All About Adsorption – Physisorption and Chemisorption

Adsorption is an exothermic reaction, which means that energy is emitted. Enthalpy is the quantity of heat generated when one mole of adsorbent is absorbed on an adsorbent. A decrease in enthalpy is a negative sign. The reason for this is that when adsorbate molecules are absorbed in the surface, their freedom of movement is constrained, resulting in a reduction in entropy. Adsorption occurs abruptly at constant temperature and pressure.

When solids absorb solvents, the majority of the solution is absorbed on the surface of the solid adsorbent, resulting in a concentration of the solution on the adsorbent’s surface that is greater than that of the bulk called positive adsorption.

In some situations, the adsorbent absorbs the solvent from the solution, causing the solution’s concentration to exceed the starting concentration called negative adsorption.

The physical interactions of gas molecules to a solid or liquid surface that comes into contact with a gas at low temperatures is referred to as physisorption. This is due to Van der Waals’ forces. Weak, long-range bonding occurs between all gas molecules on any surface and is not surface specific.

Physisorption:

During the physisorption process, a weak force termed the van der Waals Force causes gas to settle on the adsorbent. The Van der Waals Force is a concept that deals with molecular interactions that aren’t caused by chemical bonds.

Important characteristics of physisorption:

• This phenomenon is frequent and can occur in any solid/liquid combination, although specific molecular interactions can develop as a result of absorption and/or adsorption of geometric or electrical features.
• An absorbent species is chemically similar to those in the liquid phase; adsorption and subsequent dehydration have no effect on the fluid’s chemical composition.
• The forces of interaction between the molecules of adsorbate and adsorbent are in the same order of magnitude as the freezing point of adsorption, but it is often stronger.
• The adsorbate and the liquid phase form an equilibrium during physical absorption. Physical absorption range increases with increasing air pressure and decreases with increasing temperature in solid/gas systems without high pressure. 
• In the case of systems with repeatability, equilibrium may be metastable; 
• The molecules in the gaseous phase are absorbed more than those in direct contact with the surface under the right pressure and temperature.

Chemisorption:

Chemical absorption, also known as chemisorption, is a process that happens when adsorbate molecules form chemical bonds with certain surface regions on a substance, known as active sites.

Chemisorption, the forces at work are the same valence forces that act on chemical compound formation, such as adsorption with strong hydrogen bonds or weak charge transfer.

Important characteristics of chemisorption:

• Chemical specificity characterises this phenomenon.
• Changes in the electronic state can be detected using appropriate physical methods (for example, ultraviolet, infrared, or microwave spectroscopy, electrical conductivity, or magnetic susceptibility).
• Surface decomposition or reactivity may alter the chemical character of adsorption (s), preventing the original species from recovering after dehydration. In this respect, chemisorption may not be reversible.
• Chemisorption energy follows the same energy change sequence as the chemical reaction between solid and liquid: hence, chemical absorption, like chemical reactions in general, can be exothermic or endothermic, and the magnitudes of energy changes can vary. From the tiniest to the most colossal;
• Absorption has highly active energy (activated absorption), but actual equilibrium is slow or impossible to reach in the application. For example, the observed range of absorption in gases by solids at a certain period, at a constant gas pressure, increases with increasing temperature as well as within particular temperature ranges. Furthermore, where the activation energy for dehydration is considerable, chemisorbed species can only be removed from the surface under extreme circumstances of temperature or high vacuum, or by chemical treatment of the surface.
• Because valence bonds bind absorbed molecules to the surface, they usually occupy some absorption area on the surface, leaving only a layer of chemisorbed molecules.

The distinction between Physisorption and chemisorption:

Conclusion:

Adsorption is an exothermic reaction because it releases energy. When one mole of adsorbent is absorbed on an adsorbent, enthalpy is the amount of heat generated. A weak force known as the van der Waals Force causes gas to settle on the adsorbent during the physisorption process. Chemical absorption, also known as chemisorption, is a process in which adsorbate molecules form chemical interactions with active sites on the surface of a substance.