Cluster dynamics, growth and syneresis during silica hydrogel polymerisation

The aggregation and syneresis of silica particles during hydrogel polymerisation has been observed in situ for the first time with near-Å resolution using a new approach based on the combined fluorescence anisotropy decay of solvated and bound dye molecules. Primary particles of mean hydrodynamic diameter ∼1.5 nm are found to be present within 20 min of mixing sodium silicate solution and sulphuric acid. Clustering then occurs during siloxane polymerisation to produce after ∼30 h secondary particles with a mean diameter up to ∼4.5 nm at a growth rate which depends on silicate concentration and time to microgelation, tg. Subsequent condensation to ∼4 nm diameter occurs within 1 week as particle syneresis dominates. The effects on particle growth of adding D2O and inorganic salts are demonstrated.