THE PREPARATION AND CHARACTERIZATION OF SOL-GEL SILICA SPHERES

Silica sor-gel spherical bodies are prepared containing 2-13 wt% SiO2 in water. The silica sol is prepared by addition of a sodium silicate solution to a dilute nitric acid solution. The silica sor thus obtained is introduced dropwise into hot paraffin oil, which results in spherical hydrogel bodies. The pore volume of the spheres is varied between 0.16 and 1.4 ml/g, and the pore radii between 0.8 and 60 nm. The pore structure of the hydrogels is investigated by thermoporometry. The hydrogels are washed and dried at 293, 333 and 393 K. The pore structure of the dried spheres is determined by nitrogen adsorption and desorption at 77 K. Drying causes a considerable shrinkage of the pore dimensions. The pore radius of the sample containing 2 wt% SiO, in water diminishes from 10.2 to 2.9 nm, while the pore volume decreases from 0.9 to 0.5 ml/g. Drying the gelled spheres at temperatures > 293 K leads to a lower shrinkage, and consequently to a higher pore volume of the dried spheres. The pore structure of both xerogel and hydrogel spheres is modified by means of a hydrothermal treatment. Hydrothermal treatment of the hydrogel spheres increases the pore volume, whereas that of the xerogel spheres remains constant. Differences observed with the hydrothermally aged hydrogel and xerogel spheres are attributed to the ripening mechanism being dependent on the initial pore structure.