Page 145 - Introduction to Colloid and Surface Chemistry

The solid-gas interface 135

600

0.2 0.4 0.6 0.8 1.0 0.1 0.2 0.3 0.4 0.5
P/PO P/PO

Figure 5.13 Isotherm and BET plot for the multilayer adsorption of nitrogen on a
non-porous sample of silica gel at 77 K

effective area occupied by each adsorbed nitrogen molecule at
monolayer capacity can be calculated from the density of liquid
3
nitrogen (0.81 g cm" ) on the basis of a model of close-packed
20
2
spheres. The value so obtained is 16.2 x 10~ m . This calculation
would seem to be oversimplified, but the effects of the approximations
170
involved are largely self-cancelling . Adoption of this value leads to
BET areas for non-porous solids which are generally in accord with
corresponding surface areas determined by other techniques.
With most other adsorbates, adjusted molecular areas (determined,
for example, by calibration with nitrogen adsorption data) which are
usually in excess of those calculated from liquid densities must be
used, and, moreover, for a given adsorbate, this adjusted value
usually varies from solid to solid. This is due mainly to a certain
amount of localisation of the adsorbed gas in the first monomolecular
layer with respect to the variously distributed potential energy
minima of the solid lattices. To avoid excessive localisation, a low
value of — A/fi is desirable; however, a high value of c (i.e. -A//i s>
-A/f L) is also required to give a well-defined point B. The main