Adsorption isotherm is the relationship
between the pressure and adsorption amount at a constant temperature. The
horizontal axis is the relative pressure (P/P0) which is the
equilibrium pressure divided by the saturation pressure. The relative pressure
can be 0 to 1 and P/P0 =1.0 means that the condensation of
adsorptive occurs in the sample cell. So an adsorption isotherm is the
measurement of adsorptive density which becomes higher than the than the bulk
(gas) phase density due to the interaction between the adsorptive and solid
surface atoms below its condensation pressure. Adsorption amount in the
vertical axis is commonly expressed as V/ml(STP)g-1 which is expressed
by the standard gas volume (at 0oC and 1 atm).
The figure indicates the classification of
adsorption isotherms defined by IUPAC. The type of adsorption isotherm is
determined by the pore size and surface character of the material.
I : Microporous materials
(e.g. Zeolite and Activated carbon)
II : Non porous
materials (e.g. Nonporous Alumina and Silica)
III : Non porous
materials and materials which have the weak interaction between the adsorbate
and adsorbent (e.g. Graphite/water)
IV : Mesoporous
materials (e.g. Mesoporous Alumina and Silica)
V : Porous
materials and materials that have the weak interaction between the adsorbate
and adsorbent (e.g. Activated carbon/water)
VI : Homogeneous
surface materials (e.g. Graphite/Kr and NaCl/Kr)
