Solubilities and Diffusivities of Carbon Dioxide in Ionic Liquids:  [bmim][PF6] and [bmim][BF4]

Gaseous carbon dioxide (CO2) absorption measurements in l-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) and l-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) were made using a commercial gravimetric microbalance at temperatures of 283.15, 298.15, 323.15, and 348.15 K and at pressures under 2 MPa. Gas solubilities were determined from absorption saturation (equilibrium) data at each fixed temperature and pressure. The experimental solubility data of CO2 in ionic liquids have been successfully correlated for the first time using a simple cubic equation of state (EOS). Some phase behaviors of CO2 and ionic liquid mixtures as well as Henry's law constants and the volume change of solutions are predicted well with the present EOS model. From time-dependent absorption data, binary diffusion coefficients of CO2 in the ionic liquids have been obtained for the first time by the use of a simple diffusion model. Magnitudes in the observed diffusion coefficients are 10-10 to 10-11 m2 s-1, which are about 10−100 times lower than typical values, found in various organic liquids.