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Seminar on Adsorption : Adsorption

Adsorption Isotherm

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


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