Microelectrochemical studies of charge transfer at the interface between two immiscible electrolyte solutions: Electron transfer from decamethyl ferrocene to aqueous oxidants

Experimental studies of electron transfer (ET) reactions at the interface between two immiscible electrolyte solutions (ITIES) were carried out with either Fe(CN)₆ ^3-, Ru(CN)₆ ^3- or IrCl₆ ^2- as oxidants in water and decamethyl ferrocene (DMFc) in 1,2-dichloroethane (DCE), using both scanning electrochemical microscopy (SECM) and microelectrochemical measurements at expanding droplets (MEMED). Either tetrabutylammonium cation (TBA⁺) or ClO₄ were employed in each phase to control the interfacial potential drop. SECM double potential step chronoamperometry was used to show that DMFc⁺, generated in the ET process, does not cross the interface in the potential range of interest. The ET rate constants were found to depend strongly on the interfacial potential drop, with an apparent measured ET coefficient of 0.38 when TBA⁺ was used and the aqueous ionic strength was ca. 0.1 mol dm^-3. However, the potential dependence of the ET rate was complicated when ClO₄ ^- was used to change the interfacial potential drop. Although the rate constant increased when the driving force was increased by changing the aqueous oxidant, the rate constant decreased for a particular oxidant when the potential of the organic phase was made more negative relative to the aqueous phase, by increasing the concentration of ClO₄ ^- in the aqueous phase. A similar effect was observed with Fe(CN)₆ ^3- as the aqueous oxidant and DMFc as the electron donor in a nitrobenzene phase. In contrast, the rate constant for ET was found to be apparently insensitive to the ClO₄ concentration in the aqueous phase (and hence potential drop) when the aqueous electrolyte concentration was increased to the salting out levels employed in earlier studies with externally-polarized ITIES. Possible reasons for the behaviour observed and the implications for further studies are discussed.