Silica gel from water glass: a SAXS study of the formation and ageing of fractal aggregates

The influence of pH, cations, F^- and temperature on the aggregation, gelation and ageing of aqueous silica gels has been investigated using small-angle X-ray scattering (SAXS). At both low and intermediate pH (pH = 4 and 6-8, respectively), fractal aggregates are formed during gelation from silicic monomers or small oligomers. The fractal dimensionality is 2.20 (5), to be expected for reaction-limited cluster-cluster aggregates. During ageing silicic ions dissolve from the more soluble peripheral primary particles and condense in the crevices of the core of the aggregates. This process results in an increase in the size of the scattering primary particles and in a decrease in the fractal dimensionality of the silica aggregates from D = 2.25 to D = 2.0 or D = 1.85. The ageing processes are strongly enhanced by a slight increase in the pH of the solution, addition of fluorine anions and a rise in temperature. The aged silica gel is constructed of an ensemble of densified aggregates of which the density variation within the aggregates is larger than that of the freshly prepared aggregates. The primary particles of which the aggregates are built grow during ageing from molecular size to dense particles in the nanometer range, the ultimate radius depending on the type and duration of the ageing process.