FRACTAL ANALYSIS OF SOL-GEL DERIVED SILICA BY ADSORPTION

A microporous silica produced from hydrolysis-condensation of tetra-ethoxy silane has been used as a model adsorbent for fractal analysis involving the adsorption of n-alkanes of varying molecular size at 295 K. Its fractal dimension is deduced to be close to 3.0 or 2.2 depending upon the precise method of deducing the sizes of the CS-C, n-alkane adsorbates, while its accessible surface area detected by their adsorption is judged to be constant at 283 +/- 8 m(2) g(-1) or subject to a decrease of 10% with increasing alkane carbon number, again depending upon the sizes assumed for the adsorbate probes. Such surface areas are less than a half of that seen using N-2 adsorption at 77 K. Only for alkanes larger than n-octane is the accessible surface area certain to be decreased by increasing alkane carbon number; this is far larger than normally considered in fractal analysis. The potential of AFM analysis of fractal dimensions in such aggregate structures is also explored. (C) 1994 Academic Press, Inc.