Quartz Crystal Microbalance Sensitivity Loss During Ionic Liquid Deposition: Insights into Film Structure and Morphology

The understanding of the sensitivity of quartz crystal microbalances (QCMs) to the mass of ionic liquid (IL) films is essential to rationalize their deposition behavior and the limits of reliable mass measurement. Herein, [C 2 C 1 im][OTf] films were vapor‐deposited on Au‐coated QCMs over a wide mass range (0.6–55 µg cm −2 ) and characterized using QCM, scanning electron microscopy (SEM), atomic force microscopy (AFM), and X‐ray photoelectron spectroscopy (XPS). At low coverages, QCM frequency shifts follow a linear trend, consistent with rigid mass coupling, while deviations from linearity and eventual sensor saturation occur at higher mass loadings. SEM and AFM show that the IL initially fills confined surface features, forming isolated patches, and progressively evolves into extended, quasi‐continuous domains. XPS indicates that low‐mass films display surface compositions deviating from stoichiometry, whereas high‐mass films (55 µg cm −2 ) show cation enrichment at the outermost surface. These findings demonstrate that QCM failure is linked to the transition from a confined, interface‐dominated regime to a microdroplet‐type, bulk‐like film, where viscoelastic IL patches decouple from the oscillating crystal. By correlating film morphology, surface composition, and QCM response, this study provides a comprehensive framework to interpret the limitations of QCM mass detection of IL films under vacuum.