Resonant frequency and dissipation factor isotherms of adsorbed molecules on solid-liquid interfaces probed by quartz...

Changes in the resonant frequency Deltaf and the dissipation factor DeltaD of a quartz crystal microbalance due to adsorption of molecules onto the electrode surface in solutions at different concentrations have been numerically analyzed. It has been found that the contribution from the solution due to the variation of the concentration can mask the overall behaviors of the Deltaf and DeltaD isotherms. However, if the solution is sufficiently dilute or the contribution of the solution can be characterized accurately, the corrected Deltaf and DeltaD , as a function of the solution concentration, will reveal characteristic features of the adsorption processes for the adsorbed films. Specifically, steplike behaviors will be displayed in Deltaf isotherms, which correspond to layer formation and layer-by-layer growth of the films, but the onsets of the steps usually do not coincide with the layer completion. Oscillatory behaviors will be revealed in DeltaD isotherms if the adsorbed molecular layer is soft at low concentration and becomes rigid near layer completion.