The molecular basis of aging of aqueous silica gel

The influence of aging on the local structure of aqueous silica gels has been investigated using small angle X-ray scattering (SAXS), solid state nuclear magnetic resonance (NMR), and nitrogen sorption techniques (BET). From SAXS experiments it is deduced that aging of aqueous silica aggregates is a process of migration of active, dissolved silicate species (e.g., monomeric silicic acid) from the more soluble and less dense places within silica aggregates (i.e., the peripheral primary particles) to the less soluble and denser parts within the aggregates (i.e., the core of the aggregates). This process results not only in an increase in size of the scattering primary particles but also in an increase in the gradient of mass density, as such corresponding to a decrease in fractal dimensionality of the silica aggregates from = 2.25 to = 2.0. The rate at which aging processes occur is strongly effected by the pH value of the solution. Addition of fluorine anions to the polymerizing silicate solutions causes an increase in rate of restructuring as well. Nitrogen sorption measurements on freeze-dried samples of the gels show that a porous silica structure is only obtained in case the silica gel network has been reinforced through reorganization of the fragile aggregates. The aged silica gel consists of an ensemble of densified aggregates formed by primary particles with sizes in the nanometer range.