Types of Adsorption

Different types of Adsorption

Adsorption may be  classified into flowing types i.e. 
(a) On the basis of concentration
(b)On the basis of nature of force existing between adsorbate and adsorbent molecule

(a)On the basis of concentration 

On the basis of concentration of adsorbate, molecule adsorption can be of following two types
(1)Positive Adsorption
(2)Negative Adsorption

(1)Positive Adsorption:

  • If the concentration of adsorbate is more on the surface as compared to its concentration in the bulk then it is called positive adsorption.
  • For example, when a concentrated solution of KCL is shaken with blood charcoal, it shows positive adsorption.

(2)Negative Adsorption: 

  • If the concentration of adsorbate is less than its concentration in the bulk then it is called negative adsorption.
  • For example, when a dilute solution of KCL is shaken with blood charcoal, it shows positive adsorption.

(b)On the basis of nature of force existing between adsorbate and adsorbent molecule:

On the basis of nature of force existing between adsorbate and adsorbent molecule adsorption again of following two types i.e.
(1)Physical Adsorption or Vanderwaals Adsorption
(2)Chemical Adsorption or Activated adsorption

(1)Physical Adsorption or Vanderwaals Adsorption:

If the physical or Vander Waals’ force of attraction hold the adsorbate molecule to the surface of the adsorbent, it is termed as physical adsorption(physisorption). Since physical forces involve Vander walls’ forces it is,
  • Reversible
  • Involves physical forces
  • Appreciable only at low temperature below the boiling point of adsorbate
  • Not very specific
  • Cause multilayer adsorption
  • Generally has geat of adsorption less than 10 KCal/mol.
  • Due to the formation of multilayers, physical adsorption decreases after sometimes.

(2)Chemical Adsorption or Activated Adsorption:

If the chemical forces hold the adsorbate molecule to the surface of the adsorbent, it is termed as chemical adsorption(chemisorption). Since activated chemisorbent involves a high degree of specificity like chemical forces, the chemisorption is 
  • Irreversible
  • Involves transfer of electrons between gas and solid
  • Appreciable at high temperature 
  • Maybe rapid as well as slow
  • May involve activation energy in the adsorption process
  • Highly specific
  • Leads almost to monolayer
  • Generally has the heat of adsorption greater than about 20 KCal/mol

          

Adsorption

What is Adsorption? 

  • The phenomenon of adsorption was introduced by Scheele during the discovery of uptake of gases by charcoal.
  • The term adsorption was given by Kayser and this concept of adsorption was developed by Kayser and Raymonds.
  • When a solid surface is exposed to a gas or a liquid, molecules from the gas or liquid accumulates at the surface. The phenomenon of concentration of a gas or a liquid at a solid surface rather than in its bulk is called Adsorption.
  • Adsorption is caused by a type of Vanderwall’s force which exists between the molecule known as London Dispersion Force
  • The substance that concentrates at the surface is called adsorbate and the solid at the surface of which the concentration occurs is called adsorbent.

Example of Adsorption:

    • Finely divided charcoal if stirred into a dilute solution of methylene blue, the dye molecule are adsorbed by the charcoal particles and the depth of color of the solution decreases appreciably.
    • If gas like SO2 , Cl2 , NH3S is treated with powdered charcoal in a closed vessel, the gas molecule adsorbed on the charcoal surface and the gas pressure is reduced.
    • The common adsorbents are charcoal(vegetable and animal),  silica gel(prepared by heating a mixture of sodium silicate and 10% HCl at 50-degree centigrade), a metal such as Ni, Cu, Ag, Au, Pt, and colloids.