High-resolution small-angle x-ray diffraction study of long-range order in hard-sphere colloidal crystals.

The long-range order parameters in single crystals of hard colloidal spheres grown in sediments of colloid-polymer mixtures are determined using synchrotron small-angle x-ray diffraction with a resolution of 10(-6) of the wave vector. The interplanar positional order derived from the width of lattice reflections extends over at least 500 lattice planes. The lattice planes are orientationally correlated within approximately 0.1 degrees throughout the crystals, whereas the stacking of hexagonal planes remains random.

Mechanisms of phase separation and aggregation in colloid-polymer mixtures.

The final structure of a colloidal system is greatly influenced by the mechanisms by which phase separation and aggregation occur. The drive to phase separate can be altered in colloid-polymer mixtures (which phase separate due to the depletion interaction) by varying the polymer concentration. Here, we use small angle light scattering to follow the phase separation in such mixtures and analyze the results within a framework indicated by previous results from microscopy investigations. The mechanisms of diffusion-limited cluster aggregation, reaction-limited cluster aggregation, and nucleation and growth are found to provide good descriptions of the phase separation regimes. The growth rate in the nucleation and growth regime is shown to be dependent on the polymer concentration.

Direct observation of crystallization and aggregation in a phase-separating colloid-polymer suspension.

The depletion-induced phase separation in a mixture of colloidal particles (PMMA-latex) and nonadsorbing polymers [poly(styrene)] in a solvent (mixture of tetralin, cis-decalin, and carbon tetrachloride) was investigated in real space with confocal scanning laser microscopy in the initial, intermediate, and final stage. It was found that the kinetics and the morphology of the phase separation strongly depend on the polymer concentration, and thus on the strength of the depletion-induced attraction between the colloidal particles. At moderate polymer concentrations, crystallization of the PMMA particles is enhanced. At higher polymer concentrations, only aggregation is observed, resulting in amorphous sediments. The aggregation is diffusion-limited or reaction-limited, depending on the polymer concentration. Digital image processing was used to determine the dependence of the aggregation rate and the size of the clusters on the polymer concentration.