In this work, three transport properties (viscosity, diffusion coefficient, and electrical conductivity) were experimentally determined from 298 K to 343 K in four pure imidazolium-based ionic liquids with two anions and different alkyl chain lengths on the cation: 1-ethyl-3-methylimidazolium methylsulfate, [C1C2Im]­[CH3SO4], 1-butyl-3-methylimidazolium methylsulfate, [C1C4Im]­[CH3SO4], 1-ethyl-3-methylimidazolium triflate, [C1C2Im]­[CF3SO3] and 1-butyl-3-methylimidazolium triflate, [C1C4Im]­[CF3SO3]. Higher viscosities, lower diffusion coefficients, and electrical conductivities were measured when the alkyl chain length was increased or a sulfate anion was present. From these experimental data, the ionic association was discussed using the qualitative approach of the Walden plots and the quantitative ionicity concept. An increased ionic association was observed when the alkyl chain length on the cation was increased, while comparable ionicities were measured for both anions. Finally, the applicability of the Stokes–Einstein equation (relation between the diffusion coefficient and the viscosity) was also discussed in these systems.

In this study, we have focused on binary mixtures composed of 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)-imide, [C(4)C(1)im][Ntf(2)], and a selection of six molecular components (acetonitrile, dichloromethane, methanol, 1-butanol, t-butanol, and water) varying in polarity, size, and isomerism. Two Kamlet-Taft parameters, the polarizability π* and the hydrogen bond acceptor β coefficient were determined by spectroscopic measurements. In most cases, the solvent power (dipolarity/polarizability) of the ionic liquid is only slightly modified by the presence of the molecular component unless large quantities of this component are present. The viscosity and electrical conductivity of these mixtures were measured as a function of composition and the relationship between these two properties were studied through Walden plot curves. The viscosity of the ionic liquid dramatically decreases with the addition of the molecular component. This decrease is not directly related to the volumetric properties of each mixture or its interactions. The conductivity presents a maximum as a function of the composition and, except for the case of water, the conductivity maxima decrease for more viscous systems. The Walden plots indicate enhanced ionic association as the ionic liquid gets more diluted, a situation that is the inverse of that usually found for conventional electrolyte solutions.