Composition-structure relations in organically modified silica gels

Monolithic silica aerogels modified by functional organic groups were prepared by base-catalyzed sol-gel processing of Si(OR)(4) / R'Si(OMe)(3) mixtures (R' = organofunctional group), followed by drying of the wet gels with supercritical CO2. When the functional organic group has only weakly or no basic properties, the microstructure of the obtained aerogels is similar to that of an unmodified silica aerogel prepared under the same conditions and quite independent of the kind of functional group. The experimental findings are explained by a two-stage process in which the R'Si equivalent to units condense to a pre-formed gel network obtained by hydrolysis and condensation of Si(OR)(4). An increasing portion of R'Si(OMe)(3) has the same effects on the hydrolysis and condensation reactions as decreasing the bulk density of an unmodified silica aerogel and the same structural consequences as increasing the water / silane ratio and the catalyst concentration acting on Si(OR)(4). This leads to larger primary particles and, associated with that, smaller specific surface areas. The two-stage process is not observed when R' contains a strongly basic substituent such as NH2 or NHCH2CH2NH2. The structural parameters indicate that in these cases both R'Si(OMe)(3) and Si(OR)(4) are involved in the built-up of the gel network. This can be explained by extensive hydrogen bonding between the amino groups and silanol groups.