Evolution of Sodium Silicate Sols through the Sol-to-Gel Transition Assessed by the Fluorescence-Based Nanoparticle Metrology Approach

The fluorescence-based nanosize metrology approach, proposed recently by Geddes and Birch (Geddes, C. D.; Birch, D. J. S. J. Non-Cryst. Solids 2000, 270, 191), was used to characterize the evolution of primary silica particles through the sol-to-gel transition and during aging of sodium silicate (SS) derived silica. In this study, the evolution of silica particles within SS derived silica was examined as a function of pH and glycerol doping through the sol-to-gel transition and up to 10 months after gelation. Time-resolved anisotropy decays were measured for the cationic dye rhodamine 6G, which was strongly adsorbed to the silica nanoparticles, and for the anionic probe pyranine, which provided accurate data on the microviscosity of the internal aqueous solution within sols and gels. The data provide evidence for the presence of nonaggregated primary particles far beyond the gelation point and even after prolonged aging of the resulting silica when aging is done either at low pH or in the presence of glycerol. Both the fraction of stable primary particles and the final size of the primary particles within the aged silica were dependent on the pH and the presence of glycerol. In general, lower pH values (pH 3.5) or the presence of glycerol increased the fraction of primary particles relative to samples at prepared at pH 6.5. Gelation pH did not have an affect on the final particle size, which was 1.5 ± 0.4 nm at both pH 3.5 and 6.5. On the other hand, a smaller particle size (0.9 ± 0.2 nm) was observed for SS sols and hydrogels containing 50% glycerol.