Page 144 - Introduction to Colloid and Surface Chemistry

134 The solid-gas interface
(Figure 5.6) shows at least two distinct steps, each corresponding to
the formation of an adsorbed monomolecular layer. In most cases of
multilayer physical gas adsorption, however, the adsorption energies
are such that there is a greater or lesser tendency for .the first
monomolecular adsorbed layer to be completed prior to any
adsorption into the second monolayer, but little tendency for the
second monolayer to be completed prior to adsorption into the third
and subsequent monolayers.

Surface areas

The monolayer capacity, F m, is a parameter of particular interest,
since it can be used for calculating the surface area of an adsorbent if
the effective area occupied by each adsorbate molecule is known.
If the BET equation is applicable to a multilayer physical
adsorption isotherm, a plot ofp/V(p Q-p) versus p/p 0 gives a straight
line of slope (c— l)/V mc and an intercept of l/V mc on the p/V(pQ—p)
axis - i.e.

V
m = i (5.13)
slope + intercept
Using the appropriate gas equation, the monolayer capacity can be
calculated in terms of adsorbed molecules per unit mass of adsorbent.
Although the BET equation is open to a great deal of criticism,
because of the simplified adsorption model upon which it is based, it
nevertheless fits many experimental multilayer adsorption isotherms
a
particularly well at pressures between about 0.05 p 0 °d 0-35 p 0
(within which range the monolayer capacity is usually reached).
However, with porous solids (for which adsorption hysteresis is
characteristic), or when point B on the isotherm (Figure 5.5) is not
very well defined, the validity of values of V m calculated using the
BET equation is doubtful.
With a small loss of accuracy, the straight-line BET plot (Figure
5.13) can be assumed to pass through the origin and V m can be
calculated on the basis of a single gas-adsorption measurement
between 0.2 and 0.3). This procedure is frequently
(usually with p/p 0
adopted for routine surface area measurements.
The adsorbate most commonly used for BET surface area
determinations is nitrogen at 77 K (liquid nitrogen temperature). The