Photoinduced demulsification of emulsions using a photoresponsive gemini surfactant

This Article reports on the influence of light irradiation on the stability of emulsions prepared using a photoresponsive gemini surfactant (C7-azo-C7) having an azobenzene skeleton as a spacer. When mixtures of trans C7-azo-C7 aqueous solution and n-octane are homogenized, stable emulsions are obtained in a specific region of weight fraction and surfactant concentration. Fluorescence microscopy observations using a small amount of fluorescent probes show that the stable emulsions are oil-in-water (O/W)-type. UV irradiation of stable O/W emulsions promotes the cis isomerization of trans C7-azo-C7 and leads to the coalescence of the oil (octane) droplets in the emulsions, that is, demulsification. While the equilibrated interfacial tension (IFT) between aqueous trans C7-azo-C7 solution and octane is almost the same as that between aqueous cis C7-azo-C7 and octane, the occupied area per molecule for C7-azo-C7 at octane/water interface decreases with the cis photoisomerization of trans isomer. Dynamic IFT measurement shows that UV irradiation to the interface between aqueous trans C7-azo-C7 solution and octane brings about an increase in the interfacial tension, indicating that the Gibbs free energy at the interface increases. From these results, the cis isomerization of trans C7-azo-C7 molecules at the O/W interface due to UV irradiation leads to direct contact between the water and octane phases, because of the reduction of molecular area at the interface, and subsequently makes the emulsions demulsified.