RESPONSE OF THE ELECTROCHEMICAL QUARTZ-CRYSTAL MICROBALANCE FOR GOLD ELECTRODES IN THE DOUBLE-LAYER REGION

The response of the electrochemical quartz crystal microbalance (EQCM) was studied in 0.1 M HQlO(4), KNO3, and ROH. The gold coating on the crystal served as the working electrode, and the frequency was determi ned as a function potential in the double-layer region. Frequency shif ts up to -5 Hz were observed, even though the ions of the electrolytes employed are not specifically adsorbed and there is no faradaic react ion which could lead to the formation of adsorbed species through char ge-transfer. The added weight which would cause a similar shift in fre quency in our experimental setup is ca. 60 ng/cm(2), which is equivale nt to about a monolayer of chlorine atoms adsorbed on the surface. Thu s, elucidation of the origin of this effect is essential for the prope r use of the EQCM in the submonolayer region. The effects observed are due to the surface excess of ions in the diffuse double layer. A mode l was developed in which the liquid in a thin layer near the surface h as a higher viscosity than in the bulk, because of the high concentrat ion of ions and the high electrical field in this region. The value of this viscosity can neither be calculated independently nor measured e xperimentally, since the liquid in this region is charged (electroneut rality is maintained across the interface but not on each side separat ely). Using the viscosity of the film as a parameter we were able mode l the experimental behavior.