What Determines CO2 Solubility in Ionic Liquids? A Molecular Simulation Study

Molecular dynamics (MD) simulations of 10 different pure and CO2-saturated ionic liquids are performed to identify the factors that determine CO2 solubility. Imidazoliumr,based cations with varying alkyl chain length and functiorialiation are paired with anions of different hydrophobicity and Size. Simulations are carried- out With an empirical force field based on tquid-phase charges. The partial molar volume of CO, in ionic liquids (ILs) varies from 30 to 40-cm(3)/mol. This indicates that slight ion displacements are necessary to enable CO, insertions. However, the absorption of CO2 does not affect the overall organization of ions in the ILs as demonstrated by almost equal cation anion radial distribution functions of pure ILs and ILs saturated with CO2. The solubility of CO2 ILs is not influenced by direct CO2 ion, interactions, Instead, a strong correlation between the ratio of unoccupied space in pure ILs- and their ability to absorb CO2 is found. This preformed- unoccupied space is regularly dispersed, throughout the, T.La and needs to be expanded by slight ion displacements to accommodate, CO2. The amount of preformed unoccupied apace is a good indicator for ion cohesion in weak electrostatic catibn anion interaction densities in ILs, i.e., weak ion Cohesion, leads to larger average distances 'between ions and hence to more unoccupied space. Weak ion cohesion facilitates ion displacement to enable an expansion of empty space to accommodate CO2. Moreover, it is demonstrated that.the strength of ion 'cohesion is primarily,determined by the, ion density, which in tutu is given by the ion sizes. Ion cohesion is influenced additionally to a smaller extent by local electrostatic interactions among ion moieties between which CO2 is inserted and which do not depend on the ion density, Overall, the factors that determine the solubility of CO2 in ILs are identified consistently across a large variety of constituting ions through MD Simulations.