STRUCTURAL CHARACTERIZATION OF CONCENTRATED ALKALINE SILICATE SOLUTIONS BY SI-29-NMR SPECTROSCOPY, FT-IR SPECTROSCOPY, LIGHT-SCATTERING, AND ELECTRON-MICROSCOPY - MOLECULES, COLLOIDS, AND DISSOLUTION ARTIFACTS

Sodium and potassium silicate solutions (water glass; molar ratios SiO 2:Na2O = 2.0 and 3.3, SiO2 content = 25 wt%; molar ratios SiO2:K2O = 3 .3 and 4.0, SiO2 content = 25 and 20 wt%, respectively) were prepared and characterized, e.g. by Si-29-NMR spectroscopy, FT-IR spectroscopy, Raman spectroscopy, or dynamic light scattering. One type of sodium s ilicate solution (molar ratio SiO2:Na2O = 2.0) was prepared by three d ifferent methods from different precursors (two glasses with different chemical compositions and quartz). The structure of these concentrate d alkaline silicate solutions is determined by the ''molecular'' and t he ''colloidal'' fraction of silica. For the investigation of the mole cular silicate components the Si-29-NMR spectroscopy and the FT-IR spe ctroscopy yielded the most valuable results. Dynamic light scattering allowed to detect minute amounts of artefacts (possibly remnants of th e dissolving species) of the production process with diameters > 100 n m. By filtering off these artefacts it was possible to characterize th e colloidal fraction of the sodium silicate solutions. Monomodal parti cle size distributions with maxima at 30 to 35 nm were found. The solu tions with the higher sodium contents have the smaller particles. Elec tron microscopy supported the light scattering results concerning the particle sizes and gave an impression that the major part of silica is present in colloidal form. The preparation method had almost no influ ence on the molecular structure of the alkaline silicate solutions. Th is and the monomodal size distributions of the colloids can be interpr eted by the assumption of a (metastable) equilibrium state.