# Reversible Adsorption and Interfacial Photoisomerization of Azobenzene Surfactants Studied by QCM

## TL;DR

This study explores how light can control the behavior of azobenzene surfactants at liquid interfaces, revealing how their structure changes under UV and blue light.

## Contribution

The study provides new insights into the dynamic adsorption and photoisomerization mechanisms of azobenzene surfactants at interfaces.

## Key findings

- Trans isomers of AzoC6 show strong adsorption at the interface, while cis isomers are less surface-active.
- Photoisomerization kinetics at the interface depend on trans isomer concentration and interfacial structure.
- Light-driven isomer gradients generate diffusio-osmotic flows, demonstrating remote control of interfacial processes.

## Abstract

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 trans isomer
exhibits strong adsorption, while the cis 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 trans isomer concentration
due to a transition from a diffuse multilayer to a confined double-layer
structure. At higher concentrations, the rapid exchange between trans and cis 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.