Mass and Surface Fractal in Supercritical Dried Silica Aerogels Prepared with Additions of Sodium Dodecyl Sulfate

Silica wet gels were prepared from hydrolysis of tetraethoxysilane (TEOS) with additions of sodium dodecyl sulfate (SDS). The surfactant was removed after gelation. Wet gels exhibited mass-fractal structure with mass-fractal dimension D (typically around 2.25) in a length scale extending from a characteristic size ξ (typically about 10 nm) of the mass-fractal domains to a characteristic size a 0 (typically between 0.3 and 0.4 nm) of the primary particles building up the fractal domains. ξ increased while D and a 0 diminished slightly as the SDS quantity increased. Aerogels with typical specific surface of 1000 m2/g and density of 0.20 g/cm3 were obtained by supercritical drying of the wet gels after washing with ethanol and n-hexane. The pore volume and the mean pore size increased with the increase of the SDS quantity. The aerogels presented most of the mass-fractal characteristics of the original wet gels at large length scales and exhibited at a higher resolution level at about 0.7 nm a crossover to a mass-surface fractal structure, with apparent mass-fractal dimension D m ∼ 2.4 and surface-fractal dimension D s ∼ 2.6, as inferred from small-angle X-ray scattering (SAXS) and nitrogen adsorption data.