Effect of gravity on the gelation of silica sols

Acid-catalyzed silica polymerization was carried out under gravity conditions varied from 0.01g aboard a reduced gravity aircraft to 70g in a centrifuge. The silicate connectivity of the final xerogels was analyzed using solid state Si-29 NMR and the porosity using nitrogen adsorption. Gravity was found to have an important influence on the occurrence of intra- and intermolecular condensation reactions. Under reduced gravity buoyancy driven-free convection is limited and silica polymerization occurs in a diffusion-limited regime. Intramolecular condensations and densifications prevail and extended coils of cages are formed through cyclization reactions in the sol particles. In terrestrial and high-gravity conditions, bimolecular reactions compete more favorably with internal condensations. More open structures composed of chains and rings are formed. These flexible species delay the onset of gelation, leading to an increase in gelation time as the gravity level is increased. During the subsequent drying procedure, pore collapse changes the structure from meso- to microporous. The porosity of the finally obtained xerogels is mostly defined by the drying conditions of the gel, irrespective of the silicate connectivity.