The Influence of Water on the Solubility of Carbon Dioxide in Imidazolium Based Ionic Liquids

We explore the influence of small amounts of water dissolved in 1-alkyl-3-methyl-imidazolium based ionic liquids (Ms) on the solubility of CO2 as a function of water content and temperature. We observe a decreasing solubility of CO2 as function of increasing water content in almost quantitative agreement with recent experimental data by Husson et al. [Fluid Phase Eq. 294, 98 (2010)]. In addition, a purely repulsive variant of CO2 is employed as a test-case to elucidate the solvation behavior of small sized molecules in absence of favorable interactions. We find that the decreasing solubility is about one third due to the increasing solvent number density, and about two thirds due to the change in solvation free energy. In addition, we observe that the computed solvation free energy of a purely repulsive test-molecule increases even more strongly with increasing water content, indicating that small favorable "interaction sites" within the liquid, well suited for accommodating a CO2 molecule, become increasingly rare due to the competition with the water molecules.