  Theoretical framework for the isosteric heat of adsorption is developed
treating the effects of the non-ideal gas behavior and the adsorbed phase
volume. Rigorous thermodynamic treatment for the adsorbed phase volume is
presented for multi-layer adsorption from low to high pressures. The proposed
model for the isosteric heat of adsorption along with the adsorbed phase volume
is validated and verified using experimental data for several, judiciously
selected adsorbent + adsorbate (nonpolar molecules) systems. The predictions by
the model exhibit excellent agreement with the experimental data for both
adsorption isotherms and the isosteric heat of adsorption.
