Long-range repulsive charged colloids in and out of equilibrium

Abstract

The research described in this thesis focusses on the behaviour of charged colloidal particles with long-range repulsive interactions. The experimental model system we used consisted of sterically stabilised charged poly(methyl methacrylate) (PMMA) colloids suspended in cyclohexyl bromide (CHB). In this low-polar solvent very low ionic concentrations can be achieved, resulting in a large Debye screening length (on the order of the particle diameter), i.e. long-range interactions. The pair interaction in these systems is often modelled by the hard-core repulsive Yukawa (or screened Coulomb) potential, which we used for our computer simulations. In Monte Carlo simulations we investigated the effect of size polydispersity on the crystal-fluid transition. We observed a shift to higher volume fraction of the crystal-fluid transition upon increasing the polydispersity. At high polydispersities parts of the higher charged systems that were initially crystalline became amorphous. The amorphous parts had a higher polydispersity than the crystalline parts, indicating the presence of a terminal polydispersity beyond which the homogeneous crystal phase was no longer stable. We investigated the slow dynamics close to the glass transition as a function of the range of the potential, also in simulations. We determined the long-time structural relaxation time and long-time self-diffusion coefficient, which could be well fitted by a power law predicted by mode-coupling theory (MCT) and by the Vogel-Fulcher-Tammann (VFT) equation. In experiments we used confocal microscopy to study the system quantitatively in 3D on a single-particle level. We performed micro-electrophoresis measurements to determine the surface potential and charge of the PMMA particles at a volume fraction around 0.02. The surface potential was roughly independent of the diameter of the particle and was found to be lower for unlocked particles (steric stabiliser adsorbed to surface) than for locked particles (stabiliser covalently attached to surface). In binary systems we observed the spontaneous formation of alternating strings, with the two charged species having charges of the same sign, but of different magnitude. We investigated long-range repulsive glasses with volume fractions 0.16-0.64, which were obtained upon compression by centrifugation and subsequent expansion of the sediment in gravity. Crystallisation of the glasses was prevented by the presence of small clusters formed during centrifugation. The structure of the glasses was found to be remarkably similar to that of hard-sphere glasses, despite the much longer-range interaction potential. After more than ten weeks the fraction of clustered particles decreased due to spontaneous dissociation of the clusters, and bulk crystallisation of the glasses was observed. We studied experimentally the equilibrium sedimentation profiles in binary mixtures. In a well-tuned experimental system we observed the colloidal Brazil-nut effect, an ordering of the particles contrary to what would be expected on the basis of their buoyant mass: in equilibrium the heavier particles stayed further from the surface onto which the particles sedimented than the lighter particles.