The role of cations in the formation of aqueous silica gels

Abstract

The influence of alkali metal cations and polyvalent cations (i.e., Al 3+ Mg 2+) on the aggregation kinetics of aqueous silicate solutions is discussed in terms of the fractal properties of the silica aggregates formed. At pH = 4.0, alkali metal cations appear to have little or no effect on the aggregation kinetics and fractal dimensionality of the aggregates. Aluminum and magnesium on the other hand retard the aggregation process considerably. In this study, it is shown that aluminum gives rise to the most predominant effect as a result of the highest ionic charge. At low aluminum-cation concentration (Si/Al = 100) the aggregation process is inhibited by charge stabilization of the primary particles by selective adsorption of aluminum cations on the surface of oligomeric silicate anions. The aggregates initially formed at these low aluminum contents show very low fractal dimensionalities ( D ≊ 1). Increasing the aluminum content also increases the rate of aggregation, and aggregates with higher dimensionalities are observed. Homogeneous formation of amorphous, uncharged alumino-silicate oligomers prevents charge stabilization at the polymerization conditions chosen.