Smart Air–Water Interfaces with Arylazopyrazole Surfactants and Their Role in Photoresponsive Aqueous Foam

[Image: see text] A new light-switchable azo-surfactant arylazopyrazole tetraethylene glycol carboxylic acid (AAP-E(4)) was used as a molecular building block to functionalize macroscopic foams. AAP-E(4) was studied in the bulk solution with UV/vis spectroscopy and at the interface with sum-frequency generation (SFG) as well as tensiometry. Additional foaming experiments were performed with a dynamic foam analyzer to study the role of AAP-E(4) surfactants at the ubiquitous air–water interface as well as within macroscopic foam. In the bulk, it is possible to switch the AAP-E(4) surfactant reversibly from trans to cis configurations and vice versa using 380 nm UV and 520 nm green light, respectively. At the interface, we demonstrate the excellent switching ability of AAP-E(4) surfactants and a substantial modification of the surface tension. In addition, we show that the response of the interface is strongly influenced by lateral electrostatic interactions, which can be tuned by the charging state of AAP-E(4). Consequently, the electrostatic disjoining pressure and thus the foam stability are highly dependent on the bulk pH and the charging state of the interface. For that reason, we have studied both the surface net charge (SFG) and the surface excess (tensiometry) as important parameters that determine foam stability in this system and show that neutral pH conditions lead to the optimal compromise between switching ability, surface excess, and surface charging. Measurements on the foam stability demonstrated that foams under irradiation with green light are more stable than foams irradiated with UV light.