Many people get confused between adsorption and absorption. The phenomenon of molecules being drawn to the surface of a solid or a liquid due to a higher concentration is called adsorption. In contrast, absorption is when one substance absorbs another substance by absorbing its energy. Adsorption can occur in different physical, natural, chemical, and biological systems. For example, rainwater condensing on the window of your car is an example of adsorption. 

There are two types of adsorptions: physisorption and chemisorption.

What is Physisorption?

• Physisorption is when molecules or atoms bond to the surface of a solid or liquid that they come into contact with under certain low temperatures. The force responsible for this process is called Van der Waals forces. 
• These weak, long-range bonds are achieved quickly and are not necessarily surface-specific on any particular substance but happen between gas molecules on any given surface. 
• The bonding process does not involve any reaction and offers overall quicker coverage at these lower temperatures but decreases as temperatures increase. 
• Physisorption is helpful for measuring the total surface area of materials in pore size analysis, mesopores, micropores, and nanopores because it attracts these gases to desired locations for analysis purposes more efficiently than thermally activated adsorption processes.
• Physisorption is also called physical adsorption.

Examples of Physisorption 

The adsorption of gases like hydrogen, nitrogen, and other gases on the surface of adsorbents, including charcoal, is an example of physisorption.

Characteristics of Physisorption:

• Physisorption is an exothermic process. However, weak Van der Waals forces of attraction have low enthalpy values (20– 40 kJ/mol).
• Physisorption also has very low activation energy, so we can say that physisorption is reversible.
• Physisorption is an exothermic process that happens more easily at lower temperatures and decreases as temperature rises.
• The amount of adsorption increases with increasing pressure as the volume of the gases reduces during adsorption in the case of physisorption of gases over solids.
• The volume of gases drops during adsorption, as does the amount of heat liberated. As a result, adsorption is more favourable at high pressures and low temperatures. Desorption is more favourable at low pressures and high temperatures.

Difference Between Physisorption and Chemisorption

Cause of Physisorption

Physical adsorption occurs due to physical attraction. Therefore, the binding energy for physisorption is weak, making it very easy for items to become desorbed from these bonds if enough heat or a high enough concentration of labile ions are introduced into their environment. Thus the physisorption is reversible.

Applications of Physisorption

• Generation of high vacuum:

To generate very high-level vacuum, vacuum pump evacuated the 

residual adsorbed by charcoal from a vessel.

• Gas masks: 

gas mask has activated charcoal surface or mixture of adsorbents which 

adsorb toxic gases.

• Control of humidity:

To removing moisture and controlling humidity Silica & aluminium gels 

are utilised as adsorbents.

• Eliminating of colouring matter from solutions:

To removing colour impurities of solution, Animal charcoal used as 

adsorbate.

• Heterogeneous catalysis:

There are several gaseous reactions for industrial importance including 

solid catalysts in which on the solid surface adsorption of R(reactant) 

takes place and the rate of reaction(ROR) increases due to the catalyst.

Example:

• Haber’s process i.e. preparation of ammonia by using iron as a 

catalyst.

• Preparation of H₂SO₄ with contact process.
• hydrogenation of oils by using finely divided nickel.

These are excellent examples of heterogeneous catalysis.

• Segregation of inert gases:

Some of inert gases like argon, krypton and xenon at different 

temperature, can be removed by adsorption on coconut charcoal. 

Because of the variation in the degree of adsorption of gases by charcoal. 

• In curing diseases:

Various drugs are used to kill germs by adsorption.

• Froth floatation process:

In this process sulphide ore mixed with pine oil and frothing agent and 

distilled by Segregation from silica.

• Adsorption indicators:

Precipitate like silver halides have tendency of adsorption of certain 

dyes like eosin, fluorescein, etc. therefore it will helpful for colour 

changing at the end point and used as indicators.

•  Chromatographic analysis:

On the basis of adsorption Chromatog is used to finds a number of applications in 

various analytical and industrial fields

Overview of Physisorption

• Many solids have surfaces not covered by other atoms, making them vulnerable to gravity and temperature. Van der Waals force imbalance creates a pressure differential between the surface atoms and the oppositely charged ions on their outermost shell, causing them to pull in gases, liquids, or small molecules that are cheaply available, like water vapour, for instance. 
• Atoms within porous solids are affected by many intermolecular forces, including covalent bonds and hydrogen bonding, among other types of van der Waals forces, which can cause an imbalance in the charged atoms located on the solid’s surface, similar to how they’re affected under atmospheric pressure. 
• The electrical and geometrical properties of the gas or fluid molecules are unaffected by their adhesion to the surface of a solid. However, the atoms may undergo a minor but reversible distortion. 
• Gas molecules are physically adsorbed on solid and fluid surfaces (adsorbents).
• Solid-to-solid adhesion can also occur when fluid molecules cling to solid surfaces.
• Surface areas and porosity of solid surfaces are determined using the physisorption impact.

Conclusion

In this article, we discussed that physisorption is a type of physical change in the state of the substance by transferring energy from one substance to another.

We also discussed some important features and examples of physisorption; the main difference between chemisorption and physisorption is that physisorption is the process by which molecules or atoms bond to the surface of a solid or liquid that they come into contact with under certain low temperatures, and chemisorption is a type of adsorption in which the molecules of the adsorbate are bound to the surface of the adsorbent by chemical forces.