Gas permeabilities, solubilities, diffusivities, and diffusivity correlations for ammonium-based room temperature ionic liquids with comparison to imidazolium and phosphonium RTIL data

Like our previous work with imidazolium- and phosphonium-based ionic liquids, we report diffusivities over a range of viscosities (71-532 cP) and develop a predictive diffusivity correlation. Reported are the permeability, solubility, and diffusivity data for nine gases in nine ammonium RTILs liquids at 30 degrees C, as determined with a lag-time technique. The gas solubilities and diffusivities of the ammonium RTILs are of the same magnitude as those for the phosphonium and imidazolium RTILs. The ammonium RTILs used, in this study, included cations with both N-alkyl groups and branched alkyl groups. We also report on ammonium-based RTILs derived from quaternary ammonium surfactants. These surfactants-derived ammonium-based RTILs offer a relatively inexpensive alternative to imidazolium-based RTILs. We compare and contrast the thermodynamic (solubility) and transport (diffusivities) phenomena in the ammonium-based RTILs with both the imidazolium and the phosphonium RTILs in the context of being working fluids in a chemical process. From this comparison came certain "universal" trends for diffusivity in RTILs. Specifically, diffusivity scales roughly with the inverse of the square-root of viscosity and inversely with solute molar volume to the power of 1-1.3. This means that diffusivity, in RTILs, is less dependent on viscosity, and more dependent on solute size than predicted by the conventional Stokes-Einstein model. The gases tested were carbon dioxide, nitrogen, oxygen, methane, ethylene, propylene. 1-butene, butane, and 1,3-butadiene. (C) 2008 Elsevier B.V. All rights reserved