Morphology of silica aerogels obtained from the process catalyzed by NH₄F and NH₄OH

Twelve samples of silica aerogels were synthesized using different concentrations of catalysts (NU₄F, NH₄OH), and their morphology was investigated by means of small-angle X-ray scattering (SAXS), N₂ adsorption, and transmission electron microscopy. Both SAXS and the nitrogen adsorption method consistently show a different influence of the catalysts; fluoride anions promote formation of a tenuous branched-polymeric structure, with mass fractal dimension D_m @#2242; 2.3-2.5, while addition of hydroxyl ions results in a sparser structure (D_m @#2242; 2.1-2.2) self-similar over a narrower range. The morphology 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.