Sol-gel particle growth studied using fluorescence anisotropy: An alternative to scattering techniques

The aggregation of silica particles during hydrogel polymerization has been observed in situ with angstrom resolution using the combined fluorescence anisotropy decay of solvated and bound dye. Primary particles of mean hydrodynamic radius approximately 1.5 nm are found to be present within 20 min of mixing sodium silicate solution and sulfuric acid. Clustering then occurs during siloxane polymerization to produce after approximately 30 h secondary particles with a mean radius up to approximately 4.5 nm at a growth rate that depends on silicate concentration and time to microgelation t(g). Subsequent condensation to approximately 4 nm radius occurs within 1 week as particle syneresis dominates.