Interplay between Colloids and Interfaces : Emulsions, Foams and Microtubes

Abstract

The central theme of this thesis is the interplay between colloids and interfaces. The adsorption of colloids at fluid-fluid interfaces is the main topic and covers Chapters 2-6. Pickering emulsions where colloidal particles act as emulsion stabilizers in the absence of surfactants are studied in a number of systems with different colloids, particle shapes and oils. Interfacial particle adsorption is widely employed in food, cosmetic and pharmaceutical applications and plays a pivotal role in oil recovery and metallurgical refining. The conditions under which Pickering emulsions may form spontaneously are discussed in Chapter 2. The spontaneous formation and the stability of the Pickering emulsions of varying composition are achieved by a collective effect of solid particles, amphiphilic ions and interfacial tensions of the bare oil-water interface of ~10 mN/m or below. The observed stability and formation of emulsions of different composition point to a new class of solid-stabilized meso-emulsions. The effect of particle shape anisotropy in the stabilization of emulsions is investigated in Chapter 3 by employing for the first time cubic, ellipsoidal and peanut-type hematite microparticles. The interfacial packing and orientation of these anisotropic microparticles are revealed at the single particle level by direct microscopic observations. Emulsions are stable against further coalescence for at least one year. The creation of surfactant-free Pickering foams with anisotropic hematite microparticles and their dependence on ionic strength are studied in Chapter 4. The microparticles cover bubbles by densely packed interfacial monolayers that provide high stability against disproportionation and coalescence. Moreover, solid-stabilized air bubbles are used as scaffolds for the creation of free-standing particle films that are, in fact, inorganic bilayers that only consist of cubes. In Chapter 5 the self-assembly of colloidal silica cubes at oil-water interfaces is discussed. A combination of optical and laser scanning confocal microscopy enables the in-situ study of both the packing as well as the orientation of the colloidal cubes at the interface. Single layers of cubic particles arrange in ordered domains, displaying a packing intermediate between cubic and hexagonal. Moreover, the cubes show a preference for orienting parallel with the interface. Solid-stabilized emulsions from natural resources are presented in Chapter 6 by using colloids, prepared from the water-insoluble corn protein zein, and soy bean oil. Zein colloids are synthesized via an anti-solvent precipitation procedure and employed in the formation of stable oil-in-water Pickering emulsions as a function of particle concentration, pH and ionic strength. Finally, a thermo-reversible colloid-in-tube co-assembly approach that couples molecular self-assembly with colloidal self-assembly is introduced in Chapter 7. While surfactant and cyclodextrin molecules form microtubes, colloids assemble into a library of dynamic colloidal structures within those microtubes. Isotropic spheres form straight, zigzag, and zipper chains depending on the tube-sphere size ratio. Double and triple helical structures, a common occurrence in nature, were even generated from colloidal spheres. Moreover, we demonstrate that the co-assembly of microtubes and colloids is generic for colloids with different shapes and materials. The hierarchical colloid-in-tube co-assembly provides a novel route to temperature-sensitive particle alignment and their release near human-body temperature.