Understanding of charge effects in pickering emulsions and design of double pickering emulsion templated composite microcapsules
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Particle stabilized emulsions, also known as Pickering emulsions, have been widely used in many industry applications. While the breadth of potential applications for Pickering emulsions keeps growing, our fundamental understanding of Pickering emulsions is still poor. My thesis work addresses both fundamentals and applications of particle stabilized emulsions. In the fundamental part of this thesis work, we investigated the effects of particle charge on particle adsorption and the particle contact angle, and to investigate their ensuing consequences for the stability of Pickering emulsions. We provided the first experimental hint that the widely overlooked image charge repulsion can hinder the adsorption of particle to the oil-water interface and prevent the formation of Pickering emulsions. Consistently with the experimental suggestion, our theoretical model also confirmed that the image charge repulsion has the right order of magnitude, relative to the other forces acting on the particle, to impede particle adsorption and Pickering emulsification. For the conditions in which particle adsorption to the liquid interface does occur, the particle contact angle will play an important role in influencing the stability and type of Pickering emulsions. Our experimental work showed that the equilibrium contact angle of particles at interfaces and the type of emulsions preferentially stabilized by these particles can be strongly affected by the particles' charging state, which we attribute to a free energy contribution from the electric field set up by the charged particle and its asymmetric counterion cloud. A very simplistic calculation considering only the dipole field as the leading contribution and treating the water phase as a perfect conductor, found that the energy stored in the field is indeed strong enough and shows sufficient variation with the particle position to shift the equilibrium position significantly from where it would be based on interfacial tension alone. In a separate, more application oriented part of this thesis work, we have fabricated microcapsules from double Pickering emulsions and demonstrated that the combined use of hard silica particles and pH-responsive dissoluble polymer particles at the emulsion interface imparts a combination of pH-responsiveness (stimulated pore opening) and structural integrity to resulting capsules. We have further demonstrated the first double Pickering emulsion templated capsules in which interfacial polymerization was carried out at both emulsion interfaces, yielding a capsule with two composite shells, composed of polyurethane and silica particles, and characterized the transport of a model cargo through the capsules walls as well as the capsules' mechanical properties.