Surfactant-free oil-in-water nanoemulsions with nanopore membrane and ultrasound

Phase separation of heterogeneous liquids can occur in the presence of sufficient shear force between a solid interface and a liquid. We hypothesize that a continuous flow of oil through nanopores into water in combination with a given ultrasound energy can generate stable free standing oil emulsions in water in the absence of surfactants. An oil solution was pumped through a membrane with 100 nm pores at a controlled flow rate and introduced into an aqueous solution while being subjected to sonication. The resulting emulsion samples were collected at specific time intervals and characterized by dynamic light scattering and nanoparticle tracking analysis techniques. We evaluated the storage stability of the emulsions with and without surfactant at room temperature or 4 °C storage. Introducing oil into an aqueous phase using our Nanopore system in conjunction with ultrasound gave rise to surfactant-free milky oil nanodroplet solutions. Dynamic light scattering and nanoparticle tracking analysis showed that the produced surfactant-free oil nanodroplets were monodispersed in water at sizes less than 200 nm. Control experiments without the use of nanopores did not result in phase mixing. Nanoemulsions were also generated in a surfactant containing solution resulting in a similar size range of nanodroplets. This work demonstrates that our Nanopore system can generate stable nanoemulsions in the absence of a surfactant, persisting in phase separation for several days up to two weeks at room temperature and 4 °C storage. This work suggests that nanopores are effective in producing surfactant-free nanoemulsions, which offer a wide range of valuable applications varying from drug delivery and food engineering, to paints and pesticide development.