Scanning electrochemical microscopy .34. Potential dependence of the electron-transfer rate and film formation at the liquid/liquid interface

The potential drop across the interface between two immiscible electrolyte solutions (ITIES). Delta(w)(o) phi, can be quantitatively controlled and varied by changing the ratio of concentrations of the potential-determining ion in the two liquid phases. This approach was used to study the potential dependence of the rate constant for electron transfer (ET) at the ITIES (k(f)) by scanning electrochemical microscopy (SECM) with no external potential bias applied. The Tafel plot obtained for ET between aqueous Ru(CN)(6)(4-) and the oxidized form of zinc porphyrin in benzene was linear with a transfer coefficient, alpha = 0.5, determined from the slope of a plot of ln k(f) vs Delta(w)(o) phi, in agreement with conventional ET theory. The observed change in the ET rate with the interfacial potential drop cannot be attributed to concentration effects and represents the potential dependence of the apparent rate constant. This result is discussed in relation to the interface thickness and structure. The SECM was also used to study solid phase formation at the interface at high concentrations of supporting electrolyte (tetrahexylammonium perchlorate, THAClO(4)) in benzene. The precipitation of the THA(+) and Ru(CN)(6)(4-) compound occurred when its solubility product was exceeded. This process leads to the formation of a thin three-dimensional interfacial layer, which can be unambiguously distinguished from monolayer adsorption. The approach curve analysis yields the composition of such a layer. Its thickness can also be probed.