Transport Properties of <i>N</i>-Butyl-<i>N</i>-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide

Abstract

The viscosities (η) and ion self-diffusion coefficients (Di) of the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Pyr14][Tf2N] or [C4mpyr][Tf2N]) are reported between (0 and 90) °C and at pressures to 103 MPa and between (25 and 80) °C to 250 MPa, respectively. A falling-body high-pressure viscometer was employed, supplemented by a Stabinger rotating-cylinder viscometer for atmospheric pressure. Self-diffusion coefficients were measured by the steady-gradient spin–echo NMR technique. The overall uncertainties are ± 2 % and ± 3 %, respectively. Conductivities (κ) and densities obtained between (0 and 90) °C at 0.1 MPa are reported. Ion self-diffusion coefficients and densities at 0.1 MPa are included for N-methyl-N-propyl-3-azabicyclo[3.2.2]nonaneium bis(trifluoromethylsulfonyl)amide ([3-ABN13][Tf2N]). The transport properties are examined using the fractional Stokes–Einstein and Nernst–Einstein relations, velocity correlation coefficients, and density scaling. For [Pyr14][Tf2N], the fractional Stokes–Einstein exponents are (0.93 ± 0.05). There is very good consistency between the diffusion and molar conductivity (Λ) data, with a slope of (0.99 ± 0.05) for a plot of ln(TΛ) against ln(D++D–). The Nernst–Einstein deviation parameter, Δ, is (0.313 ± 0.013): it is consistent with the relation f+- > (f++ + f–-)/2 for the ion–ion velocity cross-correlation coefficients, fij. Density scaling to the temperature-volume function (TVγ) has been applied to both unreduced and reduced viscosities and self-diffusion coefficients for [Pyr14][Tf2N]. Consistent scaling parameters γ are obtained for each of η, D+, and D– with both the Casalini-Roland model and a simple polynomial fit in (TVγ), and for the reduced quantities, η̃, D̃+, and D̃–. At a fixed temperature, the self-diffusion coefficients for the series [BMIM][Tf2N], [Pyr14][Tf2N], and [3-ABN13][Tf2N] decrease with increasing salt molar volume, but the ratio of D(cation) to D(anion) changes from values greater than unity to less than unity, in common with what is generally observed for ionic liquid families.