Correcting for solvent replacement effects in quartz crystal microbalance measurements

The quartz crystal microbalance (QCM) is frequently used to measure the adsorption of molecules from a liquid bulk phase onto surfaces. However, determining the true mass adsorbed onto the surface from the raw frequency shift values often involves various correction factors. In this paper, we highlight yet another correction factor that arises from correcting for the bulk, Kanazawa-Gordon-like solvent contribution to the total frequency shift. This correction factor depends on the cross-sectional size of the solute and solvent molecules on the surface, and shows that the apparent mass adsorbed reported by the QCM can significantly differ from the true mass. By performing molecular dynamics simulations of adsorption from a liquid phase, we are able to accurately estimate the molecular sizes of the solvent and solute. Using stearic acid in heptane solutions, and iron oxide surfaces, we quantify the magnitude of the correction factor by comparing the uncorrected and corrected QCM data. This paper emphasizes the importance of accounting for deviations from widely used models in analyzing measured frequency shifts in QCM experiments, to accurately obtain the true adsorbed mass.