Light-switchable emulsion control and phase transfer enabled by a morpholine-functionalised azobenzene surfactant

Surfactants are widely used to stabilise emulsions across chemical, pharmaceutical, and environmental processes, yet conventional systems often lack controllability once formed. Smart surfactants capable of reversible emulsification and demulsification therefore offer a promising route toward adaptive formulations, especially when activated by non-invasive light stimuli. In this study, a morpholine-functionalised, photoswitchable azobenzene surfactant (MBAB) was synthesised to enable controllable reversible emulsion through trans – cis photoisomerisation. Using MBAB, a stable, reversible oil-in-water emulsion (MBAB/CTAB/n-hexane/water) was constructed, exhibiting repeatable demulsification and re-emulsification under alternating ultraviolet and visible light irradiation. Kinetically, the hydrophilicity of morpholine ring enhances interfacial hydration, resulting in robuster interfacial membrane and slower photoresponsive behaviour compared with linear alkyl analogues. Beyond demulsification and re-emulsification function, a reversible phase-transfer emulsion system (MBAB/n-octanol/water) was also established, in which UV-induced trans – cis isomerisation increases MBAB’s polarity, driving its migration from the n-octanol phase into water, with visible light restoring the original phase distribution. Both systems display excellent cycling stability, maintaining performance over at least ten switching cycles without significant loss of efficiency. Overall, this work provides an effective strategy for designing reversible photoresponsive surfactants and light-switchable emulsions, offering new opportunities for studying stimulus-responsive interfacial assembly and enabling intelligent separation and controlled-release technologies.