Physisorption and their Characteristics

The physisorption process is also known as physical adsorption. It depends on the geometry of the adsorbent and the temperature of the adsorption. It is the physical bonding of the gas molecules to a surface that encounters the low temperature gas. Generally, it is due to van der Waals forces. It takes place between all the gas molecules on any surface. When the process does not involve any surface reaction, it can be achieved swiftly at low temperatures. The process of physisorption involves mechanically locking up molecules in other solid materials. Activated charcoal is commonly used in this process. 

Physisorption plays a very important role in nature. It is used to measure the total surface area and to analyse the pore size of mesopores, micropores, and nanopores.

What is Physisorption?

The physisorption is typically adsorption by van der Waals force that is a weak intermolecular attraction taking place below the critical temperature of the adsorbate. It results in the development of a monolayer or multilayer. In short, it is the accumulation of molecular species at the surface rather than in the bulk of a solid or liquid. The molecular substances that accumulate at the surface are called an adsorbate. The surface area on which the physical adsorption takes place is called an adsorbent.

For instance, the van der Waals forces have attraction between the surfaces and the foot-hairs of geckos that provide the ability to climb up the vertical walls. These forces are formed by the interactions between induced, permanent, or transient electric dipoles. Even though the van der Waals has an attractive interaction, the electron starts to overlap with the surface atoms as the adsorbed atom moves closer to the surface of the wavefunction. Later, the system energy will keep on increasing due to the orthogonality of wavefunctions of the approaching atom and surface atoms.

The physical adsorption technique is widely used in different areas such as nanotechnology, metallurgy, pharmaceuticals, and material sciences.

Characteristics of Physisorption

• Lack of specificity

The van der Waals’ force is universal. Hence, a given surface of an adsorbent does not show any preference for a particular gas.

• Adsorbate’s nature

The total gas being adsorbed by a solid depends on the nature of the gas. The van der Waals’ forces are much stronger near the critical temperature surface areas. That is why the gases with higher critical temperatures get adsorbed first. Hence, in comparison with methane which has a critical temperature of 190K, activated charcoal easily adsorbs more sulphur dioxide with the critical temperature of 630K.

• Reversible nature

Generally, the physisorption of a gas by a solid is reversible. Hence, it can be shown as:

Solid + Gas  ↔ Gas/Solid + Heat 

According to Le–Chateliers’s principle, the gas volume decreases and can also be removed by reducing the pressure. Hence, when pressure is increased, more gas is adsorbed. The whole adsorption process is somewhat exothermic. This is why physisorption takes place at a low temperature and decreases with the increasing temperature.

• The surface area of the adsorbent

The proportion of physisorption increases with the increase of the surface area of the adsorbent. Hence, finely divided metals and porous substances with large surface areas are considered good adsorbents. 

• Physisorption’s enthalpy

Physical adsorption is already an exothermic process. However, it has a low enthalpy of adsorption somewhere around 20– 40 kJ mol-1. This is because of the attraction between the gas molecules. Due to weak van der Waals’ forces, the solid surface is formed.

BET Theory

BET (Brunauer–Emmett–Teller) theory describes the process of physical adsorption of the gas on a solid surface area. It plays an important role in analysing the measurement of the specific surface area of materials. The BET theory is applied to the systems of multilayer adsorption that usually uses an adsorbate that doesn’t react chemically with the adsorptive to quantify specific surface area. 

In short, the concept of the theory is all multilayer adsorption. Some of its hypotheses are given below:

• The gas molecules are physisorbed on a solid surface in infinite layers.
• Here, the gas only interacts with the adjacent layers.
• The single-layer theory can be applied to each of these layers.
• The adsorption’s enthalpy for the first layer is constant. Comparatively, the first layer is much greater.
• The second layer’s adsorption enthalpy is similar to that of liquefaction.

The resulting equation of the BET theory can be represented as:

θ = cp / (1- p/p0) (p0 + p(c-1))

Here, c is referred to as the BET C-constant, p0 is referred to as the vapour pressure of the adsorptive bulk liquid phase that would be at the adsorbate’s temperature, and θ is referred to the coverage of the surface area.

Mostly, it is used in the field of material science to calculate the different surface areas by the physisorption of gas molecules.

Conclusion

The process of physical adsorption is all about the accumulation of the van der Waals forces on the surface area. The process is used to determine the surface areas and the porosity of the solid surfaces. The surface area that is determined with the help of this technique can be complex at times. However, it determines a true measurement irrespective of the width and length of the object. Due to ripples, pores, and other imperfections, this process analyzes all the additional areas. This process of physical adsorption is widely used in different research, development, and quality control industries.