| The goal of the proposed research is to elucidate the microscopic origin of the recently discovered spontaneous emulsification of certain mixtures of oil, water, and nano-particles. Moreover, we aim to exploit this insight to develop new self-assembled structures. Emulsions are dispersions of oil droplets in an aqueous continuum, or vice versa, and are abundant in many food- and daily life products. A fundamental feature of these dispersions is their thermodynamically instability: energy input is required to disperse the oil phase, usually with poor control of the droplet size. Oil droplets inevitably coalesce and eventually emulsions demix into separate oil and water phases. In Pickering emulsions, droplet coalescence is suppressed by adsorption of solid particles at the oil-water interface. Ever since their discovery one century ago, Pickering emulsions have also been classified as unstable: emulsion science invariably attributed the retarded coarsening to a mechanical barrier formed by the adsorbed particles. This would imply that Pickering emulsification is essentially different from the spontaneous formation of equilibrium structures such as vesicles and micro-emulsions. Recently, however, the applicants have found [Phys. Rev. Lett. 98 (2007) 158301] that under appropriate conditions, mixtures of a methacrylate oil phase and iron-oxide colloids of 10 nanometer in size, self-assemble in water to truly stable Pickering emulsions, with reproducible mono-disperse droplet diameters in the range 30-200 nm. Also other nano-particles including amorphous silica were found to induce spontaneous Pickering emulsification, suggesting that many more exceptions may exist to the conventional distinction between stable micro-emulsions and unstable normal emulsions. While our work bears the experimental evidence for the existence of thermodynamic Pickering emulsions, the molecular origin of the stability is still a mystery. The aim of this project is therefore to investigate and explain the microscopic origin of spontaneous Pickering emulsification, and to exploit this insight to develop new self-assembled structures, including, e.g., bi-continuous or lamellar ones of oil, water, and nano-particles. The two key questions we address are: (1) what is the microscopic origin of thermodynamic stability of Pickering emulsions? (2) which factors determine size and shape of the emulsified objects? We have strong indications that negative tension of the oil-nanoparticle-water contact line is the driving force behind spontaneous emulsification. We will at first focus on factors that are expected to influence the contribution of the line tension to the free energy of formation of a Pickering emulsion. These factors are the size of the adsorbing particles, and, possibly, the nature of the oil, and the presence of charged species. As for the second question, we will concentrate on factors that influence the preferred curvature of the extended oil-water interface covered by adsorbed nanoparticles. By being able to control preferred curvature, in principle, size and shape of the emulsified objects can also be controlled, with very interesting options for new materials. |
