STRUCTURE OF SILICA-GELS

The structure of silica hydrogels has been studied by elastic light scattering. Like colloidal silica gels and neutrally catalyzed aerogels, these gels show mass fractal behavior at length scales below a crossover length-xi and scatter like a spatially random distribution of fractal objects for length scales > xi. Their fractal dimension D depends on xi and, unlike the other gels for which xi is-proportional-to phi-1/(D-3), xi-depends on both silica volume fraction-phi and gelation conditions. However, all our data are consistent with the correlation function <delta-phi(0)delta-phi(r)> = A-phi-2e-r/xi/(r/xi)3-D, with A = 1.8 +/- 0.13.