The use of additives to prepare low-density xerogels

Low-density xerogels have been prepared by incorporation of an additive directly during the synthesis of the gel. The silica precursors (main reagent) were: tetramethylorthosilicate (TMOS), tetraethylorthosilicate (TEOS) and tetrapropylorthosilicate (TPOS) and the additives were: 3-(2-aminoethylamino)propyltrimethoxysilane (EDAS) and 3-aminopropyltriethoxysilane (AES). With EDAS as additive, a nucleation mechanism by the additive takes place and exactly the same properties (pore volume, specific surface area, particle and aggregate size) are obtained either with TEOS or TPOS as main reagent. The nucleation mechanism is related to the difference in reactivity between additive and main reagent. With the AES/TPOS series, pore volumes up to 17 cm3/g have been obtained with pore sizes up to nearly 10 μm. The particle size is more than 100 nm. The couple AES–TPOS seems not to give rise to nucleation. It is likely that the difference in reactivity between ethoxy groups and propoxy groups is not sufficient to generate the nucleation mechanism by the additive.