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 electro lyte solutions (ITIES). Delta(w)(o) phi, can be quantitatively control led and varied by changing the ratio of concentrations of the potentia l-determining ion in the two liquid phases. This approach was used to study the potential dependence of the rate constant for electron trans fer (ET) at the ITIES (k(f)) by scanning electrochemical microscopy (S ECM) with no external potential bias applied. The Tafel plot obtained for ET between aqueous Ru(CN)(6)(4-) and the oxidized form of zinc por phyrin 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 conce ntration effects and represents the potential dependence of the appare nt rate constant. This result is discussed in relation to the interfac e thickness and structure. The SECM was also used to study solid phase formation at the interface at high concentrations of supporting elect rolyte (tetrahexylammonium perchlorate, THAClO(4)) in benzene. The pre cipitation of the THA(+) and Ru(CN)(6)(4-) compound occurred when its solubility product was exceeded. This process leads to the formation o f a thin three-dimensional interfacial layer, which can be unambiguous ly distinguished from monolayer adsorption. The approach curve analysi s yields the composition of such a layer. Its thickness can also be pr obed.