Emulsions are metastable systems where one liquid (dispersoid) is dispersed as particles in another liquid (dispersion medium), and have been used in many fields, including cosmetics and paints. Many research has focused on the enhancement of emulsion stability. On the other hand, Demulsification, which is a phase separation phenomenon of stable emulsions, also plays an important role in industry. For example, aqueous waste fluids containing hazardous substances can be mixed with an oil to obtain an emulsion, and then the emulsion is demulsified to extract hazardous substances into the oil phase. This process is called “emulsion liquid membrane extraction”. Demulsification has been so far performed by adding chemicals or by physical techniques such as the application of high electrical fields, or mechanical external forces and the variation of temperature. In this work, we will focus on the control over the stability of emulsions by an external stimulus, which is light. As a photo-responsive surfactant, a cationic surfactant having an azobenzene group (C4AzoTAB) has been used. When aqueous mixtures of trans-C4AzoTAB with SDS and n-octane are homogenized, stable oil-in-water (O/W) emulsions are obtained in specific regions of the mole fraction of SDS and surfactant concentration. UV light irradiation to stable O/W emulsions promotes the cis isomerization of trans-C4AzoTAB and leads to the demulsification. The difference in surface tension between trans- and cis-C4AzoTAB aqueous solutions and the measurement of diameters of emulsion droplets indicate that the Ostwald ripening is caused under UV light irradiation and subsequently makes stable emulsions demulsified.

Active demulsification of stable emulsions prepared from mixtures of azobenzene surfactant/SDS using light

256th National Meeting (2018)

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