SAXS INVESTIGATION OF SILICA AEROGELS OBTAINED FROM A SINGLE-STEP PROCESS BOOSTED BY FLUORINE ANIONS. Lucjan Pajak^a and Andrzej Jarzebski^b, Silesian University, Institute of Physics and Chemistry of Metals, Bankowa 12, PL-40007 Katowice, Poland^a, Institute of Chemical Engineering, Polish Academy of Sciences, Baltycka 5, PL-44100 Gliwice, Poland^b

The structure of aerogels depends on the rate of gelation (reaction of hydrolysis and condensation) which is markedly affected by the choice of catalyst. Fluorine anion catalysts dramatically accelerate this process but as yet have been used quite rarely in aerogel technology. A homogeneous series of aerogels synthesized using TEOS, EtOH and a complex catalyst comprising ammonium hydroxide and ammonium fluoride and dried at EtOH supercritical conditions, were investigated by means of the nitrogen adsorption method and SAXS. Since aerogels are hyperporous solids exhibiting disordered structure, the results were quantified using the fractal geometry approach. It was found that:

-aerogels obtained from the process exhibit polymeric rather than colloidal structure usually observed in aerogels obtained from the conventional basic process,

-primary particles are markedly smaller than observed in conventional basic aerogels,

-lighter samples show a pure mass fractal structure extending over a decade in the length scales from 40 Å upwards with D_m= 1.8-2.3 depending on water content,

-small water content leads to the rough surface fractal structure consistently with poisoned-Eden growth mechanism [1],

-mass/volume fractal characteristics obtained from SAXS and power-law behaviour of pore volume vs pore diameter differ substantially despite a certain similarity in values of D_m/D_V.

[1L] K.D. Keefer, D.W. Schaefer, Phys. Rev. Lett. 56, 2376 (1986)