Colloidal superballs

Abstract

This thesis is organized in four parts as follows. Part 1 focuses on the synthetic aspects of the colloidal model systems that will be used throughout the work described in this thesis. In Chapter 2 we describe synthetic procedures for the preparation of polycrystalline hematite superballs and superellipsoids. The internal structure of the particles is also investigated and will be used later to understand the magnetic properties of colloidal hematite. The same hematite particles are used as templates for the preparation of silica hollow superballs and superellipsoids as described in Chapter 3. The particles are coated with a layer of silica that is porous and permits the dissolution of the internal hematite cores by acidic treatment. The technique is convenient to obtain lighter micron sized superballs and superellipsoids useful for the study of anisotropic shape interactions. In Chapter 4, we employ the hematite colloids that possess a permanent dipole moment and therefore behave as micro-magnets, in order to prepare spherical colloids with centered and shifted magnetic dipoles. To this purpose, the magnetic hematite is encapsulated just below the surface of polymer droplets that can be subsequently polymerized. When the polymer droplet is small, they can be coated with silica to obtain centered dipolar spheres. In Part 2 we focus our attention on non-magnetic silica superballs and particularly how the superball shape influences the phase behavior of the colloidal particles. In Chapter 5 we show that silica superballs with relatively high shape parameters (m) readily crystallize into the rare simple cubic crystal structure when they are dispersed in the presence of small non-adsorbing depletants. In Chapter 6, we extend the study to superballs with different shape parameters (m) focusing on their interaction to non-adsorbing polymers of various sizes. The result of this work is presented in the first experimental phase diagram of colloidal superballs in the presence of depletants. Part 3 deals with the study of magnetic colloids. In Chapter 7 we focus our attention in the magnetic behavior of dipolar hematite superballs and superellipsoids under different conditions. We studied the structure formation at low and high particle concentrations, in the Earth’s magnetic field as well as an externally applied magnetic field. To perform the experiment we devised a magnetic set-up that allows precise control on the direction and strength of the applied field. Using this magnetic set-up, we have developed a technique that allows cancellation of any residual magnetic fields in the environment to ensure that dipolar structure formation can really be studied in zero field. In Chapter 8 we study the self-assembly behavior of colloids with magnetic-patches (spheres with shifted dipoles). The self-assembly of the patchy colloids can be tuned by changing the size of the polymer particles, the salt concentration in solution and by application of an external field. In Part 4, we explore the preparation and behavior of food-grade colloids specifically designed for application as food-additives. In Chapter 9 we study the synthesis of colloidal pyrophosphates nanoparticles as possible additives for iron-fortification in food. Because of the novelty of the material, we have performed extensive characterization of the physico-chemical properties of the nanoparticles. In Chapter 10 we focus our attention on the control of the shape and size of the colloidal pyrophosphate. We employ the porous hollow silica colloids prepared in Chapter 3 as templates for the synthesis of pyrophosphate in their inner hollow part. In Chapter 11 we develop another kind of colloidal particles using phytosterol molecules. The particles are synthesized, characterized and preliminary in vitro experiments are performed to study their capability to lower the adsorption of cholesterol during digestion. Because the synthetic method used for the particle synthesis produces phytosterol particles with a characteristic rod-like shape, in Chapter 12 we study their phase behavior at different concentrations. We show that at certain concentrations the particles self-assemble to form a cholesteric liquid crystalline phase