Self-assembled structure and dynamics of solid particles at Pickering emulsion interfaces

Emulsions stabilized by solid particles are known as Pickering emulsions. Although Pickering emulsions are encountered in various natural and industrial processes such as crude oil recovery, oil separation, cosmetic preparation, and wastewater treatment, many fundamentals related to Pickering emulsions are poorly understood. Recently, there is an increasing interest in Pickering emulsions because of their potential in numerous novel applications.

Using confocal laser scanning microscopy, the self-assembly of colloidal-sized polystyrene particles in polydimethylsiloxane (oil)-in-water Pickering emulsions was studied. Monodisperse polystyrene microparticles, when included in the emulsions at low concentrations, were found to form small patches with local hexagonal order, separated by other particle-free domains. Various interparticle interactions were calculated and it was found that the electrostatic repulsion was the dominant force and responsible for the long-range ordering. Also, evidence of partial coverage of emulsion interfaces was observed.

Pickering emulsions offered a simple template for the self-assembly of solid particles. Polystyrene particles of different sizes (0.2 micron and 1.1 micron; 1.1 micron and 4 micron) and different wettability (sulfate-modified and carboxylate-modified) could simultaneously segregate to the emulsion interface. In a most intriguing result, even mixtures of hydrophobic and hydrophilic solid particles were found to self-assemble to the same interface.

The multiphase interactions and self-assembled structure of dodecanethiol-capped silver nanoparticles (1-5 nm) at trichloroethylene-water interface were investigated using an environmental transmission electron microscope (E-TEM). The nanoparticles formed randomly distributed multilayers at the liquid-liquid interface with an inter-particle distance varying from close contact to approximately 25 nm. This was in contrast to the monolayer observed for colloidal-sized polystyrene particles. This work, to the best of the author's knowledge, offers the first experimental data revealing the detailed self-assembled structure of very small nanoparticles (less than 10 nm) at liquid-liquid interfaces using an E-TEM.

Furthermore, the Pickering emulsions have been used as a model system to study the particle mobility, aggregation, and the mechanism of aggregate growth at the two-dimensional level. Remarkably, the rate of diffusion of the colloidal-sized polystyrene particles at the oil (5 cSt)-water interface was only moderately slower than in the bulk water phase. At oil (1 cSt)-water interface of comparable viscosities the diffusion constant was comparable to that in the bulk water phase. The diffusion constant of solid particles was significantly reduced by increasing the viscosity of the oil phase and was dependent on the interfacial curvature and temperature. Also, the diffusion of multi-particle clusters at oil-water interfaces was a strong function of cluster-size and oil phase viscosity, and could be quantitatively related to fractal dimension.

Finally, the formation of stable 2-D colloidal lattices of solid particles at the oil-water interface was successfully observed. The short-time diffusion constant increased with increasing lattice spacing and the oil phase viscosity influenced the diffusion only at large interparticle distances. At longer times, the diffusion constant reached a plateau indicating a "cage effect." Interestingly, the equilibrium lattice structure could be disturbed by increasing the output laser intensity in a confocal laser scanning microscope which led to the collapse of colloidal lattices under small radiation pressure forces.

In summary, in this dissertation Pickering emulsions are described as an experimental model system to study the self-assembly and dynamics of solid particles at liquid-liquid interfaces.