Model Emulsions Stabilized with Nonionic Surfactants: Structure and Rheology Across Catastrophic Phase Inversion

The catastrophic phase inversion process of model emulsions (water/Span 80-Tween 80/heptane) from oil-in-water to water-in-oil emulsion was investigated. During this process, the phase inversion of the emulsion was monitored through Fourier transform infrared spectroscopy (FT-IR). In emulsions without NaCl, oil-in-water gel emulsions are formed prior to phase inversion. As the HLB value increases, the oil volume fraction required for phase inversion becomes higher. Polydisperse distribution of the gel emulsion is observed from microscope optical images. The Turbiscan Lab stability analyzer indicates that O/W gel emulsions before the phase inversion has good stability at 50 °C. Rheological measurements reveal that emulsions exhibit non-Newtonian behavior. The viscosity of the gel emulsions increases significantly prior to phase inversion. As the oil volume fraction increases, the storage modulus and loss modulus of the gel emulsion increase to a maximum, at which catastrophic phase inversion occurs. In emulsions with NaCl, there is no oil-in-water gel emulsion formed before phase inversion. The physicochemical properties of the emulsion play a crucial role in whether gel emulsions are produced during catastrophic phase inversion. These gel emulsions have the potential to diversify the applications in crude oil extraction, drug delivery systems, packaging materials, and other fields.