatial, and angular distribution functions point to significant structural reorganization of the anions around cations in the [C₄mim]Cl _x[NTf₂]_{1- x} system. Because of the weakly coordinating ability of the [NTf₂]^- anion with the cation, the [NTf₂]^- anion is displaced from the equatorial plane of the imidazolium ring and occupies positions above and below the ring, enabling enhanced CO₂-[NTf₂]^- association. The rearrangement also weakens the cation π-π interactions, resulting in the formation of increased local free volume aiding CO₂ accommodation. On the contrary, such structural transitions are absent in the [C₄mim]Cl _x[MeSO₄]_{1- x} mixture system.