RESUMEN
Porous composite powders, prepared by spray drying of silica and polybromostyrene nanoparticles, were calcined at various temperatures up to 750 degrees C. The structure in these powders are quantitatively investigated by ultra small-angle X-ray scattering, thermogravimetric analysis, and nuclear magnetic resonance measurements. It has been found that the polybromostyrene latex is efficient in templating mesopores. However, polybromostyrene remains almost completely in the interstitial micropores in the grain after the spray-drying process. A post thermal treatment of the powders has been applied from 250 up to 750 degrees C. We found that the hydrocarbon part of the polybromostyrene is decomposed and leaves the micropores at around 350 degrees C. However, it is demonstrated that a significant amount of bromine remains in the interstitial micropores between the silica particles. At around 600 degrees C, the silica nanoparticles start to fuse with each other and a coalescence of the micropores takes place. At still higher temperature, around 750 degrees C, the micropore network totally disappears, and the growth in pore size occurs due to the coalescence of the mesopores with a significant decrease of the total porosity. During this process, the silica network densification is accompanied by a lowering of the specific surface area.
RESUMEN
Nanoporous powders are prepared in a single step by spray-drying mixtures of small inorganic and larger organic nanoparticles. The structure of these powders has been studied as a function of the mixture's composition using silica and polybromostyrene nanoparticles. Scanning electron microscopy reveals the presence of an increasing concentration of mesopores as the concentration of polybromostyrene increases. By coupling thermogravimetric analysis and ultra-small-angle X-ray scattering, the structure is quantitatively linked to the composition of the porous grains. Experimental USAXS intensities are compared to scattering models for the composite powders. It allows to demonstrate that (i) all mesopores are empty even in the center of the grains, (ii) part of the polymer remains in the micropores of the dried grains. A quantification of the fraction of micropores filled by residual polymer is presented. Such a synthesis procedure can be used to produce hydrophobic porous powders in a one-step process.
RESUMEN
Colloidal cerium oxide particles of nanometer size are irreversibly adsorbed on molecularly smooth mica sheets from bulk dispersions. The approach to equilibrium, the homogeneity and stability of the adsorbed films, and the effects of pH and solution conditions, are determined by means of the surface force apparatus and atomic force microscopy techniques. Driven by electrostatic interactions between oppositely charged substrate and particles, a dense and relatively homogeneous flat film composed of ceria nanoparticles assembled in a single layer can be obtained. Coating substrates with nanometric particles gives a surface smooth at the nanometer scale only, but which is fully representative of a colloidal oxide/water interface. Force-distance profiles between such layers and colloidal stability can thus be compared. The long-range electrostatics repulsion is consistent with the zeta-potential values measured independently in bulk dispersions. An adhesion at contact between ceria is evidenced and its relation with the intricate colloidal stability of the oxide dispersions is demonstrated. The chemical origin for the adhesion between ceria surfaces is further supported by the effect that complexant molecules play. In their presence the ceria layers are protected and adhesion is prevented, with an efficiency increasing in the order nitrate < acetate < acetyl acetone. It appears that the stability of nanometric dispersions is better achieved by controlling the adhesion at contact rather than affecting the long-range electrostatic repulsion.
RESUMEN
The stability of a colloidal dispersion of nanometric zirconia particles has been studied during a compression process. Using the osmotic stress method, cycles of compression and reswelling were applied to the dispersion to test the reversibility of the process. Original dispersions are stable in a very limited pH range (0.5-2). At pH 3, the bare particles aggregate irreversibly under compression as checked by osmotic pressure and light and X-ray scattering measurements. To improve the stability, small organic complexing molecules (acetylacetone) were added to the original dispersion. The adsorbed monolayer on the particle surfaces acts as a steric barrier and prevents the two colloids from contacting. As a consequence, the dispersion becomes more compressible and the compression cycle is totally reversible. The experimental data are quantitatively reproduced with a classical theory of statistical mechanics of liquids based on a DLVO-like colloid-colloid potential.
