59th Annual Report on Research 2014

Overview:

The ACS-PRF grant (type ND) has helped support two separate projects over the last year. The focus of the first project is to understand the adsorption of nanoparticles to the oil-water interface. The goal of the second project was to understand the role of interconnected vertical channels on the fluid pressure necessary to separate (move) two surfaces. Both projects are relevant to the petroleum industry. For the first one, nanoparticles at fluid interfaces are important for oil recovery and, more generally, for transport in porous media. The second project has direct implications to hydrofracturing. The ACS-PRF-supported work has led to additional funding from the Office of Naval Research, and the National Science Foundation, as well as a long-term collaboration with another faculty. Additionally, three additional manuscripts are will soon be submitted reporting on the finding of the work supported by this grant.

Summary of Activities:

Project 1) Nanoparticles at interfaces

Pickering emulsions are oil-water emulsions kinetically stabilized by solid particles[1]. They are important in flotation, oil recovery, and catalysis. As with all emulsions, Pickering emulsions are only kinetically stable due to the energetic cost associated with the creation of oil-water interfacial area. In contrast to larger colloids, the attachment of nanoparticles (< 10 nm) at an oil-water interface is often comparable to thermal energy (kT), the inherent energy scale of any reversible phenomena[2-3]. The weaker energy can make nanoparticle adsorption at fluid interfaces dynamic and reversible, a necessary criteria for thermodynamic equilibrium. This regime of interactions also provides the capability to develop reversible Pickering emulsions, which could enable reconfigurable, and adaptive multiphase fluid-based materials and devices. Nanoparticles provide access to unique optical and electrical properties as well as the possibility to create much smaller droplets with increased surface area with advantages for catalysis and mass transport.

Achievements. We previously demonstrated the mechanisms driving the interfacial adsorption of 5 nm ion-pair functionalized gold nanoparticles to the toluene-water interface.[4] In the last year we performed extensive characterization of the equilibrium and dynamic of nanoparticle adsorption to a fluid interface using pendant drop tensiometry (Fig 1A-B). We combined the interfacial tension measurements with UV-Vis absorption spectroscopy to determine the partitioning of the particles between the interface and bulk phases (Fig. 1C). When then used 2D equations of state to relate the relationship between the surface pressure and bulk nanoparticle concentrations to the adsorbed amount at the interface. Using the Singer 2D equation of state we found good agreement between the tensiometry measurements and the adsorption obtained from UV-Vis (Fig. 1B-C).  Finally we developed an electrochemical method to modulate externally the adsorption of the particles to the interface (see Fig. 1D). Currently we are working on developing a rigorous thermodynamic model for the reversible nanoparticle adsorption. We also aim to investigate how nanoparticles adsorption is altered by dynamic transport, as what is observed in unsaturated porous media.