Temperature and Pressure Dependence of the Transport Properties of the Ionic Liquid Triethyloctylphosphonium Bis(trifluoromethanesulfonyl)amide, P222,8Tf2N

Viscosities of triethyloctylphosphonium bis(trifluoromethanesulfonyl)amide, P222,8Tf2N, are reported as a function of temperature (273 to 363) K and pressure (maximum 302 MPa) with a falling-body viscometer together with electrical conductivities (κ) measured by impedance spectroscopy at (283 to 348) K, to 250 MPa maximum pressure. pVT data were determined with a vibrating tube densimeter from (298 to 353) K to 50 MPa. Ion self-diffusion coefficients (DSi) were measured by steady-gradient spin-echo NMR (313 to 365) K, and densities (273 to 363) K were determined with a vibrating tube densimeter, both at atmospheric pressure. The results were correlated with Walden and Stokes–Einstein–Sutherland relations. Velocity cross-correlation coefficients (VCC), distinct diffusion coefficients (DDC) and Laity resistance coefficients (LRC) were calculated from DSi and κ at 0.1 MPa. The DDC and LRC for P222,8Tf2N and the pentyl-substituted analogue, P222,5Tf2N, showed differences for cation–cation and cation–anion velocity anti correlations, presumably due to the different cation structures. The high-pressure viscosities were used to predict the pressure dependence of the glass-transition temperature for P222,5Tf2N and P222,8Tf2N. Density scaling was applied to the high-pressure viscosities and conductivities of P222,8Tf2N for comparison with P222,5Tf2N. The scaling parameters are consistent with the theoretical treatment of Knudsen et al. for ionic liquids.