Microelectrochemical measurements of electron transfer rates at the interface between two immiscible electrolyte solutions: Potential dependence of the ferro/ferricyanide-7,7,8,8-tetracyanoquinodimethane (TCNQ)/TCNQ˙− system

Abstract

The reduction of 7,7,8,8-tetracyanoquinodimethane (TCNQ) in both 1,2-dichloroethane (DCE) and nitrobenzene (NB), by aqueous ferrocyanide, and the back reaction have been studied by scanning electrochemical microscopy (SECM) and microelectrochemical measurements at expanding droplets (MEMED). The main focus has been to elucidate the effect of galvanic potential at the interface between two immiscible electrolyte solutions (ITIES) on electron transfer (ET) rates, with different electrolyte concentrations in the organic phase. SECM studies show that the ET rate constants for both the forward and back reaction depend strongly on the interfacial potential drop, with an apparent ET coefficient close to 0.5. MEMED demonstrates that TCNQ˙⁻ is confined to DCE, but transfers from NB to water under certain experimental conditions, which could complicate kinetic analysis. The ET kinetics for the water/DCE system have been analysed further using Marcus theory. Close to zero driving force, the rate constants for the forward and back reaction are found to be similar and in good agreement with predictions from Marcus theory with a sharp liquid/liquid interface. The results suggest that Butler–Volmer kinetics describe ET at the ITIES when the driving force is low and the ionic strength in both the aqueous and organic phases is relatively high.