Solid Surfaces—Adsorption - Surface and Colloid Chemistry: Principles and Applications - page 119

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Hysteresis

Desorption

Adsorption

0

0 1

Relative Pressure

FIGure 5.8 Adsorption (Ns/Nsm = relative adsorption) versus pressure ( p/po = relative
pressure) of a gas on solid.

N s = K p (5.16)

where K is a constant. This is the well-known Henry's law relation, and it is found
to be valid for most isotherms at low relative pressures. In those situations where the
ideal Equation 5.16 does not fit the data, the van der Waals equation type of correc-
tions have been suggested.
The adsorption-desorption process is of interest in many systems (such as with
cement). The water vapor may condense in the pores after adsorption under cer-
tain conditions. This may be studied by analyzing the adsorption-desorption data
(Figure 5.8).

5.5.1.5 multilayer Gas adsorption
In some systems, adsorption of gas molecules proceeds to higher levels where mul-
tilayers are observed. From data analyses, one finds that multilayer adsorption takes
place (Figure 5.9).
The BET equation has been derived for multilayer adsorption data.
The enthalpy involved in multilayers is related to the differences, and was defined
by BET theory as

E BET = exp[(E 1 − Ev)/RT] (5.17)

where E 1 and Ev are enthalpies of desorption. The BET equation, after modification
of the Langmuir equation, becomes

p /(N s ( p o − p )) = 1/E BET N sm + [(E BET − 1)/(E BET N sm )( p / p o) (5.18)

A plot of adsorption data of the left-hand side of this equation versus relative pressure
(p/po) allows one to estimate N sm and E BET . The magnitude of E BET is found to give