The Effect of Hydrolysis Conditions on the Structure and Growth of Silicate Polymers

Abstract

ABSTRACTSmall angle scattering experiments have demonstrated that the structure of the silicate species produced by the hydrolysis of silicon alkoxides in non-aqueous solvents ranges from extended, weakly cross-linked polymers to highly condensed, colloidal particles. In contrast, inorganic, aqueous silicate solutions yield primarily colloidal particles because the silicate species have a number of different silanol sites available and the preferred condensation reaction is that of weakly condensed species with highly cross-linked branch sites, such as those on an amorphous silica surface. It is proposed that in the alkoxide systems, however, the hydrolysis reaction may control the number and type of silanol sites available for condensation. In acid catalyzed reactions, the rate of hydrolysis of a silicate tetrahedron tends to decrease as alkoxide groups are removed. This favors the production of silanol sites on the end of chains, thus generating linear polymers. In base catalyzed reactions, it is argued that each subsequent hydrolysis of a tetrahedron should proceed more rapidly than the previous one, producing numerous branch points which are the preferred sites for condensation.