The structure, dynamics, and strength of aqueous boehmite gels were studied by yield stress measurements and light scattering as a function of the particle aspect ratio and ionic strength. The yield stress was found to be a strongly varying function of the salt concentration. At low ionic strength, yield stress results from extensive double layers surrounding the rods, leading to a "colloidal glass". Addition of LiCl decreases the yield stress and enhances the particle dynamics in the gel when [LiCl] < 10mM. For 10 mM < [LiCl] < 75 mM, particle aggregation induces the formation of a space-filling heterogeneous network with fractal dimension d ) 2.35, causing a rise in the yield stress and renewed freezing of the particle dynamics. When the LiCl concentration exceeds 75 mM, the yield stress decreases again and the now very turbid gels show syneresis. For the attractive rods, the peculiar salt dependence of the gel strength can be explained by polydispersity in interactions, caused by inhomogeneities in particle surface chemistry and shape.

A synthesis method is introduced for very small uniform gold particles (diameter less than 5 nm), based on the reduction of hydrogen tetrachloroaurate(III) in ethanol in the presence of (γ-mercaptopropyl)trimethoxysilane (MPS). The surface layer of MPS molecules gives the gold particles a high colloidal stability and allows in principle further reaction with any silane coupling agent. Decrease of the HAuCl4:MPS ratio allows a controlled reduction of gold particle size, resulting in remarkably uniform gold clusters of (sub)nanometer size, observed with high-angle-annular-dark-field scanning transmission electron microscopy. After attachment of (γ-aminopropyl)triethoxysilane (APS) to the MPS surface layer, other molecules may be covalently bound to the gold colloid via the amine group of APS. As an illustrative example we prepared in this manner gold particles labeled with a fluorescent dye. The chemical structure of the surface silanes was studied with Fourier transform infrared spectroscopy.

We report the observation of depletion-induced phase separation in a mixture of colloidal silica spheres and colloidal silica rods with light microscopy and confocal scanning laser microscopy. We show that very low rod concentrations are sufficient to induce sphere crystallization. This qualitatively agrees with theoretical predictions and demonstrates that rodlike colloidal particles are highly efficient depletion agents.

The concentration dependence of the sedimentation rate has been studied for a variety of colloids with a magnetic core and nonmagnetic shell. The nonmagnetic shell was either a silica layer or, in the case of magnetic fluids, a surfactant layer. It was found that the (linear) concentration dependence is very much affected by the thickness of the nonmagnetic shell and varied from negative for repulsive particles to positive for a sufficiently strong net attraction. This change in sign is in accordance with a recent theory for sedimentation of dipolar hard spheres. The sensitivity of concentration-dependent sedimentation to interaction details is also illustrated by the significant influence of any van der Waals attraction and surface charge, which competes with the effect of magnetic attraction. Our study shows that surface charge may be important in nonaqueous magnetic fluids.