Formation and Structure of Gel Networks from Si(OEt)₄/(MeO)₃Si(CH₂)₃NR‘₂ Mixtures (NR‘₂ = NH₂ or NHCH₂CH₂NH₂)

Monolithic silica aerogels modified by amino-substituted organic groups with bulk densities between 0.07 and 0.16 g cm - 3 were prepared by sol−gel processing of Si(OEt) 4 /(MeO) 3 Si(CH 2 ) 3 NR‘ 2 mixtures (NR‘ 2 = NH 2 or NHCH 2 CH 2 NH 2 ), followed by drying of the wet gels with supercritical CO 2 . The Si(OEt) 4 /(MeO) 3 Si(CH 2 ) 3 NR‘ 2 ratio was varied between 9:1 and 6:4. The ≡Si(CH 2 ) 3 NR‘ 2 groups were completely incorporated in the aerogels in each case. Raman spectroscopic investigations showed that the precursors were rapidly consumed upon addition of water to the alkoxysilane mixtures. Gelling was slowed with an increasing portion of (MeO) 3 Si(CH 2 ) 3 NR‘ 2 . The structure of the aerogels was investigated by nitrogen sorption, small-angle X-ray scattering, and transmission electron microscopy. The polarity of the inner surface of the aerogels was rather low. The particle radii were in the range of 6.0−6.3 nm, the fractal dimension in the range of 1.3−1.6, and the BET surface area in the range of 300−430 m 2 g - 1, rather independent of the Si(OEt) 4 /(MeO) 3 Si(CH 2 ) 3 NR‘ 2 ratio. The only exception was an aerogel prepared from 90% Si(OEt) 4 and 10% (MeO) 3 Si(CH 2 ) 3 NH 2, which contains significantly larger primary particles and a smaller specific surface area. The results were interpreted that both precursor silanes are involved in the build-up of the aerogel network, contrary to results with organically substituted alkoxysilanes R‘Si(OR) 3 without basic properties.