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

A monodisperse silica powder prepared by hydrolysis and condensation of a tetraalkoxysilane in an alcoholic solution with a large concentration of ammonia is characterized by beads of high and uniform density, a very small surface area measured with argon and a concentration of uncondensed silanols which may be higher than that in most gels, with respect to the sample mass (up to 6.2 mmol OH g-1, of which 29% are geminal). Most of these hydroxyl groups are deuterated with D2O, although they are located inside the beads and despite the very small porous volume of a bead. These properties are consistent with a growth mechanism dominated by the aggregation of primary polyhedra Si10O12(OR)16 rather 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 the large concentration of ammonia that determines a deprotonated state of most of the hydrolyzed species.