IN-SITU-MONITORING OF ELECTROCHEMICAL DOUBLE-LAYER STRUCTURE CHANGES AT GOLD WITH A PHASE-CONTROLLED QUARTZ MICROBALANCE

Continuum hydrodynamics and no-slip boundary conditions apply to Continuum hydrodynamics and no-slip boundary conditions apply to electrochemical quartz microbalances of the thickness shear mode type with gold in aqueous contact. Electrochemical phase-stabilized quartz microbalance measurements can give insight into ion adsorption, solvation, hydrogen bonding, and water clustering at charged surfaces. Species in the outer and inner Helmholtz layer can be treated as rigidly coupled masses. Frequency changes on polycrystalline gold electrodes in alkaline aqueous contact, and in the potential range from the hydrogen evolution up to the bulk oxide formation, are primarily caused by ion solvation and ion pair formation. Specifically adsorbed anions, like sulfate and hydroxide, are stripped of nearly half of their original solvation shell, and function as counter charge carriers analogously to completely solvated anions in the outer Helmholtz plane. Their saturation coverage is limited by lateral electrostatic repulsion and steric crowding by their solvation shells. Specifically adsorbed anions which form neutral ion pairs with alkali metal cations practically do not exhibit electrostatic repulsion and solvation shell crowding. They contribute much stronger to the double layer loading than specifically adsorbed partially hydrated ions and non-specifically attracted species. In the oxide monolayer potential region, electrosorbed hydroxyl functions are deprotonated, and become neutralized by alkali metal ions at high pH. In a first order approximation, viscous fluid coupling and roughness changes do not have to be invoked to explain the observed frequency data.