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Introduction:

Particle stabilized emulsions (Pickering emulsions) are frequently formed during the extraction and processing of organic fluids in the presence of water and solid particles (microparticles or nanoparticles). Like surfactants, fine solid particles are enriched at the oil-water interface due to a favorable energy of adsorption. Emulsion stabilization can be electrostatic or steric. In electrostatic stabilization, the adsorption of charged particles creates a repulsive interaction between droplets of similar surface charge. Steric stabilization occurs through the creation of an elastic shell around the oil that prevents coalescence and maintains the stability of the emulsion. Thus, emulsion stability is controlled primarily by the specific structure and organization of the solid particles at the interface. The primary aim of this project is to develop the experimental ‘toolbox' to probe the interfacial organization of solid particles in Pickering emulsions. Experimental access to the buried oil-water interface is critical to the understanding of the parameters that control emulsion stability. In particular, we aim to understand the transitions that occur in Pickering emulsions that are subjected to freeze-thaw phase transition processes and relate this back to their interfacial structure. The freeze-thaw process is found to destabilize emulsions but the governing physical processes are poorly understood.

Use and impact of PRF funds:

PRF funds are essential for the education of our future scientific workforce. This program supports the dissertation research of Chemical Engineering graduate student Kjersta-Larson Smith. Funds have provided Kjersta with all necessary supplies and resources that are needed for her PhD research. With PRF funds, Kjersta has also been able to travel to NIST facilities in Gaithersburg MD to be trained and to carry out neutron scattering experiments (SANS and USANS). Three new proposals for neutron experiments have been recently accepted at the NIST center for neutron research and these will be carried out in the upcoming year. Funds have also been used for Kjersta to present her research at the International Congress on Neutron Scattering (Knoxville, TN). The PRF award has also been instrumental in the continued development and growth of the PI's research group and has enabled the pursuit of additional funding for the continuation of this new research program.

Project Progress:

During the first year of this project, focus has been primarily placed on the development of a solid scientific framework to characterize the interfacial structure of Pickering emulsions. This has resulted in the formulation of a new analytical model for the quantitative analysis of the neutron scattering experiments. This model is an essential tool for the effective design and planning of the small angle scattering experiments. In addition, we have also developed and evaluated several model systems consisting of surface functionalized nanoparticles stabilizing model oils. Preliminary small angle X-ray and neutron scattering experiments have already been performed to validate the analytical model and to demonstrate experimental feasibility. So far, the results suggest that the primary stabilization mechanism for nanoparticle stabilized emulsions is of the electrostatic type. All particles that have been studied so far have negative surface charges. Adsorbed amounts have been found to be quite low while still resulting in emulsions that are stable for several days. The stability of the emulsion was also found to be greatly affected by the addition of very small amounts of monovalent salt (< 1 mM). Steric stabilization is expected to occur when particle adsorption is strong so that a solid elastic shell can form. The size and wetting properties of the particles are the primary parameters controlling adsorption strength.

Future project development:

In the following year the focus will shift primarily to the use the established scattering techniques and model systems for the characterization of various emulsion interfaces. We will use neutron and x-ray scattering to probe the interface of emulsions stabilized with particles of different size and surface treatment. This information will be correlated to their long term stability and their susceptibility to destabilization after undergoing phase transitions (freeze-thaw). In addition, we will also pursue the use of neutron reflectivity to measure the penetration depth of the nanoparticles at the oil-water interface. These combined experiments will provide a general and fundamental toolbox for the analysis of emulsion interfaces.