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Physisorption is caused by the intermolecular forces present between adsorbates and adsorbents. Adsorption is an integral part of surface chemistry. Physisorption is a type of adsorption. Adsorption is different from absorption. A solution is formed when solute particles are dissolved in the solvent. However, in the case of adsorption, the solute becomes adsorbate, and the solution becomes adsorbent. Adsorption can be defined as the selective transfer of solute particles in the fluid phase to the surface of a solid which is insoluble. Note that only the surface of the solid is concerned, not its full volume. Adsorption is an exothermic process. Physisorption is a type of adsorption in which the force acting between the adsorbate and adsorbent is Van der Waal forces.
Mechanism of physisorption
Physisorption is caused by the intermolecular forces present between adsorbates and adsorbents. The adsorption is known as van der Waals adsorption and the force is called the van der Waals force. Since van der Waals forces exist between any two molecules, physical advertising can occur in any solid environment.
Mechanism of adsorption
The atoms of the surface of a solid are affected by uneven forces called the residual attractive forces. Because of these forces, the atoms attract atoms of other gases, liquids or solids to even out their energy. The atoms whose energy levels are uneven constitute the adsorbent molecules, and the atoms attracted because of them constitute the adsorbate molecules. Adsorption occurs in this way.
The process of physisorption also takes place similarly. In this process, gas and liquid adsorbate fill out the pores of the solid adsorbent and are held together by the Van der Waal force of attraction. This process does not involve any surface reaction and can occur at a low temperature. It is a fast process. Gas-solid, liquid-solid and even solid-solid adherence is possible. During the process of physisorption, dissipation of heat or enthalpy occurs according to the formula:
ΔG= ΔH –TΔS
We know ΔG is the Gibbs free energy. ΔH is the change in enthalpy, and ΔS is the change in entropy of the adsorbate. There are some important points regarding this formula:
For the reaction to be spontaneous, Gibbs’s free energy ΔG is always negative.
ΔS is negative, and –TΔS has a positive value.
ΔH is also negative.
As the process gradually reaches equilibrium, ΔH eventually becomes less negative and becomes equal to the value of TΔS. As a result, ΔG becomes zero.
Difference between Chemisorption and Physisorption
The difference between chemisorption and physisorption are as follows:
In physisorption, molecules are attached to the surface of the adsorbent by Van der Waals’s forces of attraction.
In chemisorption, molecules are attached to the surface of the adsorbent by chemical bonds (ionic or covalent).
Physisorption is relatively weak.
Chemisorption is relatively strong.
In physisorption, the enthalpy heat is low and lies between 20-40 KJ MOL-1
In chemisorption, the enthalpy heat is high and lies between the range of 40-200 KJ MOL-1
Physisorption is reversible.
Chemisorption is irreversible.
In physisorption, multilayers of adsorbed particles are formed.
In chemisorption, a monolayer of adsorbed particles is formed.
Physisorption is not specific. Hence it may be found in all instances.
Chemisorption is specific. It only occurs when there is a chance of forming a chemical bond between The adsorbate and adsorbent molecules.
Physisorption only occurs at low temperatures. The extent of adsorption is decreased as the temperature of the surface rises.
Chemisorption occurs at all temperatures. And with the temperature rise, the extent of adsorption also rises.
Physisorption examples
As physisorption is a type of adsorption, its examples will be a part of the examples of adsorption. Some of the examples are:
Adsorption of various gases on the surface of charcoal is an example of physisorption. Because of this, the mask worn by miners contains charcoal, so when polluted air enters the mask, the heavy and harmful gases get absorbed by the charcoal.
This process makes the storage of hydrogen and other gases like methane easy. Physisorption of hydrogen on the surface of carbon nanostructures provides a fast and low-pressure storage facility. It is also easily reversible. Thus the gases can be released easily.
Various molecular interactions take place on the surface of a solid. If we adhere noble gases to the surface of the solid by physisorption, we can explore those molecular interactions.
Conclusion
Physisorption is a type of adsorption. Adsorption can be defined as the selective transfer of solute particles in the fluid phase to the surface of a solid which is insoluble. Physisorption is a type of adsorption in which the force acting between the adsorbate and adsorbent is Van der Waal forces. The principle of adsorption is put to use in pollution masks to adsorb dust particles..
