Twelve samples of silica aerogels were synthesized using different con
centrations of catalysts (NH4F, NH4OH), and their morphology was inves
tigated by means of small-angle X-ray scattering (SAXS), N-2 adsorptio
n, and transmission electron microscopy. Both SAXS and the nitrogen ad
sorption method consistently show a different influence of the catalys
ts; fluoride anions promote formation of a tenuous branched-polymeric
structure, with mass fractal dimension D-m similar or equal to 2.3-2.5
, while addition of hydroxyl ions results in a sparser structure (D-m
similar or equal to 2.1-2.2) self-similar over a narrower range. The m
orphology of aerogels obtained from the process catalyzed by fluoride
hydroxide resembles that of aerogels produced by the two-step method.
This may suggest similar mechanisms of gel formation in both processes
, and,consequently, a dramatic acceleration of the hydrolysis reaction
caused by NH4F.