Volmer

144

ADSORPTION

Vol. 1

Fig. 2. Difference between Gibbs and layer model, ę, specific property; x, distance from adsorbent surface; ABC, Gibbs excess properties; and AEDC, layer model properties.

characterization. The process results in a decrease in the number of degrees of freedom of bound molecules and in a decrease in free energy.

The appropriate function is

AG=AH-TAS    (1)

It follows that AH must be negative when AG and AS are negative. Using the traditional sign convention in thermodynamics (i.e., AH is also negative), it can be concluded that the adsorption process is exothermic, while the desorption process is endothermic, regardless of the naturę of forces.

Two theoretically fundamental representations are possible (1): the layer model and the so-called Gibbs representation. The difference between the two is shown in (Fig. 2). The surface excess amount in the Gibbs representation is given by area ABC while the same in the case of the layer model is defined by area AEDC.

Isotherms

Adsorption isotherms are widely used for the quantitative characterization of adsorption. At eąuilibrium, the specific adsorbed amount is defined as

(2)

mte

n - -

mai

Adsorption from gas phase is not the same as adsorption from liąuid phase and the treatment too is different in the two cases. Many isotherm equations and models can be found in the literaturę. The main goal is to find appropriate expressions for

(3)

n = f(p) or n = f(p/po) or n — f(x)