Reversibly Adsorbed Monolayers on Microelectrodes: Effect of Potential on the Adsorption Thermodynamics

Abstract

Monolayers of 2-hydroxyanthraquinone (2OH-AQ) have been formed by reversible adsorption onto mercury microelectrodes. Cyclic voltammetry of these monolayers in contact with HClO4 as supporting electrolyte is nearly ideal and is consistent with reduction proceeding by a two-electron, two-proton transfer mechanism. Cyclic voltammetry reveals that the dependence of the surface coverage Γ on the bulk concentration of 2OH-AQ is accurately described by the Langmuir isotherm over the concentration range from 2 × 10-8 to 2 × 10-6 M. A limiting surface coverage Γs of (1.0 ± 0.08) × 10-10 mol cm-2 and an adsorption coefficient β of (6.3 ± 0.6) × 106 M-1 are observed. Since adsorption is reversible in this system, the effect of changing the potential at which the monolayer is formed on the surface coverage, or the adsorption thermodynamics, cannot be investigated by ex situ measurement of Γ in a blank electrolyte solution. To address this issue, microsecond time scale chronoamperometry has been used to time-resolve ...