Probing the Depth-Resolved Structure of Adsorbed Azobenzene Surfactant Films by QCM-D.

Adsorbed films of photoswitchable surfactants exhibit complex internal organization that critically determines their interfacial mechanics and functionality. Here, we use quartz crystal microbalance with dissipation monitoring (QCM-D) to resolve the depth-dependent structure of azobenzene-based surfactant layers adsorbed at borosilicate glass-water interfaces. Analysis of frequency and dissipation responses across multiple overtones allows us to distinguish contributions from film regions that are rigidly coupled to the oscillatory shear field from those that remain weakly bound and are highly hydrated. We observed a pronounced concentration-dependent reorganization of the adsorbed layer. Below the critical micelle concentration (CMC), adsorption results in compact, mechanically rigid mono- to bilayer structures formed by tightly packed surfactant molecules. Above the CMC, excess surfactant accumulates in an extended, water-rich overlayer that contributes strongly to dissipation at low overtones but only marginally to the shear-coupled mechanically active film thickness. These results provide quantitative insight into the vertical viscoelastic profile of azobenzene surfactant films and demonstrate the capability of QCM-D to resolve the internal structural heterogeneity in responsive interfacial systems.