Reversible Adsorption and Interfacial Photoisomerization of Azobenzene Surfactants Studied by QCM

Photoresponsive surfactants offer a versatile approach for remotely controlling interfacial properties through light-triggered isomerization. Among them, azobenzene-based surfactants are particularly attractive due to their structural reversibility and stability under repeated irradiation. In this study, we investigate the dynamic adsorption and desorption behavior of the azobenzene-containing surfactant AzoC₆ at a glass-water interface under controlled UV and blue-light illumination. Using quartz crystal microbalance (QCM) measurements, we show that the interfacial mass change is governed by the isomeric composition in the bulk solution: the <i>trans</i> isomer exhibits strong adsorption, while the <i>cis</i> isomer is significantly less surface-active. We further quantify the photoisomerization kinetics at the interface, revealing that the isomerization rate constant decreases with a lower <i>trans</i> isomer concentration due to a transition from a diffuse multilayer to a confined double-layer structure. At higher concentrations, the rapid exchange between <i>trans</i> and <i>cis</i> isomers sustains dynamic interfacial rearrangements, facilitating the formation of spatial isomer gradients. These gradients generate light-driven diffusio-osmotic flows, with a time evolution that reflects the interfacial photoresponse. Our findings provide mechanistic insight into light-induced interfacial processes and highlight the potential of azobenzene surfactants for designing stimuli-responsive systems and soft materials with remote, reversible control.