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 Conti nuum hydrodynamics and no-slip boundary conditions apply to electroche mical quartz microbalances of the thickness shear mode type with gold in aqueous contact. Electrochemical phase-stabilized quartz microbalan ce measurements can give insight into ion adsorption, solvation, hydro gen bonding, and water clustering at charged surfaces. Species in the outer and inner Helmholtz layer can be treated as rigidly coupled mass es. Frequency changes on polycrystalline gold electrodes in alkaline a queous 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 an d hydroxide, are stripped of nearly half of their original solvation s hell, and function as counter charge carriers analogously to completel y solvated anions in the outer Helmholtz plane. Their saturation cover age is limited by lateral electrostatic repulsion and steric crowding by their solvation shells. Specifically adsorbed anions which form neu tral ion pairs with alkali metal cations practically do not exhibit el ectrostatic repulsion and solvation shell crowding. They contribute mu ch stronger to the double layer loading than specifically adsorbed par tially hydrated ions and non-specifically attracted species. In the ox ide monolayer potential region, electrosorbed hydroxyl functions are d eprotonated, and become neutralized by alkali metal ions at high pH. I n a first order approximation, viscous fluid coupling and roughness ch anges do not have to be invoked to explain the observed frequency data .