Adsorption

Adsorption can be termed as a process where molecules, ions, or atoms of a solid, liquid or gas form a layer around the surface of a solid or a liquid. Coined by the great physicist Heinrich Kayser, the term adsorption was first used back in the year 1881. 

In simpler terms, adsorption is a process that involves the accumulation of any substance on the surface of a molecular species. The accumulation is done in higher concentration which makes it different from absorption, and the best example of adsorption is when we look at gases like hydrogen, nitrogen, and oxygen. These gases are adsorbed on activated charcoal and accumulate on its surface. 

How is adsorption different from absorption?

The basic difference between absorption and adsorption is that in absorption, the fluid is completely dissolved with the help of a liquid or a solid, also known as an absorbent. While on the other hand, adsorption is a process in which all the atoms, ions, and molecules of the substance are accumulated on the surface of the material.

Absorption is a bulk phenomenon and an endothermic process that is not affected by temperature. However, adsorption is a surface phenomenon and an exothermic process, thus favoured by low-temperature processes.

For adsorption, two basic components are required: 

• Adsorbent: Substances that are deposited on the surface of another substance, for example, the gases hydrogen, nitrogen, and oxygen
• Adsorbent: The surface of the substance on which the adsorbent forms a layer

Uses of adsorption

There are many uses of adsorption. Some of them are given below:

• Adsorption is used in the air conditioning units for cooling water
• Silica gel, another use of adsorption, is used to prevent moisture from damaging several electronic products and clothes
• Adsorption is also used in the pharmaceutical industry to leverage the exposure time of specific drugs
• Adsorbents are used to produce a non-stick coating on several surfaces

Types of adsorption:

Now we know about what are some of the uses of adsorption, let us take a look at what are types of adsorption. Adsorption is classified into 2 types:

1. Physisorption: Physisorption can be termed as the condition when the adsorbate is held onto the surface of the adsorbent by weak physical forces, i.e., van Der Waals force. Physisorption is also known as van der Waals adsorption. For example, adsorption of a layer of water vapour on the upper surface of silica gel and adsorption of various gases on the upper layer of animal charcoal.
2. Chemisorption: Moving forward to the next type of adsorption, when the molecules, ions, or atoms of gases, solids, or liquids are held on the upper surface of the absorbent by a strong chemical bond, it is known as chemisorption.For example, The most common example of chemisorption is rusting of iron. The rust holds the upper surface of the iron because of a strong chemical reaction.  

Importance of adsorption

The importance of adsorption can be noted from the few points mentioned below: 

1. In adsorption, many chemical reactions speed up because of the presence of adsorptive surfaces. For example, gases like oxygen and hydrogen are adsorbed on the surface of platinum black and are then combined at a very fast speed at ordinary temperature to form water.
2. An important factor of adsorption is that the surface, which helps in adsorption, also promotes the combination of en­zymes with substrates. This combination is responsible for giving us reaction products.
3. Adsorption that takes place on the membranes of any cell promotes quite a few vital chemical reactions. It also promotes changes in surface tension and cell consistency.
4. Lastly, the drugs and poisons which are adsorbed on the surfaces of a cell portray all their effects from the same location. 

Adsorption isotherms

Temperature plays a significant role in the adsorption process, and thus, adsorption is usually described by isotherms. Mentioned below are several isotherms that are used to describe the adsorption technique: 

Freundlich Theory 

During adsorption, Freundlich’s theory of adsorption isotherm is obeyed when the adsorbate forms a molecular layer on the surface of the adsorbent. Coined by Herbert Freundlich in 1909, this theory represents adsorption’s isotherm variation of a quantity of gas adsorbed by a solid surface. 

The Freundlich theory of adsorption isotherms has a major drawback. It fails at high-pressure situations and during the multi-layered adsorption over a solid surface. 

Langmuir Theory

According to the Langmuir theory, proposed in 1916 by the US Chemist Irving Langmuir, during the adsorption of gases, every solid surface has several elementary sites, each of which absorbs one particle of gas. 

Some assumptions that are considered while studying Langmuir’s theory of adsorption are:

i) It is assumed that all the adsorption sites considered to be on a solid surface are equivalent, and their ability to absorb a gas molecule is independent of whether their neighbouring site of equal length is occupied.

ii) It is assumed that between the adsorbed and non-adsorbed gas molecules, a dynamic equilibrium exists.