An isotropic-nematic phase separation is observed in dispersions of sterically stabilized rodlike boehmite particles. The phase separation process is studied with polarization microscopy. Also, the phase volumes and particle concentrations in the coexisting phases are determined. The dependence of these quantities on the total concentration of the dispersion is analyzed in terms of the Onsager theory for the isotropionematic phase transition extended to bidisperse mixtures of rodlike particles. Qualitative agreement between theory and experiment is obtained. The triphasic isotropic-nematic-nematic equilibrium as predicted by the theory is observed after 6 months.

We report the phase behavior of aqueous dispersions containing rodlike boehmite particles with two different average lengths, 130 and 280 nm, with length-to-width ratios of approximately 8 and 20, over a range of ionic strengths. At low salt concentrations, where the particle interaction is predominantly repulsive, a separation into an isotropic upper phase and a birefringent lower phase occurs in the system containing the high-aspect-ratio particles. This phase separation was not observed in the case of dispersions containing the shorter particles. This seems to indicate that the phase separation is driven by the excluded volume effect. Above a critical (low) particle concentration, the repulsive 280 nm longrods form a monophasic birefringent glassy phase. This concentration is much lower than that at the isotropic-nematic transition as predicted from Onsager's theory for charged rods. Above a critical ionic strength a regime is entered where the phase behavior is largely governed by the attractive interaction. Then for both aspect ratios a space-filling amorphous gel is formed.

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.

Fairly monodisperse colloidal boehmite fibrils with a high aspect ratio were synthesized by hydrothermal treatment at 150°C of an acidified aqueous alkoxide solution, prepared by adding an aqueous HCl solution to an aluminum alkoxide precursor. The average particle length could be controlled between about 100 and 500 nm by varying the initial amounts of alkoxide and acid. Using two different alkoxides in a 1 : l molar ratio yielded the most needlelike product, having a particle length standard deviation of 40%. The boehmite particles were polycrystalline and contained 0.14 mol of excess H 2 0 per mol of AlOOH, bound to the particle surface. [

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