Experimental Measurement of the Solubility and Diffusivity of CO₂ in Room-Temperature Ionic Liquids Using a Transient Thin-Liquid-Film Method

In this paper, results from an experimental investigation of carbon dioxide (CO₂) solubility and diffusivity in the ionic liquids 1-n-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf₂N]), 1,2-dimethyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([pmmim][Tf₂N]), 1-butyl-3-methylpyridinium bis(trifluoromethyl sulfonyl)imide ([bmpy][Tf₂N]), 1-(3,4,5,6-perfluorohexyl)-3-methylimdazolium bis(trifluoromethyl sulfonyl)imide ([perfluoro-hmim][Tf₂N]), and 1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]) are reported. A transient thin liquid film method was developed, which enables one to determine the Henry's law constant and the diffusivity at low pressure simultaneously. Measurements were performed at temperatures in the range of 283−323 K. Henry's law constants were in the range of 25.5−84 bar and were in general agreement with measurements reported by other researchers for these and similar ionic liquids. The entropies and enthalpies of absorption were determined to increase as gas solubility decreased. The measured CO₂ diffusion coefficients in the five ionic liquids were ∼10^-6 cm²/s, which is an order of magnitude smaller than the coefficients for CO₂ diffusion in traditional organic solvents. In contrast to the gas solubility results, measured diffusion coefficients were determined to be dependent on the ionic liquid cation as well as the anion. In addition, CO₂ diffusion coefficients were considerably more sensitive to temperature than were CO₂ solubilities in these ionic liquids. Results were used to develop a correlation relating CO₂ diffusion to ionic liquid properties and system temperature.