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 degrees C. The measured CO2 diffusion coefficients were similar to 1 x 10(-6) cm(2)/s, an order of magnitude smaller than coefficients for CO2 diffusion in traditional organic solvents. Combining, these results with diffusivity values reported in the literature for CO2 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 CO2 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 CO2 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 CO2 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.