Reactions between aluminate ions and orthosilicic acid in dilute, alkaline to neutral solutions

We studied the formation and polymerization of silicatoaluminate (SA) monomer ions in dilute, aqueous solutions of aluminate ions, orthosilicic acid, and sodium hydroxide by stepwise neutralization with hydrochloric acid. We inferred SA formation from the increase in the proportion of “monomer aluminum” and the decrease in removal of silicon and aluminum from the solution by polymerization, with increasing silicon-to-aluminum ratio. The products that were formed at 100 percent neutralization and retained on the Millipore filter were similar to allophane; they had silicon-to-aluminum ratios of 0.4 to 1.2 and appeared as hollow spheres with diameters of 30 to 50 angstroms, though there were some differences between them and allophane on the infrared spectra. The strong surface acidity of some products suggested that they contained four-fold coordinated aluminum.