Characteristics of Physisorption

Adsorption is a surface phenomenon where the fluid molecule gets trapped in the surface of the solid. The adsorption process happens due to chemical bond formation or physical forces of attraction. Usually, this phenomenon is reversible and known as desorption. Some adsorption examples are silica and aluminium gel, charcoal gas mask and purification of water using alum. Adsorption of gas on a solid is an exothermic event that occurs spontaneously. The heat of adsorption is the amount of heat released when a unit mass of a gas is adsorbed on a surface. The two types of adsorption are physisorption and chemisorption.

Physisorption, also known as physical adsorption, is the process of adsorption in which the electronic structure of the atom or molecule is minimally altered. Physisorption is a type of adsorption where there is van der Waals force between the adsorbent and adsorbate. Physical adsorption can occur on any solid surface since the van der Waals interaction occurs between two molecules. Because intermolecular forces cause physisorption, the binding energy is small, resulting in low adsorption heat and a quick adsorption rate and desorption. Physical adsorption is reversible since the adsorbed substance is also easier to desorb. For example, adsorbed gas on activated carbon can be easily extracted without causing any environmental impacts.

Adsorption molecules (adsorbate) can come from various sources, including gases, liquids, and solids. Gas molecules can undergo physisorption on solid and liquid surfaces (adsorbents). Solid-to-solid adhesion can occur when liquid molecules cling to solid surfaces.

Characteristics of Physisorption

• Physisorption is caused due to the weak van der Waal’s force of attraction between the absorbent and adsorbate molecules.
• Because of the weak force, this is a reversible process.
• They have lower energy of activation (20kJ – 40kJ)
• There is no specificity in physisorption; all gas molecules get adsorbed on the solid surface.
• Physisorption is a multi-layered process.
• Requires a low enthalpy of adsorption.
• A low temperature favoured process.

Factors Affecting Physisorption

• Temperature: The extent of physisorption of gas increases with the decrease in temperature. The lower the temperature, the more will be the rate of physisorption.
•  Surface Area: The rate of physisorption increases with surface area. Surface area is directly proportional to the extent of physisorption. When the surface is is more, more particles will undergo adsorption. Porous materials and finely divided metals work well as adsorbents.
• Pressure: When pressure is increased, the volume of gas drops and more gas is adsorbed. As a result, gas can be removed from the solid surface by lowering the pressure. As the pressure increase, the adsorption also increases.
• The energy of Activation: Physisorption is a low energy process and hence does not require an appreciable amount of activation energy.
• Nature of Adsorbate: The more liquefiable gases or the easily soluble in water are adsorbed readily compared to others as the intermolecular forces are more prominent in the easily liquefiable gases. Thus, they are adsorbed more fastly.
• Nature of Adsorbent: The adsorption rate differs even under similar conditions for the same adsorbate. The porous and finely divided adsorbent molecules adsorb faster. The adsorbent with a rough surface also adsorbs more when compared to the smooth surface. 

Equilibrium is created between the adsorbate and the fluid phase in physical adsorption. Physical adsorption increases with rising gas pressure and usually decreases with increasing temperature in solid/gas systems at not too high pressures. When a system exhibits hysteresis, the equilibrium may be metastable.

Uses of Physisorption

Surface areas and porosity of solid surfaces are calculated using the physisorption effect. Unlike the area obtained by an object’s width and length, the surface area determined by this study is more complex, but it is an accurate measurement. The research considers all of the different regions created by ripples, pores, and other flaws.

In nanotechnology, material science, metallurgy and pharmaceuticals,   physisorption analysis is widely utilised in research, development, and quality control.

Examples of Physisorption

The hydrogen adsorption on finely dived nickel is the best example of physisorption. Another example is the adsorption of nitrogen on charcoal.

Conclusion

Adsorption is the accumulation of molecules on the surface rather than in bulk. Physisorption or physical adsorption is a type of adsorption. The intermolecular force that occurs between adsorbates and adsorbents causes physisorption. 

The physisorption is a reversible process due to the presence of weak interactions. The physical adsorption requires only low activation energy and low energy of adsorption. The method of physisorption is not specific, and it is also low temperature favourable. Physisorption forms a multilayer. 

The extent of physisorption of gas increases with a decrease in temperature, increase in surface area and pressure. Physisorption is employed in vare fields of science and research. The adsorption of hydrogen or nitrogen on charcoal is an example of physisorption.