Small-angle x-ray scattering study of the fractal morphology of porous silicas

Small-angle x-ray and neutron scattering measurements have shown that on a length scale smaller than the average pore diameter but larger than the diameters of atoms or small molecules, the pore surfaces in four commercial porous silica gels with average pore diameters ranging from approximately 200 to 2500 Å are fractal and have a fractal dimension D = 2.15 ±0.10. When these gels were manufactured, the nonequilibrium micropore structure was relaxed by thermal methods. The scattering data indicate that in the gels with average pore diameters of about 200 and 500 Å, and perhaps also in the two gels with larger average pore diameters, the relaxation process leads to a pore structure nearly identical in form but on a larger scale than the structure in a gel with an average pore diameter of 60 Å that was the material from which the other four gels were produced.