Photoresponsive aqueous foams with controllable stability from nonionic azobenzene surfactants in multiple-component systems

The controllability of foam stability is a vital feature that allows for practical applications of foam systems. Light, as an external stimulus, offers unique opportunities to tune the foam stability in a non-invasive manner with high spatiotemporal precision. However, most of the reported photoresposive foams were generated from ionic type surfactants, limiting their applications in industrial complex systems with multiple components. Herein, we design and synthesize a series of nonionic azobenzene surfactants with different polyoxyethylene glycol (EO) chain lengths (BEO-n-Azo, n, referring to the EO chain length, is 14, 19 and 23, respectively) to prepare photoresponsive foams. Detailed insights into the effects of EO chain length on photoisomerization properties, surface tension, as well as foamability and controllable stability of photoresponsive foams are presented. The results demonstrate that photoresposive foams are generated not only from single-component solutions of BEO-n-Azo, but also from multiple-component complex systems doped with BEO-n-Azo, providing a promising strategy to broaden applications of photoresponsive foams in industrial processes.