Adsorption Isotherm Relationship between pressure and temperature

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 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 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 0 oC 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.

Type:

I(a) : Materials with only microporous (e.g. Zeolite)

I(b) : Microporous materials (e.g. Activated carbon)

II : Non porous materials (e.g. Nonporous Silica and Magnetic powder) III : Non porous materials and materials which have the weak interaction between the adsorbate and adsorbent (e.g. Graphite/water)

IV(a) : Mesoporous materials (e.g. Mesoporous Silica and Alumina)

IV(b) : Mesoporous materials with pore diameters smaller than 4 nm (e.g. MCM-41)

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)

Generally, in nitrogen or Ar adsorption, the isotherm of Type I, II, and IV is mainly displayed

Reference

F. Rouquerol, J. Rouquerol and K. Sing, "Adsorption by Powder & Porous Solid", Academic Press, London, (1999).

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