The structure directing effect of cations in aqueous silicate solutions - a Si-29-NMR study

Studies of the dissolution of amorphous silica gel in aqueous hydroxides by 29Si-MAS-NMR, give information on the oligomerisation of dissolved monomeric silicic acid and on the interaction between cations and the silica gel surface. The dissolution rate increases in the order (LIOH ˜ CsOH) <(RbOH ˜ NaOH) <KOH, indicating different promoting effects of the corresponding cations in the dissolution reaction. We attribute these differences to hydration properties of adsorbed alkali metal cations. Formation of silicate oligomers at high pH-values proceeds via cyclic trimeric silicate anions which, at high pH-values are more stabilised than the linear silicate anions. Further, the structure of highly polymerised silicate species depends on the alkali metal cation. Here we show for the first time that tetramethylammonium (TMA) hydroxide initially inhibits the dissolution of amorphous silica gel, which is caused by strong adsorption of rather hydrophobic TMA-cations on the silica gel surface. The effect of sodium/TMA-ratios on the dissolution rate was studied, showing that in the case where sodium is present, the rate of silica gel dissolution substantially increases. Studies on the reaction mechanism for the formation of cubic octameric silicate anions stabilised by TMA-cations are also reported.