Barrosse-Antle Laura E, Hardacre Christopher, Compton Richard G
Department of Chemistry, Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom.
J Phys Chem B. 2009 Mar 5;113(9):2805-9. doi: 10.1021/jp810926u.
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.
卤化物和水等溶质对室温离子液体（RTILs）物理性质的影响已得到广泛研究。本工作考察了溶质二氧化碳对RTIL 1-乙基-3-甲基咪唑双（三氟甲烷磺酰）亚胺（[C2mim][NTf2]）的影响及其对重要氧化还原对二茂铁/二茂铁鎓（Fc/Fc+）电化学表征的影响。使用循环伏安法、计时电流法和电子自旋共振（ESR）光谱对该体系进行了研究。向[C2mim][NTf2]中的Fc溶液中加入100%的CO2，导致Fc的极限氧化电流和扩散系数均大幅增加。在纯离子液体和CO2饱和离子液体中Fc扩散系数的阿伦尼乌斯图显示，平移扩散的活化能从29.0（±0.5）kJ mol(-1)降至14.7（±1.6）kJ mol(-1)，这表明加入CO2后离子液体的粘度降低。然后使用ESR光谱计算探针分子2,2,6,6-四甲基-1-哌啶氧基（TEMPO）的旋转相关系数，为该假设提供支持证据。在纯离子液体和CO2饱和离子液体中旋转相关系数的阿伦尼乌斯图导致活化能类似地从28.7（±2.1）kJ mol(-1)降至18.2（±5.6）kJ mol(-1)。这种溶质对离子液体[C2mim][NTf2]以及对Fc/Fc+对的电化学测量的影响强调了严格样品制备的必要性，因为很明显，所研究的电活性物种的伏安电流受溶液中CO2存在的影响。不能认为RTILs中电活性物种的伏安响应与其他溶质无关。
