Structure of Microparticles and Nanoparticles in Solid-Stabilized Emulsions

Emulsions of oil and water stabilized by adsorbed solid particles are known as solid-stabilized emulsions (often referred to as Pickering emulsions). Using confocal microscopy and environmental transmission electron microscopy, we have studied the self-assembly of colloidal-sized polystyrene particles and alkanethiol-capped silver nanopaticles in Pickering emulsions. Colloidal samples of monodisperse size, when exposed to the emulsion at low concentrations, were found to form small patches with local hexagonal order; these crystalline domains were separated by other particle-free domains. Polystyrene particles with different sizes (1 micron and 4 microns) and different wettability could simultaneously segregate to the emulsion interface and form mixtures on it. In contrast to microparticles that form monolayers, the dodecanethiol-capped silver nanoparticles of 1-5 nm form randomly distributed multilayers at the Pickering emulsion interface, with an interparticle distance varying from close contact to approximately 25 nm. Our work offers the first direct observation of nanoparticles in a liquid medium using the environmental transmission electron microscope (E-TEM). About a century ago, Pickering discovered that fine solid particles could be used as stabilizers in emulsion technology. Pickering emulsions, composed of droplets of one immiscible liquid in another liquid stabilized by solid particles, are often encountered in the recovery, separation, and cleaning of oil, in cosmetic preparation, and in wastewater treatment. In Pickering emulsions, the solid particles are strongly adsorbed at the liquidliquid interface, and the Gibbs free energy to remove one solid particle away from the interface is significantly higher than those of surfactants in conventional emulsions. Recently, there has been growing interest in Pickering emulsions because they open new avenues of emulsion stabilization and have numerous practical applications. In addition, Pickering emulsions provide novel and simple templates for the self-assembly of solid microparticles and nanoparticles. Much relevant literature observes or assumes a monolayer of uniformly distributed spherical particles at the fluid-fluid interface in Pickering emulsions. It is also controversial whether or not a full coverage of solid at the oil-water interface is necessary to effectively stabilize Pickering emulsions. Here, using confocal microscopy and environmental transmission electron microscopy (E-TEM) we have studied the structure of microparticles and nanoparticles at Pickering emulsion