Experimental Measurements and Molecular Dynamics Simulations of Self-Diffusivity in Mixtures of [C₂C₁im][Tf₂N] with...

Thermophysical properties of hydrofluorocarbon (HFC) gas and ionic liquid (IL) mixtures have previously been shown to exhibit large deviations from ideal solution behavior. Understanding self-diffusivity can help elucidate the contributing dynamic and structural properties. ¹H and ¹⁹F NMR pulsed-field gradient stimulated echo techniques were utilized to measure the self-diffusion coefficients of the ions of 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide ([C₂C₁im][Tf₂N]) with either difluoromethane (HFC-32) or pentafluoroethane (HFC-125) at saturation with temperatures from 25 to 75 °C and pressures to 62 bar. Molecular dynamics (MD) simulations were performed for mixtures of difluoromethane in [C₂C₁im][Tf₂N] across a wide range of HFC compositions. MD simulations demonstrated good qualitative agreement with experiments, with better quantitative agreement at high HFC compositions. MD predicted diffusivities were generally lower and viscosities higher than experimental values, probably due to polarizability effects. Experiments and simulations illustrate that the HFC diffusivity is the highest of all mixture constituents at all investigated conditions and the cation diffuses faster than the anion. A sharp increase in the liquid self-diffusivity of all constituents was observed at HFC compositions above 80% mole_HFC in the difluoromethane/IL system. From previous viscosity measurements and MD simulations, the anion generally follows Stokes-Einstein behavior while the cation and HFC exhibit increased deviations.