A volumetric and intra-diffusion study of solutions of AlCl3 in two ionic liquids – [C2TMEDA][Tf2N] and [C4mpyr][Tf2N]

Abstract

Intra-diffusion coefficients (D_Si) have been measured for the ionic liquid constituent ions and aluminium-containing species in aluminium chloride (AlCl₃) solutions in the ionic liquids 1-(2-dimethyl-aminoethyl)-dimethylethylammonium bis(trifluoromethylsulfonyl)amide ([C₂TMEDA][Tf₂N]) and N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([C₄mpyr][Tf₂N]), to investigate whether spectroscopically detected interactions between the ions and AlCl₃ 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 (V₁) volumes for AlCl₃ have been calculated from solution densities. For [C₂TMEDA][Tf₂N] solutions, V_ϕ,1 increases with increasing solute concentration; for [C₄mpyr][Tf₂N] solutions, it decreases. In pure [C₂TMEDA][Tf₂N], the cation diffuses more quickly than the anion, but this changes as the AlCl₃ concentration increases. In the [C₄mpyr][Tf₂N] 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 AlCl₃. AlCl₃ complexation by [Tf₂N]⁻ and [C₂TMEDA]⁺, confirmed by ²⁷Al, ¹⁵N and ¹⁹F 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.