Voltammetric currents in room temperature ionic liquids can reflect solutes other than the electroactive species and are influenced by carbon dioxide.
The effects of such solutes such as halides and water on the physical properties of room temperature ionic liquids (RTILs) have been extensively studied. This work examines the effect of the solute carbon dioxide on the RTIL 1-ethyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide ([C2mim][NTf2]) and its influence on the electrochemical characterization of the important redox couple ferrocene/ferrocenium (Fc/Fc+). The system was studied using cyclic voltammetry, chronoamperometry, and electron spin resonance (ESR) spectroscopy. Addition of 100% CO2 to a solution of Fc in [C2mim][NTf2] resulted in a substantial increase in both the limiting oxidative current and diffusion coefficient of Fc. Arrhenius plots of Fc diffusion coefficients in the pure and CO2-saturated ionic liquid revealed a decrease in activation energy of translational diffusion from 29.0 (+/- 0.5) kJ mol(-1) to 14.7 (+/- 1.6) kJ mol(-1), suggesting a reduction in the viscosity of the ionic liquid with addition of CO2. ESR spectroscopy was then used to calculate the rotational correlation coefficients of a probe molecule, 2,2,6,6-tetramethyl-1-piperinyloxyl (TEMPO), to add supporting evidence to this hypothesis. Arrhenius plots of rotational correlation coefficients in the pure and CO2-saturated ionic liquid resulted in a similar drop in activation energy from 28.7 (+/- 2.1) kJ mol(-1) to 18.2 (+/- 5.6) kJ mol(-1). The effect of this solute on the ionic liquid [C2mim][NTf2] and on the electrochemical measurements of the Fc/Fc+ couple emphasizes the necessity of fastidious sample preparation, as it is clear that the voltammetric currents of the electroactive species under study are influenced by the presence of CO2 in solution. The voltammetric response of the electroactive species in RTILs cannot be assumed to be independent of other solutes.
Barrosse-Antle
Laura E
LE
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom.
Hardacre
Christopher
C
Compton
Richard G
RG