High-temperature and low-temperature supercritical drying of aerogels - structural investigations with SAXS

Base-catalyzed silica aerogels are composed of particles with a mean size of about 5 nm, which form a chain-like porous network. Up to now it has been assumed that supercritical drying (SCD) of highly porous gels in autoclaves leaves the structure nearly unchanged. Small-angle X-ray scattering (SAXS) measurements provide evidence that this is only true for the low-temperature CO₂ drying process with a critical temperature T_c = 304.2 K and a critical pressure p_c = 73.9 × 10⁵ Pa. In the high-temperature methanol process (T_c = 512.5 K and p_c = 80.9 × 10⁵ Pa) with remaining water and catalyst, however, structural changes are introduced. The SAXS measurements can be explained by a narrowing of the particle-size distribution during the heating period of the autoclave process. In a double-logarithmic representation of the scattered intensity I versus the scattering vector q, intermediate slopes smaller than -4 in the Porod region as well as oscillations in an Iq⁴ vs q plot appear. On the contrary, SCD hardly affects the particle structure of acid-catalyzed gels and, if the basic solvent is exchanged for pure methanol, of base-catalyzed gels.