IMPEDANCE SPECTROSCOPY OF SELF-ASSEMBLED MONOLAYERS ON AU(111) - SODIUM FERROCYANIDE CHARGE-TRANSFER AT MODIFIED ELECTRODES

The Faradaic impedance of a Au(lll) electrode modified by three differ ent self-assembled monolayers has been studied over a wide frequency r ange in aqueous NaClO4 solutions in the presence of 1 mM [Fe(CN)(6)](4 -/3-). The impedance characteristics of the bare electrode are compare d with those of the same electrode modified with decanethiol, omega-hy droxydecanethiol, and 4'-hydroxy-4-mercaptobiphenyl. The 4'-hydroxy-4- mercaptobiphenyl molecule is susceptible to pi-pi interactions and is of special interest as a mediator of electron transfer. In the case of the decanethiol and omega-hydroxydecanethiol systems the interfacial impedance can be modelled by the Randles circuit. However, for the 4'- hydroxy-4-mercaptobiphenyl system, the impedance behavior is more comp lex with a resistance in the monolayer parallel to and distinguishable from the expected Faradaic impedance. This behavior is quite differen t from that discussed to date in the literature and assumed in the int erpretation of voltammetric data for modified electrodes. The paramete rs for these systems are reported here as a function of the type of el ectrode modification. The consequences of the present observations are discussed with respect to the phenomena observed.