A volumetric and intra-diffusion study of solutions of AlCl 3 in two ionic liquids - [C 2 TMEDA][Tf 2 N] and [C 4 mpyr][Tf 2 N].

Intra-diffusion coefficients ( D Si ) have been measured for the ionic liquid constituent ions and aluminium-containing species in aluminium chloride (AlCl 3 ) solutions in the ionic liquids 1-(2-dimethyl-aminoethyl)-dimethylethylammonium bis(trifluoromethylsulfonyl)amide ([C 2 TMEDA][Tf 2 N]) and N -butyl- N -methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([C 4 mpyr][Tf 2 N]), to investigate whether spectroscopically detected interactions between the ions and AlCl 3 affect these properties. Such electrolyte solutions are of interest for the electrowinning of aluminium. The temperature, composition and molar volume dependences are investigated. Apparent ( V ϕ ,1 ) and partial molar ( V 1 ) volumes for AlCl 3 have been calculated from solution densities. For [C 2 TMEDA][Tf 2 N] solutions, V ϕ ,1 increases with increasing solute concentration; for [C 4 mpyr][Tf 2 N] solutions, it decreases. In pure [C 2 TMEDA][Tf 2 N], the cation diffuses more quickly than the anion, but this changes as the AlCl 3 concentration increases. In the [C 4 mpyr][Tf 2 N] solutions, the intra-diffusion coefficient ratio remains equal to that for the pure ionic liquid and the aluminium species diffuses at approximately the same rate as the anion at each composition. The intra-diffusion coefficients can be fitted to the Ertl-Dullien free volume power law by superposing the iso-concentration curves with concentration dependent, but temperature independent, molar volume offsets. This suggests that they are primarily dependent on the molar volume and secondarily on a colligative thermodynamic factor due to dilution by AlCl 3 . AlCl 3 complexation by [Tf 2 N] - and [C 2 TMEDA] + , confirmed by 27 Al, 15 N and 19 F NMR spectroscopy, seems to play a minor role. Our results indicate that the application of free volume theories might be fruitful in the study of the transport properties of ionic liquid solutions and mixtures.