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

Molecular dynamics (MD) simulations were conducted to investigate the variation of Henry's constant of CO in two binary ionic liquid mixtures. One of the mixtures is formed by pairing the cation 1- n-butyl-3-methylimidazolium [Cmim] with chloride Cl and methylsulfate [MeSO], whereas the other binary ionic liquid mixture contains [Cmim] in combination with the anions Cl and bis(trifluoromethanesulfonyl)imide [NTf]. 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 CO saturation were performed at 353 K and 10 bar. Our calculations indicate that the Henry's constant for CO follows a highly nonlinear, although expected based on ideal solubility, trend with respect to the increasing concentration of Cl in [Cmim]Cl [NTf], whereas the Henry's constant is almost independent of the anion composition in the [Cmim]Cl [MeSO] 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 [Cmim]Cl [NTf] system. Because of the weakly coordinating ability of the [NTf] anion with the cation, the [NTf] anion is displaced from the equatorial plane of the imidazolium ring and occupies positions above and below the ring, enabling enhanced CO-[NTf] association. The rearrangement also weakens the cation π-π interactions, resulting in the formation of increased local free volume aiding CO accommodation. On the contrary, such structural transitions are absent in the [Cmim]Cl [MeSO] mixture system.