Advantages Available in the Application of the Semi-Integral Electroanalysis Technique for the Determination of Diffusion Coefficients in the Highly Viscous Ionic Liquid 1-Methyl-3-Octylimidazolium Hexafluorophosphate
By Bentley, Cameron L.; Bond, Alan M.; Hollenkamp, Anthony F.; Mahon, Peter J. & Zhang, Jie
Published in Analytical Chemistry
2013
Abstract
While it is common to determine diffusion coefficients from steady-state voltammetric limiting current values, derived from microelectrode/rotating disk electrode measurements or transient peak currents at macroelectrodes, application of these methods is problematic in highly viscous ionic liquids. This study shows that the semi-integral electroanalysis technique is highly advantageous under these circumstances, and it has allowed the diffusion coefficient of cobaltocenium, [Co(Cp)2]+ (simple redox process), and iodide, I - (complex redox mechanism), to be determined in the highly viscous ionic liquid 1-methyl-3-octylimidazolium hexafluorophosphate (viscosity = 866 cP at 20 °C) from transient voltammograms obtained using a 1.6 mm diameter Pt electrode. In such a viscous medium, a near-steady-state current is not attainable with a 10 μm diameter microdisk electrode or a 3 mm diameter Pt rotating disk electrode, while peak currents at a macrodisk are subject to ohmic drop problems and the analysis is hampered by difficulties in modeling the processes involved in the oxidation of iodide. The diffusion coefficients of [Co(Cp)2]+ and I - were determined to be 9.4 (±0.3) × 10 -9 cm2 s -1 and 7.3 (±0.3) × 10 -9 cm2 s -1, respectively. These results highlight the utility of the semi-integral electroanalysis technique for quantifying the diffusivity of electroactive species in high viscosity media, where the use of steady-state techniques and transient peak currents is often limited.