The elementary mechanistic model of adsorption and sorption is based on a simple hypothesis: the adsorption sites are uniformly distributed on the surface of the pore walls in the adsorbent

the sorption sites are uniformly distributed in the volume of the polymer. In this first paperwewill analyze the simple casewhere one solute molecule is only allowed to occupy a single adsorption or sorption site. A common elementary occupation law of the free sites is assumed: the differential increase of the number of the adsorbed/sorbed molecules is proportional to the differential increase of the activity of the solute and the concentration of the free (non-occupied) sites in the solid. The proportionality coefficient is called affinity coefficient depending on the solid/solute couple and on the temperature and independent of the concentration of the solute. In adsorption the concentration of the free sites is a surface concentration on the porewalls and in sorption it is expressed by themolarity. The simple monolayer adsorption lawof Jovanovi? is obtained: n=n

0

(1-e

-KP

)where n is the number ofmoles adsorbed when the pressure is P. n

0

is the total number of a

dsorption sites and K the affinity coefficient for adsorption. The sorption law writes: a = 1/k [φ/(1-φ)

]

+ (1-r)/k ln[1 + 1/r φ/(1-φ)

]

where φ

r and k hold respectively for the volume fraction of the solvent in the polymer

for the ratio of the molar volumes of the solvent to the elementary polymer chain containing one single adsorption site and for the sorption affinity coefficient. The confrontation of these equations to experimental isotherms is satisfactory in comparison with the classical Langmuir and Flory-Huggins equations: the best results are obtained for adsorption of vapors on a 5A zeolite and for all analyzed sorption results.