Photocontrol of Foam Stability Using Photoresponsive Cationic Surfactants

Photoresponsive surfactants provide an effective approach for controlling interfacial phenomena with light. In this study, we investigated the light-driven modulation of foam stability using a series of photoresponsive cationic surfactants containing an azobenzene moiety, CmAzoCnTAB (m/n = 2/2, 2/4, 2/6, 4/2, and 6/2). Trans-cis photoisomerization under UV irradiation was confirmed by UV-vis and 1H NMR analyses. Surface tension measurements revealed that the critical micelle concentration (CMC) increased upon trans-cis photoisomerization and decreased with increasing alkyl chain length. Foam stability was evaluated both above CMC and at concentrations corresponding to the maximum surface tension difference between the trans and cis isomers. Foam stability depended strongly on the terminal alkyl chain length of the hydrophobic tail. Notably, trans-C4AzoC2TAB formed highly stable foams, whereas UV irradiation markedly decreased foam stability via trans-cis photoisomerization. Localized UV irradiation further demonstrated that foam collapse occurred selectively in the irradiated region, enabling spatially controlled defoaming. Based on these results, photoisomerization may alter the interfacial adsorption state and generate interfacial nonequilibrium, which may contribute to foam-film destabilization and subsequent foam collapse. These findings demonstrate that photoresponsive surfactants with appropriately designed alkyl chain lengths enable effective, localized, light-driven control of foam stability.