Transport properties and ionic association in pure imidazolium-based ionic liquids as a function of temperature.
Résumé
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.