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

Area of Science:

• Surface Science
• Materials Science
• Physical Chemistry

Background:

• Photoswitchable surfactants form adsorbed films with critical roles in interfacial mechanics.
• Understanding the internal organization of these films is key to controlling their functionality.

Purpose of the Study:

• To resolve the depth-dependent structure of azobenzene-based surfactant layers adsorbed at interfaces.
• To investigate the concentration-dependent reorganization of these adsorbed layers.
• To demonstrate the capability of QCM-D in resolving internal structural heterogeneity.

Main Methods:

• Quartz crystal microbalance with dissipation monitoring (QCM-D) was employed.
• Analysis of frequency and dissipation responses across multiple overtones.
• Adsorption at borosilicate glass-water interfaces was studied.

Main Results:

• Below the critical micelle concentration (CMC), compact, rigid mono- to bilayer structures were observed.
• Above the CMC, an extended, water-rich overlayer formed, contributing significantly to dissipation.
• Distinct contributions from rigidly coupled and weakly bound film regions were distinguished.

Conclusions:

• QCM-D can resolve internal structural heterogeneity in responsive interfacial systems.
• Azobenzene surfactant films exhibit pronounced concentration-dependent reorganization.
• Quantitative insight into the vertical viscoelastic profile of these films was obtained.