Electrokinetics of the amphifunctional metal/electrolyte solution interface in the presence of a redox couple.
Double layers (DL) at amphifunctionally electrified interfaces, such as that of an oxidized metal in an aqueous electrolyte solution, arise from coupling between ionic and electronic surface-charging processes. The electronic component enters the double-layer formation in the well-known situation where a potential is externally applied. In that case, the DL is fully or partly polarized depending on the possibility of interfacial electron transfer, that is, a faradaic process. This paper reports on the conjunction of the chemical/electrochemical processes at the interface in the case where the solution contains a redox-active couple. This makes it possible to polarize/depolarize a DL without invoking any external circuit. Streaming potential data obtained for the gold/(Fe(CN)6(3-)/Fe(CN)6(4-), KNO3) electrolyte interface are analyzed in terms of a recently developed theory which takes into account reversible bipolar faradaic depolarization, the inherent nonlinearity of the lateral field, and the effects of flow on the rate of the faradaic reactions. It appears that the theory largely overestimates the bipolar currents, leading to physically unrealistic zeta-potentials. A careful analysis of monopolar voltammetric data reveals quasi-reversible behavior of the redox couple under the typical convective conditions and electrolyte compositions met in electrokinetic experiments. Inclusion of reduced reversibility (the extent of which is position-dependent under the streaming-potential measurement conditions) leads to a consistent set of zeta-potentials which compare well to the values for the background electrolyte.
Duval
Jérôme F L
JF
Department of Physical Chemistry and Colloid Science, Wageningen University, Dreijenplein 6, 6703 HB Wageningen, The Netherlands. jerome.duval@wur.nl