Photo-Induced Demulsification

Demulsification is an aggressive phase-separation phenomenon of stable emulsions. Several studies have examined demulsification by chemical reactions using external stimuli, such as temperature variation, pH variation, and light irradiation. This study focused on the interfacial and emulsification properties of light-responsive surfactants because the use of light as an external stimulus is relatively simple. This chapter presents an overview of the photo-induced demulsification of emulsions prepared using light-responsive surfactants containing an azobenzene skeleton. Stable emulsions are obtained when mixtures of n-octane and aqueous solutions of azobenzene-containing surfactants are homogenized. Ultraviolet irradiation of the stable emulsions induces complete phase separation of the octane and aqueous surfactant solution phases. The phase separation, i.e., demulsification, results from a change in the interfacial properties, such as occupied areas per molecule and interfacial tension, of the light-responsive surfactants at the oil/water interface.