Diffusivity of Carbon Dioxide in Room-Temperature Ionic Liquids

A transient thin liquid film gas uptake technique was used to measure the diffusion coefficients of carbon dioxide under infinite dilution conditions in eight commercially available room-temperature ionic liquids at 10, 25, and 40 °C. The measured CO 2 diffusion coefficients were ∼1 × 10 −6 cm 2 /s, an order of magnitude smaller than coefficients for CO 2 diffusion in traditional organic solvents. Combining these results with diffusivity values reported in the literature for CO 2 diffusivity in nonionic fluids shows a generally universal relationship between diffusivity and solvent viscosity, with diffusivity inversely proportional to the square root of viscosity. Activation energies for CO 2 diffusion in ionic liquids were found to be considerably smaller than activation energies for self-diffusion of ionic liquids (determined from NMR measurements) and activation energies for ionic liquid viscosity. These relative values are different than for CO 2 diffusion, self-diffusion and viscosity in traditional nonionic solvents, where activation energy values for the three different processes are generally comparable. Results were used to develop correlations relating CO 2 diffusion to ionic liquid properties and system temperature. Different expressions were needed to describe diffusion in ionic liquids with bis(trifluoromethylsulfonyl)imide anion than for ionic liquids with a different anion. Diffusivity values predicted using these correlations agree within 25% with experimentally measured values.