(246d) Transport Properties of Ionic Liquids with Compressed Polar Gases

The molecular tunability of ionic liquids allows for their potential use in a vast array of engineering applications. Ionic liquids have negligible vapor pressures and are generally considered environmentally benign in various aspects relative to commonly used organic solvents. Coupling ionic liquids with compressed gases has been proposed in reaction engineering, separations, CO₂ capture, refrigeration, and fuel cell technology among other applications. Proper design of such systems utilizing biphasic ionic liquid/gas mixtures requires accurate thermodynamic solubility data as well as thermophysical and transport data of the pure ionic liquids as well as the mixtures. Here, high pressure vapor-liquid equilibrium data of various imidazolium ionic liquids saturated with polar gases are measured. Mixture transport and thermophysical properties including dynamic viscosity, density, kinematic viscosity, self-diffusivity, and thermal conductivity are also measured at temperatures ranging to 125 °C and pressures up to 50+ bar. Polar gases investigated here include the hydrofluorocarbons: difluoromethane (R-32), pentafluoroethane (R-125), 1,1,1,2-tetrafluoroethane (R-134a); and ammonia. The viscosity of all systems investigated demonstrate a drastic decrease with increasing gas composition. This is attributed to a diluent effect in which a component with a low viscosity, the HFC, is introduced to a component with a higher viscosity, the IL. The self-diffusivity of the cation, anion, and the dissolved gas are measured using a pulse-gradient spin-echo technique (PGSE). The measured diffusivities are well-correlated with mixture viscosity through the Stokes-Einstein relation. In contrast to trends in viscosity, the trends in density with increasing gas composition vary from one system to another based on the proximity of the liquid density of the HFC gas to that of the ionic liquid at the conditions measured and the varying mechanisms of solubility. In general, a slight decrease in thermal conductivity is observed in the IL/HFC systems with increasing HFC compositions; however, the thermal conductivity tends to remain dominated by that of the pure IL until very high compositions of the gas are achieved (90+%). It is believed that these HFCs are not polar enough to disrupt the structure of the IL and thus, drastically change the thermal conduction abilities of the IL at loadings below ~90%mol. The trends investigated for the aforementioned HFC compounds are compared to those obtained for systems of ionic liquids saturated with lower polarity gases, e.g. CO₂.