AGGREGATIVE GROWTH OF SILICA FROM AN ALKOXYSILANE IN A CONCENTRATED-SOLUTION OF AMMONIA

A monodisperse silica powder prepared by hydrolysis and condensation o f a tetraalkoxysilane in an alcoholic solution with a large concentrat ion of ammonia is characterized by beads of high and uniform density, a very small surface area measured with argon and a concentration of u ncondensed silanols which may be higher than that in most gels, with r espect to the sample mass (up to 6.2 mmol OH g-1, of which 29% are gem inal). Most of these hydroxyl groups are deuterated with D2O, although they are located inside the beads and despite the very small porous v olume of a bead. These properties are consistent with a growth mechani sm dominated by the aggregation of primary polyhedra Si10O12(OR)16 rat her than by the collapse of random polymeric clusters. It is suggested that the condensation of alkoxysilanols can be driven to such primary particles by step-by-step hydrolysis and condensation on account of t he large concentration of ammonia that determines a deprotonated state of most of the hydrolyzed species.