Solubility of CO₂ and N₂O in an Imidazolium-Based Lipidic Ionic Liquid

Imidazolium-based ionic liquids have been extensively studied for their ability to dissolve a wide variety of gases and for their potential to be used as separation agents in industrial processes. For many short chain 1-alkyl-3-methylimidazolium bistriflimde salts, CO₂ and N₂O solublities are very similar. In this work, the solubility of CO₂ and N₂O has been measured in the lipidic ionic liquid 1-methyl-3-(Z-octadec-9-enyl)imidazolium bistriflimide ([oleyl-mim][NTf₂]) at 298 K, 310 and 323 K up to ∼2 MPa. N₂O was found to have higher solubility than CO₂ under the same conditions, similar to the behavior observed when olive oil, a natural lipid, was the liquid solvent. However, the solubility of each gas on a mole fraction basis is lower in the ionic liquid than in olive oil. Comparison of the gas solubilities on a mass fraction basis demonstrates that CO₂ solubility is nearly identical in both liquids; N₂O solubility is higher than CO₂ for both liquids, but more so in the olive oil. The difference is attributed to the high mass fraction of the olive oil that is lipid-like in character. The differential solubility of N₂O/CO₂ in this ionic liquid, in contrast to that of shorter chain 1-alkyl-3-methylimidazolium bistriflimide salts, gives physical insight into the solvent properties of this class of ionic liquids and provides further support for their lipid-like character.