Stability, Photoinduced Destabilization, and Photoswitchability of Azo-CTAB-Based Emulsions

Stimuli-induced interfacial destabilization of oil-water emulsions is an attractive approach for emulsion processing, as it typically requires less energy input than conventional bulk manipulation methods. In this work, we demonstrate the photoinduced destabilization and reconstruction of <i>n</i>-decane-in-water emulsions that were rendered highly stable (>45 days) with the help of an azobenzene-containing cetyltrimethylammonium bromide (Azo-CTAB) surfactant. Phase separation was induced easily in Azo-CTAB stabilized emulsions by the illumination of ultraviolet (UV) light, which activates photoisomerization of the Azo-CTAB molecule and would, in turn, affect its interfacial adsorption. A simple analysis based on the size of the dispersed phase droplets shows that phase separation occurs by coalescence of the droplets, following second-order kinetics. The coalescence rate constant is higher by about 5 orders of magnitude when demulsification is induced by UV exposure, compared to those in samples stored under ambient conditions. The degree of phase separation can be controlled by tuning various factors, such as the power intensity of UV light, the addition of an electrolyte, and the addition of a mutually soluble solute. The coalescence rate is faster by about 4-5 times in the presence of an added electrolyte (NaCl). When UV light of low intensity (∼1 mW cm^-2) is used, complete demulsification can still be achieved if a small quantity of a volatile liquid such as ethanol is added to the emulsion. More interestingly, we show that the original emulsion can be reconstructed by subsequently illuminating the phase-separated mixture with visible light, thus demonstrating photoswitchability in Azo-CTAB stabilized emulsions. The reversible photoswitchability persists for multiple (5-7) cycles, without any appreciable change in the appearance and size of the dispersed droplets. These results have great significance in applications that require repeated and facile stabilization and destabilization of emulsions, such as oil recovery and wastewater treatment.