Photoswitchable de/adsorption of an azobenzene-derived surfactant on a silica surface

The control of surfactant adsorption/desorption, a fundamental process in colloid and surface chemistry, is of crucial importance for surfactant recycling and pollution-free waste treatment. Using first-principles simulations, we designed a photoswitchable approach to realize separation of photo-responsive surfactant from adsorbate. We chose 4-butyl-(4’-(3-trimethylammoniumpropoxy)phenyl)azobenzene cation and quartz as the model system of surfactant and adsorbate, respectively. Through first-principle calculations, we found that the trans isomer of the surfactant tends to assemble on the silica surface, while the cis isomer tends to be detached from the surface and instead surrounded by water molecules. The chemical origin of the difference arises from the interactions between the surfactant and solvent which depend on the molecular conformational change and associated redistribution of charges before and after the isomerization. Intriguingly, the interaction energy between silica surface and the surfactant does not change significantly with conformational change of the molecule. Meanwhile, appreciable void space of cis conformer owing to steric hindrance disfavor assembling of surfactants on the silica surface, and its significant polarity favors the water environment which prompts its desorption form the surface. The prediction from computations was then validated by experimental results. We expect our proof-of-concept study on phototriggered separation of azobenzene-derived surfactant from silica surface provides an alternative way of achieving stimuli-responsive separation of surfactant from adsorbates.