Physical and Transport Properties of Bis(trifluoromethylsulfonyl)imide-Based Room-Temperature Ionic Liquids: Application to the Diffusion of Tris(2, 2 ′ -bipyridyl)ruthenium(II)

Abstract

The densities, viscosities, molar conductivities, and surface tensions of room temperature ionic liquids based on the bis(trifluoromethylsulfonyl)imide anion and the 1-(1-butyl)-3-methylimidazolium, 1-butyltrimethylammonium, 1-(1-butyl)-1-methyl- pyrrolidinium, 1-(1-butyl) pyridinium, and 1-ethyl-3-methylimidazolium cations were measured as a function of temperature over the range from . The surface tension of tri-(1-butyl)methylammonium bis(trifluoromethylsulfonyl)imide is also reported. Linear equations were fitted to the experimental density and surface tension data, and the Vogel–Tammann–Fulcher equation for glass-forming liquids was fitted to the experimental viscosity and conductivity data. The surface energies, surface entropies, and critical temperatures were estimated from the temperature dependence of the surface tension data. All of the liquids studied obey the fractional Walden rule and fall only slightly below the ideal line, indicating that they possess high ionicity. In each case, the viscosity shows only modest decoupling from the conductivity as the temperature is increased. Diffusion coefficients for the oxidation of tris(2,-bipyridyl)ruthenium(II) were measured in the six ionic liquids as a function of temperature. The hydrodynamic radius of tris(2,-bipyridyl)ruthenium(II) was estimated from the Stokes–Einstein equation and was found to be remarkably close to the crystallographic radius of this species.