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Monday, December 21, 2009

Adsorption

1. Molecules on the surface of a liquid behave in a different way from molecules in the bulk.

2. This is because the molecules inside the liquid are attracted equally from all sides while the molecules on the surface are subjected to an unbalanced attraction from the liquid side which tends to drag the molecule inside the liquid.

3. The surface layer displays free energy or surface tension and has additional free energy over the rest of the liquid.

4. Similarly, attractive forces of atoms or molecules in the outermost layer of a solid are not fully utilised as in the interior of the solid.

5. The surface of a liquid is thus in a state of strain or unsaturation and that of a solid similarly has a residual or unbalanced field of force.

6. There will be a tendency for the free energy or surface tension to decrease.

7. It is this tendency which is ultimately responsible for the phenomenon of adsorption.

8. Liquid and solid surfaces tend to balance or satisfy the residual forces by attracting substances which they come in contact.

9. Thus adsorption is a surface phenomenon.

10. Adsorption should not be confused with absorption.

11. The former is solely a surface effect whereas, in the latter case the liquid or gas being absorbed penetrates into the capillary spaces of the absorbent.

12. The taking up of water by a sponge is absorption.

13. The concentration of alkaloid molecules on the surface of clay is adsorption.

14. When there is an increase in the concentration of a substance at an interface, it is called positive adsorption, and if there is a decrease in concentration, it is called negative adsorption.

15. When some charcoal powder is shaken with dilute solution of acetic acid, the latter is adsorbed.

16. The substance which adsorb is called the adsorbent (in this case charcoal) and the substance which is adsorbed is called the adsorbate or sorbate (in this case acetic acid).

17. Adsorption may be expected when two phases come in contact with each other.

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