Factors affecting the stability of water-oil-water emulsion films

Water-oil-water (oily) liquid films appear between the droplets in water-in-oil emulsions. Here we report results from systematic experiments, aimed to clarify how several important factors affect the stability and drainage of such oily films, in the presence of the nonionic oil-soluble surfactant Span 80. These results reveal that: (1) At Span 80 concentrations around the CMC, the film lifetime coincides with the duration of film drainage. When the oily films reach a critical thickness of around 40 nm, unstable thinner black spots of thickness < 10 nm are formed due to attractive van der Waals forces, and the films rupture within seconds; (2) At intermediate concentrations of 3-30 times the CMC, the films are again unstable, but their lifetime increases significantly with Span 80 concentration, due to a longer drainage time and higher stability of the formed thin black spots. The increased drainage time is explained with a faster surfactant adsorption which leads to lower interfacial tension and, hence, to lower capillary pressure driving the film thinning. (3) At even higher Span 80 concentrations (around and above 100 CMC) the drainage time approaches a constant value, approximate to 600 s. The films live much longer which is due mostly to the increased stability of the thin black films, formed after the black spot expansion. The film drainage time at intermediate and high Span concentrations agrees very well with the theoretical predictions. Surprisingly, the addition of 500 mM NaCl into the aqueous phase leads to formation of non-equilibrium thick oil films, with drainage time > 1 h. This effect is explained with a mass transfer of molecules between the aqueous and the oil phases which keeps the oil films out of equilibrium for a long period. These results can be used to explain and control the formation and stability of water-in-oil emulsions, in the presence of nonionic oil-soluble surfactants. (C) 2017 Elsevier B.V. All rights reserved.