| dc.description.abstract | Silicon-29 nuclear magnetic resonance (NMR) spectroscopy has been
used to investigate the equilibrium distribution and kinetics of silicate oligomers
in aqueous tetraalkylammonium silicate solutions. Certain tetraalkylammonium
cations (particularly tetramethylammonium cation) stabilize specific silicate
oligomers, such as the cubic octamer and prismatic hexamer, by forming a
protective shell that shields these oligomers from interaction with paramagnetic
ions and impedes Si-Si chemical exchange. For tetramethylammonium silicate
solutions, the dependence of the initial rate of cubic octamer formation on
alkalinity, silica concentration and tetramethylammonium cation concentration
has been investigated and a mechanism for the formation of cubic octamer
proposed.
For tetraalkylammonium silicate solutions with OH~:Si < 1:1 which
favour the cubic octamer, at least four resonances which occur down-frequency
of the cubic octamer resonance have been attributed to stable protonated
states of the cubic octamer.
Alkali-metal cations added to tetramethylammonium silicate solutions
destabilize the cubic octamer probably by displacing tetramethylammonium
cations which surround the oligomer. | |
