92168.pdf

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, Fm, 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(pQ-p) versus p/p 0 gives a straight

line of slope (c— l)/Vmc and an intercept of l/Vmc on the p/V(pQ—p)

axis - i.e.

Vm = 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
particularly well at pressures between about 0.05 p 0 a°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 Vm 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 Vm can be
calculated on the basis of a single gas-adsorption measurement
(usually with p/p 0 between 0.2 and 0.3). This procedure is frequently
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