Measuring Diffusion and Solubility of Slightly Soluble Gases in [CnMIM][NTf2] Ionic Liquids

We report an advanced experimental setup for the reliable determination of diffusion and solubility, especially for slightly soluble gases in ionic liquids. We were able to improve resolution and reproducibility of an isochoric saturation method by using differential pressure measurements. It was also possible to reduce the influence of wall adsorption, too. For the first time we present diffusion coefficients of hydrogen gas (H-2) in the ionic liquids [EMIM] [NTf2], [BMIM] [NTf2], [HMIM] [NTf2], and [OMIM] [NTf2] at temperatures between 293.15 and 343.15 K. Henry's Law constants were determined simultaneously by fitting the experimental pressure decay to a one-dimensional diffusion model. With rising temperature, diffusion coefficients and solubility of H-2 were increased. Increasing the length of the alkyl side chain of the cation decreased diffusion, whereas solubility of H-2 was increased. An empirical correlation equation for H-2 diffusion in [CnMIM][NTf2] ionic liquids is presented. Limitations are also pointed out. Furthermore, diffusion and solubility of carbon monoxide and carbon dioxide were investigated in [BMIM] [NTf2] and compared to that of H-2.