(167c) Solubility and Diffusivity of Hydrofluorocarbons in Room-Temperature Ionic Liquids

Authors: 
Shiflett, M., DuPont Central Research and Development


Room-temperature ionic liquids are being considered as a new class of solvents with improved environmental properties due to their immeasurably low vapor pressure. Ionic liquid research has focused on their use as solvents for a variety of reactions, separations, and material processing. Phase behavior can often determine the attractiveness of using ionic liquids in these applications. This work presents new solubility and diffusivity data for hydrofluorocarbons (HFCs) in ionic liquids. The hydrofluorocarbons include trifluoromethane (HFC-23), difluoromethane (HFC-32), pentafluoroethane (HFC-125), 1,1,1,2-tetrafluoroethane (HFC-134a), 1,1,1-trifluoroethane (HFC-143a), and 1,1-difluoroethane (HFC-152a). A variety of imidazolium based ionic liquids with fluorinated anions (ex. 1-n-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF6], 1-n-butyl-3-methylimidazolium tetrafluoroborate, [bmim][BF4], 1-ethyl-3-methylimidazolium bis(pentafluoroethylsulfonyl)imide, [emim][BEI], 1,2-dimethyl-3-propylimidazolium tris(trifluoromethylsulfonyl)methide, [dmpim][TMeM]) will be discussed. The solubility of each pair was measured using a gravimetric microbalance over a temperature range from 283 to 348 K and for pressures up to about 2.0 MPa. . Diffusivities were calculated using a 1-dimenisonal diffusion analysis of the time-dependent absorption data. Magnitudes in the effective diffusion coefficients were on the order of 10-10 to 10-11 m2 sec-1. In order to understand and predict the phase behavior of mixtures at a wide range of conditions, for the first time an equation of state (EOS) thermodynamic model has been developed. The EOS model employed here is a generic RK (Redlich-Kwong) type of cubic EOS and has been successfully applied to various refrigerant ionic liquid mixtures. Experimental gas solubility was also successfully correlated with well-known solution models (Margules, Wilson, and NRTL activity coefficient equations). Diffusivities were also well analyzed using a diffusivity model based on the Einstein-Stokes equation.

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