Preparation of low-density xerogels by incorporation of additives during synthesis

Low-density xerogels were prepared by incorporation of an additive to alcogels prior to gelation. The additives studied are 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS), 3-aminopropyltrimethoxysilane (AMS), propyltrimethoxysilane (PMS), tetramethylorthosilicate (TMOS) and 3-aminopropyltriethoxysilane (AES) using tetraethylorthosilicate (TEOS) as main silica precursor. Samples were also prepared with EDAS as additive and TMOS as main silica reagent. When the additive contains methoxy groups, it reacts first, forms nuclei on which the main reagent TEOS reacts to form the silica particles. The nucleation mechanism by the additive occurs only in case of a difference of reactivity between additive and main silica precursor. The other group of the additive (amine, alkyl group, …) influences only the gelation time. In case of ethoxy groups (series AES/TEOS) or methoxy groups (series EDAS/TMOS) for both additive and main reagent, there is no nucleation by the additive.