Silica Dissolution-Redeposition in Gels and Aerogels

Silica dissolution-redeposition phenomenon is investigated on a microscopic scale using Small Angle Xray Scattering (SAXS) method. The changes occurring in the microtexture are different according to the procedure used to dry the gels. A xerogel, a CO₂ supercritically dried aerogel and an alcohol supercritically dried aerogel were compared. Silica does not dissolve in CO₂ supercritical. The transformation of heat treated gels into CO₂ supercritically dried aerogels is demonstrated as having minor effect on the microtexture. SAXS data show oscillations around Porod law I(Q) Q^–4 where Q is the wave vector. The oscillations are more pronounced for alcohol dried aerogel than for xerogel or CO₂ dried aerogel. These oscillations are related to curvatures of the surface i.e. the surface roughness. It was found that according to silica dissolution extent, the solid network surface was continuously smoothened in alcohol supercritically dried aerogel. The average chord length of the solid phase is shifted to larger values while the distribution seems to be more narrow. This effect does not give rise to the formation of closed pores.