TRUE SURFACE-AREAS FROM NITROGEN ADSORPTION EXPERIMENTS

The surface area of amorphous silicon dioxide powders composed of monosized, compact, nonporous, spherical particles was determined by electron microscopy, It was shown that the BET evaluation of nitrogen adsorption isotherms in the relative pressure range of 0.05 less than or equal to p(e)/p(0) less than or equal to 0.23 gives the same surface area if a value of a(N-2) = 13.5 Angstrom(2) is used for the space requirement 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) evaluation on some other model surfaces. On type D surfaces the first layer is dense with a(N-2) = 13.5 +/- 0.1 Angstrom(2), on type L surfaces the first layer is loose with a(N-2) = 16.8 +/- 0.6 Angstrom(2). A possible structure is proposed for the dense as well as for the loose layer both being considered as bidimensional solids.