Types of Adsorption

Adsorption, a term coined by a German scientist, Heinrich Kayser, in 1881, is commonly referred to as a surface phenomenon where the particles of gas, liquid, or solid form a layer over the top surface of the substance they are adsorbed at. Very similar to absorption, both these scientific concepts have a very prominent difference. The liquid or gas substance gets absorbed into another solid, liquid, or gas substance in absorption. When we talk about adsorption, the liquid, gas or solid forms a covering layer over the top surface of another solid, liquid or gaseous substance that does go all the way through. 

Adsorption:

When we talk about adsorption, it usually involves molecules, atoms, or even ions of dissolved gas, liquid, or a solid that form the top layer over the surface. The process of adsorption consists of two key terms, i.e. adsorbate and adsorbent; before moving on to the types of adsorption, let us get a brief about these two terms. 

Therefore, the adsorbent can be termed as the substance (either solid, liquid, or gas) on the surface of which adsorption occurs. 

On the other hand, The term adsorbate can be termed as the substances(primarily molecules, atoms, or ions of solid, liquid, or a gas substance) that get adsorbed on the solid surface(adsorbent) due to intermolecular attractions on the top layer.

Types of Adsorption:

Adsorption is divided into two types, i.e. Physisorption and Chemisorption. 

• 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. 

Here are some examples of physisorption:

Example 1: Adsorption of a layer of water vapour on the upper surface of silica gel

Example 2: Adsorption of various gases on the upper layer of animal charcoal

Characteristics of Physisorption: 

As mentioned, in physisorption, the upper layer of the adsorbent attracts the adsorbate with a very weak physical force. Now, let us know what all are the effects on physisorption when temperature, pressure, etc., come into play. 

1: Physical change: As we all know, there is no formation of chemical bonds on the upper surface of the adsorbent in physisorption. Thus, it simply goes through a physical change. Therefore, it is called physical adsorption or physisorption.

2: Non-specific nature: Another characteristic of physisorption is that as Van Der Waals’ force is universal and not associated with a particular type of substance, physisorption becomes non-specific in nature. 

3: Reversible nature: A very important characteristic of physisorption is that it is reversible in nature. With external pressure or temperature increase or decrease, physisorption can be easily reversed. 

4: Effect of temperature: When we talk about the effect of temperature on adsorption, physisorption generally decreases with an increase in temperature. 

This is because the energy of activation of physisorption is almost equal to zero. Therefore, even if the temperature is very low, the rate of adsorption remains unchanged.

5: Effect of pressure: Physisorption increases with an increase in pressure. This is because when on a surface the pressure increases, the molecules start to hit the surface with more intensity increasing the effect of physical forces on the upper surface of the adsorbent. 

• Chemisorption

Moving forward to the next type of adsorption, we get chemisorption. Chemisorption can be defined as if the molecules, ions, or atoms of gases, solids, or liquids are held on the upper surface of the absorbent by the strong chemical bond.

Here are a few examples of chemisorption: 

Example 1: Rusting of iron – The rust holds the upper surface of the iron because of a strong chemical reaction. 

Example 2: Haber Process – In this process, nitrogen combines from the air with hydrogen derived mainly from methane (natural gas) into ammonia.

Characteristics of Chemisorption: 

We have now studied the basics of chemisorption and some of its examples. Now, moving further, let us know the characteristics of chemisorption. : 

1: Chemical change: As the name suggests, chemisorption involves the formation of a chemical bond between the adsorbate and the adsorbent; thus, during the process of chemisorption, a chemical change occurs. 

2: Nature of adsorbate: Another important characteristic of chemisorption is that while chemisorption, the adsorbate should have the ability to form a chemical bond with the adsorbent.

3: Irreversible nature: Unlike physisorption, chemisorption is irreversible in nature due to irreversible chemical change and the formation of chemical bonds between the surface and the molecules. 

4: Effect of temperature: The process of chemisorption is exothermic in nature. Thus, it is favourable to low temperatures. As the temperature during chemisorption rises, The extent of adsorption first increases and then decreases.

5: Effect of pressure: Chemisorption increases with an increase in pressure. 

During the process of chemisorption, at higher pressure, the gas molecules get attracted towards the surface of the adsorbent and leverage the formation of chemical bonds.

Conclusion: 

With this article, we hope that you got a detailed view of the types of adsorption and their characteristics. There is a major distinction between chemisorption and physisorption: chemisorption is a kind of adsorption where the adsorbed material is kept in place by chemical bonds, whereas physisorption involves intermolecular interactions. Chemical concepts such as chemisorption and physisorption can be used to describe the adsorption mechanism of substances on a surface.