APPARENT LOW SURFACE-AREAS IN MICROPOROUS SIO2-XEROGELS

Nitrogen adsorption at -195 degrees C serves as routine method for the measurement of surface area of porous materials derived by sol-gel pr ocedures. In a systematic study of the simultaneous effect of the pH a nd water-silane ratio of the starting solutions of preparation of SiO2 xerogels on their surface areas, it was found that conditions of high acidity and low water/silane ratio result in microporous materials, t he surface of which cannot be accessed by N-2 at -195 degrees C, withi n a realistic time scale, but only by CO2 at 0 degrees C. In a typical result for TMOS/water/methanol = 1/2/3 at pH = 1.0, the apparent N-2- area was zero, while the CO2-area was 414 m(2)/gr. It is recommended t o recheck apparent N-2-low surface areas of sol-gel materials by CO2 a dsorption. The behavior of pyrene-doped xerogels of this type is in ag reement with the structural characterization by adsorption.