Colloidal dispersions of repulsive nanoparticles: tunable effective interactions, phase behaviour and anisotropy

Abstract

We investigated various aspects of colloidal dispersions. In this study we not only emphasized the tunability of the particle properties (rods versus spheres, charging properties of the particle surface), but also the properties of the medium in which they reside (oil and/or water). These properties can impact the phase behaviour of colloidal dispersions, and the effective colloid-colloid or colloid-interface interactions. We applied concepts like (long-range) ionic screening, charge regulation, many-body effects, and anisotropy in particle shape, to various dispersions of charged colloidal spheres and hard needles. For the phase behaviour, we studied reentrant melting of colloidal Wigner crystals as function of colloid density, the spontaneaous demixing in binary mixtures of positively charged colloidal spheres, and the formation of string-like structures consisting of charged colloidal particles. Charge regulation, the way how colloidal particles acquire their charge by adsorption or desorption of ions, turned out to be essential not only on the level of the macroscopic phase diagram (such as the occurrence of reentrant fluid phases), but also on the type of structures that could be formed (strings or clusters). Colloid-interface interactions were studied in the context of colloidal particles in a medium that consists of two immiscible solvents, namely oil and water. We showed that so-called non-touching colloids experience an effective interaction potential with the oil-water interface, that is tunable by the Donnan potential and colloidal charge. These colloid-ion forces can not only be tuned in range and strength, but also in being repulsive or attractive at sufficiently large distances from the interface, and should be added to the well-known image-charge and van der Waals potential. Furthermore, we investigated lateral colloid-colloid interactions for colloids that can penetrate the oil-water interface. We suggested that the colloid-colloid lateral interaction is dominated by the small, weakly screened bound charges at the oil-exposed side of the colloidal surface, and these are probably more important than the many highly screened charges at the water-exposed side. Finally, we considered the anisotropy in the particle shape in the context of hard needles. These needles were described by a novel Landau-de Gennes theory that accurately mimicks Onsager theory. We tested this theory, which is based on the grand-canonical ensemble, in bulk and for the isotropic-nematic interface. We applied it subsequently to the hedgehog defect and showed that colloidal systems have an isotropic core size that is on the order of the particle size. This should be contrasted to molecular (thermotropic) systems where the core size is much larger than the molecular size. Furthermore, we investigated the density dependence of director field textures and their defects for confined rods. Ultimately, we applied our theory to tactoids: liquid crystalline droplets in an isotropic background. The experimentally observed large size aspect ratios of these droplets are, however, not explained in our theory, if we use the surface tensions from Onsager theory, revealing the need of additional ingredients in our model.