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An adsorption is a form of the surface phenomenon in which the atoms, ions, and molecules from a gas or a liquid create a film of the adsorbate on the surface of the adsorbent by adhesion; the atoms and molecules get attached to the surface during adsorption. The concept of adsorption can be defined as the accumulation of molecular species on the material’s surface rather than the bulk; for example, silica gel acts as an adsorbent for water vapours. Adsorption is of two types; physisorption and chemisorption. The article describes the phenomenon of adsorption, physisorption, characteristics of physisorption, and physisorption potential.
Adsorption – Overview
The capability of solid elements to get attracted to the surface molecules of gases and solutions in contact is known as adsorption. For instance, charcoal is an adsorbent present in the gas marks that removes impurities and poison from the air. Adsorption is of two types, chemical and physical adsorption.
Adsorption Characteristics
- The process of adsorption implies the existence of a higher concentration of a gas or liquid on a surface.
- It indicates the free energy level decrease present in any system; the adsorption equilibrium can be achieved when the change in free energy is 0.
- Physical adsorption is an exothermic process implying that energy is given out/liberated. The amount of heat evolved when one mole of adsorbate gets adsorbed on the adsorbent is the system’s enthalpy and is one of the main characteristics of physisorption.
What is Physisorption?
Physical adsorption or physisorption is an exothermic process with low adsorption enthalpy, and the adsorbate is adsorbed due to the weak Van der Waal forces. This process of adsorption can be thoroughly understood by the characteristics of physisorption explained in the subsequent section. Physical adsorption is a reversible process implying that the adsorption of a solid can reverse the adsorption of a gas by a solid. This process depends upon the surface area of the adsorbent; as we see an increase in the surface area, the adsorption also increases. For instance, finely grained metals or porous materials are suitable adsorbents since they have a large surface area. However, the physical adsorption also depends on the nature of the adsorbate on which the molecular species accumulate. It increases with a decrease in temperature, while chemical adsorption increases as the temperature increases.
Characteristics of Physisorption
- Physical adsorption is reversible and depends on the pressure and temperature; it increases as pressure decreases. The decrease in pressure causes the removal of the gas molecules from the adsorbate surface.
- Physical adsorption is an exothermic process and decreases as the temperature rises as per Le-Chatelier’s principle.
- In physical adsorption, any gas can get adsorbed on the adsorbate. Moreover, porous materials are better adsorbents due to a larger surface area that increases adsorption.
- Physical adsorption does not require activation energy; highly liquefiable gases can be adsorbed more strongly in physisorption.
Physisorption Potential
As the molecules in the adsorbate move closer to the material on which they are adsorbed, the wavefunction of the electrons starts overlapping with that of the atoms present on the surface. However, this increases the system’s energy because of the orthogonality of the wavefunctions of both adsorbate and adsorbent atoms. These influence the surface interactions due to repulsion, resulting in minimum energy of physical adsorption by balancing the Van der Wall attraction and the Pauli repulsion to calculate the physisorption potential.
Conclusion
Adsorption is the process of the accumulation of atoms, ions, and molecules on the surface of a liquid or solid material. The former is the adsorbate, while the latter is known as the adsorbent. The adsorption process is of two types: physical and chemical adsorption. The physical adsorption or the physisorption is an exothermic reversible process occurring in the presence of weak Van der Waals forces.
