Scanning electrochemical microscopy. 43. Investigation of oxalate oxidation and electrogenerated chemiluminescence across the liquid-liquid interface

To treat the problem of oxalate oxidation in an aqueous phase by a Ru(III) species (mediator) generated in an immiscible nonaqueous phase (benzonitril e) at a scanning electrochemical microscope (SECM tip), the theory of the f eedback mode of the SECM is extended to include both finite heterogeneous e lectron transfer (ET) kinetics at the substrate and homogeneous decompositi on of the mediator. As for the classical electrochemical (EC scheme, a zone diagram is constructed showing pure ET heterogeneous kinetic control, pure homogeneous kinetic (C) control (insulating behavior), and mixed EC kineti c control. The model allows interpretation of the anomalous approach curves obtained for oxalate oxidation by a ruthenium(III) coordination complex at the benzonitrile (BN)-water interface and allows calculation of the rate c onstant for the ET at the liquid-liquid interface. Attempts are made to rel ate these rates to the homogeneous rate constant in terms of Marcus theory. The second ET (oxidation of CO2. (-)) at the liquid-liquid interface gener ates an emitting excited state of the ruthenium species. The electrogenerat ed chemiluminescence process is related to the current crossing the interfa ce.