Since the 1980s there has been a trend towards surfactant-free emulsions driven by products for sensitive skin and for improved sunscreens. In recent times there has been a flurry of research, development and intellectual property activity in the field of particle-stabilized emulsions. Tailored nanoparticles are desired for these applications. The purpose of this article is to briefly review the basic principles of the particle stabilization of emulsions.
Pickering reported the first recorded scientific study of particle-stabilized emulsions in 1907 when he stabilized water and paraffin with basic copper and iron sulfates, in which the particles were precipitated in situ. He noted, “Many other precipitated substances act as emulsifiers but this property is destroyed as soon as they have been dried or have by any other means been deprived of their finegrained structure.” These emulsions are now known as Pickering emulsions. In Pickering emulsions, small particles position themselves at the oil-water interface and form a mechanical barrier to coalescence. These emulsions were known as a problematic occurrence in the recovery of oil because stable water-in-oil emulsions were formed by minerals present in the system. Other examples of Pickering emulsions are the stabilization of whipping cream by fat particles and the stabilization of ice cream by ice crystals.
In order for particles to stabilize emulsions, they must be of an appropriate size, wettability and concentration. Other factors contributing to the stability of the emulsion include the pH and presence of ions in the water phase as well as the presence of any other emulsifiers. These three factors can lead to an inversion in the type of the emulsion. The interactions of the particles with each other are also important.