Temperature and Density Dependence of the Transport Properties of the Ionic Liquid Triethylpentylphosphonium Bis(trifluoromethanesulfonyl)amide, [P_222,5][Tf₂N]

Viscosities (η), conductivities (κ, Λ), and ion self-diffusion coefficients (D_Si) for triethylpentyl­phosphonium bis­(trifluoro­methanesulfonyl)­amide, ([P_222,5]­[Tf₂N]), have been measured as a function of temperature and pressure in the ranges ([273–363] K, 243 MPa max), ([273–353] K, 251 MPa max), and (298–363 K, 225 MPa max), respectively. pVT data are also reported from (298 to 353) K to 50 MPa. The ratio of the ion self-diffusion coefficients is constant, independent of temperature and pressure. The results are discussed using velocity correlation, distinct diffusion, and resistance coefficients, and the Stokes–Einstein–Sutherland (SES: relating D_Si and η) and Nernst–Einstein equations (NE: relating Λ and D_Si). As is usual for ionic liquids the SES and NE plots for high-pressure isotherms and the atmospheric pressure isobar overlap quantitatively, that is the self-diffusion and distinct diffusion coefficients, and the molar conductivity, are functions of the viscosity. This can be used for the interpolation of these quantities within the range of the pressures and temperatures employed here, and for moderate extrapolation beyond them. The resistance coefficients are positive: there is no evidence for any ion association. Density scaling using Rosenfeld reduced variables yields (2.37 ± 0.07) for the scaling parameter, γ, for the four transport properties.