EFFECT OF ALKALI-METAL CATIONS ON THE CHEMISTRY OF AQUEOUS SILICATE SOLUTIONS

Silicon-29 NMR spectroscopy was used to monitor changes in aqueous alkali-metal- (M-) silicate equilibria (in solutions with [MOH]:[Si(IV)] greater-than-or-equal-to 1:1) as the alkali-metal cations were complexed using cryptand 2.1.1 or 2.2.2. Peak displacements up to several ppm provide a quantitative indication of appreciable silicate-M+ ion-pairing which decreases as the mass of M+ is increased. The paired cations promote silicate condensation by weakening the electrostatic repulsion between anions. However, because strongly paired cations resist subsequent formation of a siloxane linkage, the level of condensation decreases systematically from M = Na to M = Cs. Paired M+ cations demonstrably stabilize several specific oligomers by supporting their open-framework structures. Electrostrictive water-structuring by lithium ions apparently causes additional polymerization.