Anion Distributions in Sodium Silicate Solutions. Characterization by29SI NMR and Infrared Spectroscopies, and Vapor Phase Osmometry

Abstract

Soluble silicates are complex mixtures of silicate anions. The larger anions are two- or three-dimensional condensation products of silicate monomer, SiO4-4. In more siliceous silicate solutions, i.e., SiO2:Na2O ratio >2.0, some of the silicate condenses to polymeric (colloidal) silica. Negative charges on the anions are balanced by protons or by cations, typically alkali metal or quaternary alkylammonium cations. The distribution of these anions varies with the concentration of dissolved silicate solids, the relative molar concentrations of cations and silica, and to a lesser extent, trace impurities. 29Silicon NMR spectroscopy of silicate solutions has been a powerful method for studying the connectivity of silicon and oxygen atoms in silicate solutions. In this paper we will show how variations in silicate band shape and peak location in the mid-infrared spectrum from 700 to 1300 cm-1 depend on concentration and silica:alkali ratio. We will interpret the infrared data in consonance with NMR results. We will also relate these variations in anion distribution to the average molecular weight of silicate solutions.