GEL TRANSFORMATIONS IN SILICAS - A COMBINED NMR AND SAXS STUDY

H-1 NMR transverse spin-spin relaxation times (T2) of water have been measured to monitor the aggregation and aging processes of low concent ration silica gels. Along with H-1 NMR, small-angle X-ray scattering ( SAXS) and physisorption measurements; were used to obtain additional i nformation on aging. After acidification of water-glass (silica soluti on) the primary building units combine to form aggregates with fractal dimensionalities (decreasing T2). Gelation occurs when these growing aggregates form a percolating network, resulting in a minimum value of the T2 relaxation time. During and after these processes, rearrangeme nts of silica take place (aging), a crucial step in the formation of p ores. The aging of silica gel can be monitored by changing the relaxat ion behaviour (increasing T2). With SAXS a densification of the struct ure is indicated by an observed decrease in fractal dimensionalities a nd an increase in the radius of the primary building units, indicating a migration of silica from the tips of the aggregates towards the cen tre of the aggregates. Aging is a slow process compared with the aggre gation of the primary particles, but it can be accelerated by adding c atalytic quantities of fluorine or by preparation at higher pH or at h igher temperatures. By using spin-spin relaxation measurements, reacti ng aqueous silica systems with a low solid content could be studied in situ. It is shown that hydroxyl and fluoride ions have different impa cts on the silica systems, leading to different mechanisms. For aggreg ation and aging we observed (different) optimal fluorine concentration s.