Physisorption

To understand Physisorption, we must understand the concept of Adsorption. Adsorption refers to the accumulation of molecules at a surface rather than in solid or liquid bulks. Adsorption is generally a phenomenon seen on surfaces. One good example of Adsorption is air turning dry when a silica gel is exposed to it.

What happens here is that molecules such as water or other atmospheric moisture in the air adsorb on the silica gel surface. Therefore, it can be said that during Adsorption. Molecules, atoms, or ions of gases of liquids settle on the surface of adsorptive materials.

Adsorption Mechanism 

The adsorption process or mechanism is exothermic. The meaning is that energy is generated and liberated in the process. In this process, the quantity of heat generated when one mole of adsorbate is adsorbed on an adsorbent surface is called enthalpy.

The heat generation or enthalpy change is depicted as negative. This is because when an adsorbate molecule is adsorbed on an adsorbate surface, the movement of molecules becomes restricted. The result is a decrease in entropy. Adsorption can occur spontaneously at constant room temperatures and pressure. 

Different types of Adsorption

The process of Adsorption can take place in different ways. Thus, it is divided into the following categories. These two categories are: 

• Physisorption
• Chemisorption

Physisorption

The process of Physisorption can also be called physical Adsorption. The Physisorption process is usually exothermic. The adsorption enthalpy for Physisorption is usually low. It starts from 20 kJ/mol and reaches 40 kJ/mol.

Generally, in the Physisorption process, a gas gets settled on the adsorbent because of the weak force called the Van der Waals force. The Van der Waals force is a principle that discusses molecular attractions that do not occur due to chemical bonds.

The Physisorption process does not have specificity. This is because the adsorbent, which is a surface where Adsorption happens, doesn’t exert gas. The Physisorption process is reversible. This means that the Physisorption of any gas by a solid material is reversible to solid by gas.

The Physisorption process is also dependent on the surface area available on the adsorbent. When the surface area increases, the adsorption extent will also increase. For example, metals, as well as porous substances that have large surfaces, can be considered good adsorbents. Physisorption can also depend on the nature and properties and the adsorbate.

Properties of Physisorption

1. Since any and all gas could be absorbed on the area of surface, there is no specificity. 
2. Highly liquefiable gases can be adsorbed strongly through Physisorption.
3. The Physisorption process is reversible.
4. The Physisorption process depends on pressure and temperature. When pressure is increased, there is a decrease in the volume of gas and thus causes Adsorption of gas molecules. In contrast, decreased pressure removes gas molecules from the adsorbate’s surface. 
5. Adsorption is an exothermic process. This causes Physisorption to occur promptly at low temperatures. The process slows down with increasing temperature. This observation is also known as Le-Chatelier’s principle.
6. Porous substances make better adsorbents and facilitate the Physisorption process. This is because an increased surface area is good for Adsorption. 
7. The Physisorption process needs no energy for its activation.

Uses of Physisorption

The effect of Physisorption is used to determine surface areas and the porosity of solid surfaces. The determined surface area is a true measurement, in contrast to the area determined by the width and length of an object. The process helps take into account all the additional areas due to ripples, pores, and other imperfections and provides a true measurement of surface area. 

Analysis done through Physisorption can be widely used in fields of research, development, and quality control of several industries such as:

• Nanotechnology
• Metallurgy
• Pharmaceutical
• Material science

Chemisorption 

The process of Chemisorption can also be called chemical Adsorption. During the chemisorption process, Adsorption occurs on substances that have chemical bonds. The chemisorption process has a high specificity, and it can only take place if there exists a chemical bonding between both the 2 materials. 

The chemisorption process is also irreversible. It occurs under high pressure. Further, in Chemisorption, because of chemical bonds, the enthalpy is high. It starts from nearly 80 kJ/mol and reaches 240 kJ/mol. Similar to Physisorption, Chemisorption is also dependent on the available surface area of the adsorbent.

A good example of the chemisorption process is the Adsorption of elements like hydrogen and nitrogen adsorbent materials such as ferrous catalysts.

Properties of Chemisorption

1. The chemisorption process is exothermic and thus results in an increase in temperature. 
2. The process is irreversible.
3. It is characterised by the chemical bonds between the gas molecules of the adsorbent surface. This results in a unimolecular layer formation. 
4. Chemisorption has a slow rate at low temperatures. In contrast, Chemisorption has a high rate when pressure is increased.
5. The chemisorption process is specific in character. This means that it only occurs if there exists a chemical bond formation between adsorbent material and the adsorbate material.
6. Similar to Physisorption, Chemisorption is also directly proportional to the surface area present in the adsorbate.
7. As Chemisorption includes chemical bond formation, it is observed that there is high enthalpy. 
8. The chemisorption process requires certain energy for activation, unlike Physisorption.

Uses of Chemisorption

The chemisorption process is used to evaluate the physical and chemical properties of materials. Chemisorption can also be used to analyse the rate of catalysis and other chemical reactions. 

These are relevant in several industries such as:

• Oil and gas
• Petrochemical and fine chemicals
• Environmental chemistry

Conclusion

Adsorption refers to an accumulation of molecules on a surface. There are two types of Adsorptions, Physisorption and Chemisorption. They both exhibit different properties and can be used in different processes. Some main differences amongst them are that Physisorption is not specific in nature, but Chemisorption is. Physisorption is a reversible process, but Chemisorption is not. Another difference is that Physisorption does not require activation energy while Chemisorption does. Lastly, Physisorption results in multimolecular layers, while chemisorption results in unimolecular layers.