On the Adsorption Kinetics of Octanoic Acid at the Mercury/Electrolyte Interface

Abstract

The adsorption kinetics of octanoic acid at the mercury/electrolyte interface was investigated by means of capacity−time transients. For low concentrations of octanoic acid and for the short-term region, the transients can be theoretically well described with a diffusion-controlled adsorption process according to the Delahay−Trachtenberg law. The long-term transients for higher concentrations of octanoic acid exhibit an increase of the capacity with time, which can be modeled with an adsorption-controlled replacement of a condensed adsorption state by a hemimicelle adsorption state. Criteria are derived for which an increase of the double-layer capacity with time can be expected.

On Packing Constraints in the Formation of Surface Micelles of Amphiphiles at the Metal/Electrolyte Interface—The Necessity of Introducing a Generalized Packing Parameter

“…Recently, investigations with atomic force microscopy (1-9), STM (10), spectroelectrochemical methods (11), grazing incident X-ray diffraction pattern (12), and electrochemical methods (13)(14)(15) have shown that surfactant molecules adsorbed at the solid-solution or air-water interface form aggregates and that surface aggregate phase transitions occur.…”

Effect of the Supporting Electrolyte on the Adsorption of Octanoic Acid at the Mercury/Electrolyte Interface

“…The surface concentration of such hemimicelles should be proportional to that of octanoic acid as well as that of the anion, so leading to the asymmetry in the adsorption behavior observed and to the growing tendency to form such hemimicelles if the specific adsorption of the anion increases. The increase in the capacity is then only a logical consequence of the hemimicelle formation as we already stated in our last paper on octanoic acid (24).…”

“…In investigations of the kinetics of the adsorption of neutral organic molecules it is customary to start from a potential where there is no or negligible adsorption. Thus, no molecules of octanoic acid exist in the double layer (24).…”

“…The potential −1.3 V is also a potential where no adsorption of octanoic acid occurs (24) and mercury is negatively charged. This capacitance value at the initial potential is identical for all the electrolyte systems studied.…”

“…In our previous paper (24) we studied the adsorption of purified solutions of octanoic acid at the mercury/solution interface and at the air/solution interface, in the presence of KCl and HCl, and we reported on both capacitance and interfacial tension measurements. The adsorption at the mercury/solution interface was studied using phase-sensitive ac voltammetry, by measuring the change of the capacitance with time.…”