| Abstract
| - Acid-catalyzed silica polymerization was carried out under gravity conditions varied from 0.01g aboarda reduced gravity aircraft to 70g in a centrifuge. The silicate connectivity of the final xerogels was analyzedusing solid state 29Si NMR and the porosity using nitrogen adsorption. Gravity was found to have animportant influence on the occurrence of intra- and intermolecular condensation reactions. Under reducedgravity buoyancy driven-free convection is limited and silica polymerization occurs in a diffusion-limitedregime. Intramolecular condensations and densifications prevail and extended coils of cages are formedthrough cyclization reactions in the sol particles. In terrestrial and high-gravity conditions, bimolecularreactions compete more favorably with internal condensations. More open structures composed of chainsand rings are formed. These flexible species delay the onset of gelation, leading to an increase in gelationtime as the gravity level is increased. During the subsequent drying procedure, pore collapse changes thestructure from meso- to microporous. The porosity of the finally obtained xerogels is mostly defined bythe drying conditions of the gel, irrespective of the silicate connectivity.
- Gravity during gelation of acid-catalyzed silica sols has a strong impact on the final structure of the obtained porous material. The absence of buoyancy effects favors intermolecular densification of the primary silica structures before gelling can occur. The result is a faster gelation and a different structure in the xerogel obtained under reduced gravity conditions
|
