Modeling the Influence of Low Concentrations of Water on the Thermodynamics, Electron Transfer Kinetics, and Diffusivity of the [Ru(CN)6]4–/3–Process in Propylene Carbonate

Addition of low concentrations of water to an aprotic solvent can lead to large changes in the thermodynamics of some electrode processes. In contrast, the impact of water addition on the electrode kinetics of such systems has yet to be established. Herein, direct current and Fourier transformed alternating current voltammetric studies were undertaken at a glassy carbon electrode to investigate changes in the thermodynamics, kinetics, and diffusivity of the [Ru(CN)6]4–/3–process in propylene carbonate (PC, 0.10 M n-Bu4NPF6) that occur on addition of up to 1.87 M water and in fully aqueous 0.10 M KCl electrolyte media. The formal reversible potential (Ef0) for the [Ru(CN)6]4–/3–couple is −0.66 V versus Fc0/+(Fc = ferrocene) in PC and shifts substantially in the positive direction by ∼0.35 V in the presence of 1.87 M water, and it is established to be 1.0 V more positive in fully aqueous media. In comparison, Ef0for the Fc0/+process only changes by ∼20 mV versus a Pt quasi reference electrode in the presence of 1.87 M water, emphasizing the impact of charge and structure on the thermodynamics. Modeling the water-induced shift in Ef0in terms of solvent interaction implies that transfer of approximately four water molecules in the secondary coordination sphere accompanies the oxidation of [Ru(CN)6]4–to [Ru(CN)6]3–. Despite the implied large level of solvent rearrangement, the heterogeneous electron transfer rate constant only increases from 1.81 × 10–2cm s–1in “dry” PC to 3.38 × 10–2cm s–1on addition of 1.87 M water to PC. An increase in diffusivity of [Ru(CN)6]4–was observed but is not evident in the Fc0/+process under the same conditions; hence, it is probably not simply due to the small decrease in viscosity that occurs on addition of low concentrations of water. Some, but not all, aspects of the kinetics are in qualitative agreement with Marcus theory predictions.