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Summary: A neutron scattering approach has been applied to characterize nanostructures that form at the oil-water interface of emulsions. Pickering emulsions are generated when two immiscible fluids are mixed in the presence of solid nanoparticles or colloids. Such emulsification can be encountered in the petroleum industry during oil extraction, in oil desalination processes and in underwater oil-spills. Depending on the wetting behavior of the solid and the two fluids, the particles can preferentially accumulate at the oil-water interface creating a barrier that prevents coalescence. This enhanced stability is dictated by the specific organization of the solid particles at the interface so that systematic characterization is critical to understanding macroscopic phase behavior. In this project we focused on developing relationships between colloidal behavior and structural transitions occurring at the interface of Pickering emulsions when they are subjected to changing conditions such as temperature, pH or ionic strength. For example, freeze-thaw cycling is an effective destabilization mechanism for emulsions but the underlying physical principles and the associated structural changes occurring at the interface have not been properly characterized and are poorly understood. Unfortunately, the ‘buried’ interface cannot be investigated with direct microscopic visualization techniques (light or electron) due to limitations caused by high turbidity, thermal fluctuations and size resolution. In this project, we characterized the local interfacial structure of solid nanoparticles through the use of non-intrusive neutron scattering techniques capable of seamlessly probing a wide range of length scales from 1-10,000 nm. These cover all relevant features from individual nanoparticles to oil droplets of micrometer size.

We focus specifically on the characterization of Pickering emulsions stabilized by nanosized particles because little is known about them owing to their small size. Through the PRF award, we developed and published an analytical model for the analysis of neutron and x-ray scattering data from Pickering emulsions. We also used this model to evaluate the interfacial structure of emulsion systems composed of water, hexadecane and nanoparticles. Contrast variation techniques were used in neutron scattering experiments to highlight the contribution of the nanoparticles at the interface. Without this, the shape of the large oil droplets would dominate the scattering signal and it would be impossible to probe the small particles. The adsorption energy for particles decreases rapidly with size and for nanosized particles it approaches thermal energy values (kT). Therefore, the adsorbed amount decreases significantly as the particle size is reduced. Small angle X-ray and neutron scattering experiments confirm that the adsorption for nanoparticles is very low (< 10% of the area). Yet, the nanoparticles are still effective stabilizers for oil-in-water emulsions. This suggests that the primary stabilization mechanism for nanoparticle-stabilized emulsions is electrostatic and not due to the formation of an elastic shell. This is in contrast to what is found for emulsions formed from larger particles. The stability was also strongly affected by small amounts of monovalent salt (< 1 mM) indicating that a strong electrostatic effect was involved. We still continue to use small angle neutron scattering and neutron reflectivity to analyze this behavior and to identify the transition between elastic and steric stabilization. We are also evaluating and quantifying the effect of changing other parameters such as surface treatment, pH and temperature on the structure and colloidal behavior of these systems.

A side project was also initiated, with PRF support, to intentionally increase the adsorption of small nanoparticles at oil-water interfaces. Our approach consisted on generating and using amphiphilic nanoparticles that would have an increased energy of adsorption at the oil-water interface so that they would adsorb at higher levels. Previously, this would require functionalization with amphiphilic molecules such as block copolymers. The synthesis process would usually be complex and expensive. We developed a novel one-pot approach to create nanoparticles having amphiphilic surfaces but without the need for using amphiphilic surface ligands. Our new approach only utilizes commercially available homopolymers and small organic ligand molecules and does not require polymer synthesis or purification steps. As expected, the interfacial adsorption of these amphiphilic nanoparticles is significantly enhanced. Furthermore, the particles are also found to self-associate in solution in a way similar to small molecule surfactants that form micellar structures. Thus, this new approach represents a simple, scalable and inexpensive method to organize nanoparticles into stable cluster superstructures. Results from this sub-project are being prepared for publication and proposals for continued external funding have also been submitted to federal agencies. In the future we will also evaluate the organization of amphiphilic nanoparticles at the oil-water interface and relate this to emulsion stability.

Impact of PRF Award on Student and Faculty Development: This type-G PRF program directly supported the dissertation research for one female Chemical Engineering graduate student (PhD candidate). PRF funds provided her with a monthly stipend and with all resources needed for her PhD research for two years and allowed her to complete an MS degree in the process. The graduate student has traveled several times to NIST facilities (Gaithersburg, MD) for training and to perform neutron scattering experiments (SANS and USANS) related to the project. She also presented this research at the regional ACS meeting (NORM 2009) and at the International Congress on Neutron Scattering (Knoxville, TN). She will also be presenting work sprouted from the ACS-PRF award at the 2010 Annual AIChE meeting (Salt Lake, UT). Owing to her accomplishments in this grant, she recently received a prestigious fellowship that will continue to fund her PhD research through 2010-2011. The grant has also supported five undergraduate students by providing them with direct mentorship by the graduate student and the PI and with materials and supplies to work in projects related to this award. The PRF award was also instrumental to sustain the growth of the Pozzo research group and significantly increased our visibility through award recognition, conference presentations and research publications. More importantly, it allowed us to enter a new research area and sprouted other novel research directions. Overall, the PRF award has been a great catalyst for creative thinking and innovative research in our group.