Wednesday, April 27, 2011

Adsorption

Definition:

“it is a phenomenon in which accumulation of a substance at the boundary on interface between the hetergenous phases takes place.”

Explanation:
It is difficult from absorption, as the absorption is the distribution of a substance throuth the bulk solution while adsorption is a surface phenomenon.

Sorption:
It is sometimes very difficult to define clearly the interface of highly porous solids, so for these system the term sorption is used as we cannot distinguish wether it is adsorption or absorption.

The substance that is attached to the surface of the solid is called adsorbate and the surface on which it gets adsorption is called adsorbent.

Occurance:
Adsorption can occur on following interfaces:

• Solid/Liquid
• Solid/gas
• Liquid/gas
• Liquid/liquid

Since adsorption is a surface phenomenon. The most effective adsorption are those with high surface area e.g. finely divided solids.

Positive adsorption:
Adsorption shows the ratio of a substance at the interface and the bulk phase if the concentration of the substance at the interface is greater. Than the concentration of the substance in bulk phase then it is called as positive adsorption.

Negative adsorption:
If the volume concentration of substance is higher than the concentration of bulk is known as negative adsorption.

Types of adsorption:
There are two types of adsorption:

Physical adsorption

Negative adsorption

1. Physical adsorption:
In physical adsorption the adsorbate is attached with adsorbent by Vander Waals or Electrostatic weak forces and it is characterized by low heat of adsorption.

Physical adsorption of gases is common at low temperature and high pressure. The gas in the adsorbent layer is in equilibrium with the gas molecule. In the bulk gas the equilibrium depends upon the nature of the adsorbent.

2. Chemical adsorption:
This involves the chemical combination of adsorbate at the surface of adsorbent. It is characterized by high heat of adsorption and unlike physical adsorption is irreversible. In many cases the chemical adsorption is slow because the molecule has to acquire an energy of interaction before they can react with the adsorbent, the rate of uptake will increase with increase of temperature.

Factors affecting the adsorption:

Solubility of adsorbate:
The adsorption is inversely proportional to the solubility of the adsorbate in the adsorbent.

Adsorption α 1/Solubility

pH:
it does not effect the adsorption directly pH of the solution affect the degree of ionization.

Usually the drug with a single molecule has more adsorption.

Nature of the adsorption:
Nature of the adsorbent have major effect on the adsorption by increasing the surface area, the adsorption rate could be increased. It can be increased by making it porous or finely divided.

Temperature:
Adsorption is an Exothermic process so increase in temperature will decrease. The adsorption and vice versa.

Pressure:
Adsorbed amount of adsorbate is directly proportional to the pressure applied.
Post a Comment