Photoinduced Reconfiguration of Complex Emulsions Using a Photoresponsive Surfactant

Photoresponsive complex emulsions are prepared in a three-phase system consisting of two oils: hexane (H) and perfluorooctane (F). An aqueous solution of a mixed surfactant of fluorosurfactant, F(CF₂) _x(CH₂CH₂O) _yH (Zonyl FS-300), and a synthesized light-responsive surfactant, 2-(4-(4-butylphenyl)diazenylphenoxy)ethyltrimethylammonium bromide (C₄AZOC₂TAB) was employed as the continuous phase. Complex emulsions with various geometries were prepared by one-step vortex mixing and a temperature-induced phase-separation method. It was noticed that the topology of the complex emulsion was highly dependent on the mass ratio of Zonyl FS-300/C₄AZOC₂TAB. Light microscopy images showed that phase inversion from an H/F/W- to an F/H/W-type double emulsion via a Janus emulsion was achieved by gradually increasing the mass ratio of C₄AZOC₂TAB/Zonyl FS-300. Upon UV/blue light irradiation, the topology of complex emulsions was turned to switch from an F/H/W double emulsion to a Janus emulsion to an entirely inverted H/F/W double emulsion. Dynamic interfacial tension measurements showed that UV irradiation of the interface between an aqueous trans-C₄AZOC₂TAB solution and hexane brings about an increase in the interfacial tension, suggesting the nature of photoinduced morphological changes in complex emulsions. The reconfiguration process of complex emulsions was illustrated by the Marangoni effect based on heterogeneity in the interfacial tension at the complex emulsion surface induced by controlling the molecular conversion of C₄AZOC₂TAB using light irradiation. Finally, we used the complex emulsions structure to form an on-off switch to start and shut off the evaporation of one volatile phase to achieve process monitoring. This could be used to initiate and quench a reaction, which offers a novel idea for achieving switchable and reversible reaction control in multiple-phase reactions.