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

Abstract

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.

Macroscopic Differentiators for Microscopic Structural Nonideality in Binary Ionic Liquid Mixtures

“…For example, binary ionic liquid mixtures containing anions with a very substantial difference in β parameter have been reported to yield nonideal behavior in terms of excess molar volumes, viscosity, conductivity, and glass transition temperature experiments. ,, Our work and that of others have also shown that when binary ionic liquid mixtures with a common cation and two anions with a large difference in the hydrogen-bonding ability are formed, the molecular arrangement of anions around the cation in the mixture can markedly differ from those in the pure ionic liquids . We have also demonstrated that the presence of “non-native” arrangements of anions exert a profound influence on the physical dissolution mechanism of CO 2 at the molecular level …”

“…11 We have also demonstrated that the presence of "non-native" arrangements of anions exert a profound influence on the physical dissolution mechanism of CO 2 at the molecular level. 12 Despite the significance of molar volumes and the hydrogenbonding ability of the anions in predicting the nonideal behavior in ionic liquid mixtures, it is surprising that a systematic investigation evaluating the influence of these markers has been absent in the literature. In the present work, we resort to molecular dynamics simulations to assess the ability of these two descriptors in providing clues to the nonideality in binary ionic liquid mixtures.…”

Modeling solubility of CO2 gas in room temperature ionic liquids using the COSMOSAC-LANL model: a first principles study

“…This relation holds when the system is in the region x 2 or 1 -0. Since the above equation is valid when the binary system approaches x 1/2 -1, x 2/1 -0 for a species, so one will correct the calculated activity coefficient at infinite dilution (x 1/2 -0, x 2/1 -1) using the following relation ln g 1/2 (real) = ln g 1/2 (compute) = 1 + ln g 1/2 (model), (41) for which the minimum computed asymmetric interaction is less than 1 according to Appendix C.…”

Ion speciation: a key for the understanding of the solution properties of ionic liquid mixtures

“…4 Not surprisingly these ionic liquid mixtures have become very popular and are known to have a plethora of applications. [5][6][7][8][9][10][11][12][13][14] While the application of ionic liquid mixtures keeps growing, the fundamental knowledge of the interactions that occur in the system is only beginning to be acquired.…”