TRUE SURFACE-AREAS FROM NITROGEN ADSORPTION EXPERIMENTS

The surface area of amorphous silicon dioxide powders composed of mono sized, compact, nonporous, spherical particles was determined by elect ron microscopy, It was shown that the BET evaluation of nitrogen adsor ption isotherms in the relative pressure range of 0.05 less than or eq ual to p(e)/p(0) less than or equal to 0.23 gives the same surface are a if a value of a(N-2) = 13.5 Angstrom(2) is used for the space requir ement of the nitrogen molecule in the first adsorbed layer instead of the generally applied 16.2 Angstrom(2). On the basis of this nitrogen cross sectional area, parameters are given for the BET and alpha(s) ev aluation on some other model surfaces. On type D surfaces the first la yer is dense with a(N-2) = 13.5 +/- 0.1 Angstrom(2), on type L surface s the first layer is loose with a(N-2) = 16.8 +/- 0.6 Angstrom(2). A p ossible structure is proposed for the dense as well as for the loose l ayer both being considered as bidimensional solids.