LONG-RANGE ELECTRON-TRANSFER THROUGH A LIPID MONOLAYER AT THE LIQUID LIQUID INTERFACE/

The electron transfer (ET) rate at the interface between two immiscibl e electrolyte solutions was probed as a function of the driving force and distance between redox centers by scanning electrochemical microsc opy. The adsorption of phospholipids at the interface resulted in a de crease in the rate of interfacial ET between aqueous redox species and the oxidized form of zinc porphyrin in benzene. The fraction of the i nterfacial area covered with lipid (theta) was evaluated from the meas ured heterogeneous rate constants (k(f)). The dependence of theta vs l ipid concentration in benzene fit a Langmuir isotherm. For complete mo nolayers of phospholipids, k(f) was a function of the number of methyl ene groups in a hydrocarbon chain. The driving force dependencies of i nterfacial ET rates (Tafel plots) were measured for several aqueous re dox couples. They were linear; with a transfer coefficient of alpha si milar or equal to 0.5 when the driving force for ET (Delta G degrees) was not too high, in agreement with Marcus theory, and leveled off to the diffusion-controlled rate at larger overpotentials. For even highe r Delta G degrees and for the first time for heterogeneous ET at a pol arizable interface, inverted region behavior was observed.