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Seminars
Pickering Emulsions - A Paradigm Shift
Lenore L. Dai
Department of Chemical Engineering
Texas Tech University
When: Thursday, April 13, 2006
Time: 2:30 PM to 3:30 PM
Where: 1064 Duncall Hall
Abstract:
Emulsions are ubiquitous in natural and industrial processes. Conventional emulsions use organic surfactants or polymers as stabilizers. Although solid particulate stabilized emulsions (Pickering emulsions) are often encountered in crude oil recovery, oil separation, cosmetic preparation, andwastewater treatment, the phenomenon is poorly understood. There is no existing technology of using solid particulates as commercial stabilizers, although they open a new paradigm of emulsion stabilization and provide opportunities for numerous practical applications.
Using confocal microscopy and environmental transmission electron microscopy, we have studied the self-assembly of colloidal-sized polystyrene particles and alkanethiol-capped silver nanoparticles in Pickering emulsions. Monodisperse polystyrene microparticles, when included in 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. Particles with different sizes (1 micron and 4 microns) and different wettability (hydrophobic and hydrophilic particles) could simultaneously segregate to the same emulsion interface and form mixtures on it. In contrast to microparticles, the alkanethiol-capped silver nanoparticles of 1-5 nm formed multilayers and packed randomly at the emulsion interface.
Finally, we have used Pickering emulsions as a new, convenient, and unique experimental model system to investigate the dynamics of microparticles at liquid-liquid interfaces. Remarkably, the rate of microparticle diffusion at the oil (5 cSt.)-water interface was only moderately slower than that in the bulk water phase. The ambient diffusion constant of microparticles was significantly reduced from 1.1´10-9 cm2/s to 2.1´10-11 cm2/s when the viscosity of the oil phase increased from 5 cSt. to 350 cSt. In addition, we found that a highly curved emulsion interface slowed the motion of solid particles; however, the interfacial curvature effect decreased with increasing oil phase viscosity. 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.
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