Transport Properties of the 1-Hexyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide-Trichloromethane Binary System: Indication of Trichloromethane Segregation.

Abstract

Self-diffusion coefficients and electrical conductivity were studied for the binary system 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide-trichloromethane ([C6mim][NTf2]-CHCl3) as a function of composition and temperature. Self-diffusion coefficients of cation and anion are identical for ionic liquid mole fractions xIL < 0.95. The self-diffusion coefficient of CHCl3 is consistently larger than that of the ions by a factor of 4. A double logarithmic plot for the ratio of self-diffusion coefficient and temperature versus viscosity is linear for ionic liquid mole fractions 0.1 < xIL < 0.9 indicating (a) a fractional Stokes-Einstein applies where self-diffusion is inverse proportional to some power b of viscosity (D ∼ η(-b)) and (b) single average length scales are associated with the mass transport of [C6mim][NTf2] and CHCl3. However, the obtained length scale for CHCl3 is unreasonably small, which is indicative of CHCl3 segregation. The molar conductivity displays a maximum near xIL = 0.2. Evaluation of the ionicity from molar conductivity and self-diffusion coefficients indicates a gradual speciation change from charged species to neutral species for xIL < 0.5. The temperature dependencies of self-diffusion and molar conductivity follow Arrhenius behavior. The obtained xIL-dependent activation energies are found to be linear for molar conductivity and largest for the cation and anion self-diffusion coefficients. The activation energies for the self-diffusion of CHCl3 appear to be identical with those obtained from fluidity data.