Stucturall characterization of concentrated alkaline Silicate Solutions by ²⁹Si-NMR spectroscopy, FT-IR spectroscopy, lightscattering, and electronmicroscopy - molecules, colloids, and dissolution artefacts

Abstract

Sodium and potassium Silicate soludons (water glass; molar rados SiO₂: Na₂O = 2.0 and 3.3, SiO₂ content = 25 wt%; molar rados SiO₂: K₂O = 3.3 a n d 4.0, SiO₂ content = 25 a n d 20 wt%, respectively) were prepared and characterized, e.g. by ²⁹Si-NMR spectroscopy, F T - IR spectroscopy, Raman spectroscopy, or dynamic light scattering. One type of sodium Silicate Solution (molar ratio SiO₂: Na₂O = 2.0) was prepared by three different methods from different precursors (two glasses with different chemical compositions and quartz). The structure of these concentrated alkaline Silicate soludons is determined by the "molecular" and the "colloidal" fraction of silica. For the investigation of the molecular Silicate components the ²⁹Si-NMR spectroscopy and the FT-IR spectroscopy yielded the most valuable results. Dynamic light scattering allowed to detect minute amounts of artefacts (possibly remnants of the dissolving species) of the production process with diameters > 100 nm. By filtering off these artefacts it was possible to characterize the colloidal fracdon of the sodium Silicate soludons. Monomodal particle size distribudons with maxima at 30 to 35 nm were found. The Solutions with the higher sodium contents have the smaller particles. Electron 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 influence on the molecular structure of the alkaline Silicate Solutions. This and the monomodal size distributions of the colloids can be interpreted by the assumption of a (metastable) equilibrium State.