Light-Triggered Reversible Switching between Microemulsions and Nanoemulsions

Reversible interconversion between microemulsions and nanoemulsions without altering composition or temperature remains a formidable challenge. We report a light-responsive strategy relying on spiropyran-functionalized ionic liquid surfactants to construct microemulsions composed of 1-butanol and water. Visible light irradiation progressively enlarges droplets from about 7.5 nm to over 120 nm, converting the transparent microemulsion into a turbid nanoemulsion. Simply storing the turbid system in the dark for 5 min fully restores the original clear microemulsion, with excellent reversibility. Comprehensive SAXS, DLS, conductivity, and tensiometry reveal that photoisomerization modulates surfactant hydrophilicity, raising the critical aggregation concentration and slightly increasing interfacial tension. These changes, together with a reduction in the packing parameter, are consistent with a transition from a thermodynamically stable microemulsion to a kinetically trapped nanoemulsion, without inducing macroscopic demulsification. This work presents the initial noninvasive, completely reversible, isothermal, and isocompositional photoinduced transition between these two colloidal states, facilitating low-energy, spatiotemporally controllable nanoemulsion fabrication for potential applications in drug delivery and smart materials.