Factors Affecting Adsorption of Gases on Solids

Introduction

The molecules from the gas or liquid start to collect on the surface of solids when brought in contact with a solid surface. This process of collection of molecules of liquid or gas on the solid surface is called adsorption.  The molecules that get collected on the solid surface are called adsorbate, and the surface on which it gets collected is known as adsorbent.  This process usually creates a layer of ascorbate over the surface of the adsorbent. Solids can attract molecules of gases, liquids, or dissolved solids to their surface when they are in contact. However, the adsorption of molecules depends upon many factors. Let us look at some factors affecting adsorption of gases on solids.

Factors affecting adsorption of gases on solids

Nature of adsorbate (gas)

The nature of the adsorbate, gas in this case, highly matters when it comes to adsorption. It has been seen that easily liquefiable and highly water-soluble gases are adsorbed more rapidly because they have greater van der Waals forces. Whereas in physical adsorption, the boiling point of the gases determines the extent of adsorption. Adsorption of gas on a solid surface takes place more rapidly than liquids.

Nature of solid adsorbent

Among the several factors affecting the adsorption of gases on solids, the nature of solid adsorbents is a major factor. Rate of adsorption increases with the increase in the surface area of the solid. If the surface of the solid is finely divided or is rough, the surface area increases. This, in turn, increases the rate of adsorption.

The temperature of the surface

Adsorption is an exothermic process, so according to Le Chatelier’s principle, low temperature favours adsorption at a given pressure. When the temperature increases, the adsorbate molecules get removed from the surface. This process is also known as desorption.

Pressure of gas

One of the main factors affecting the adsorption of gases on solids is pressure. There is a fixed number of adsorption sites on the solid surface where gas molecules can be adsorbed. When the pressure increases, there is an increase in the rate of adsorption due to the increased number of gas molecules striking the surface. Therefore, the increase in pressure increases the rate of adsorption linearly. After some time, it reaches a point where the pressure has zero effect on the adsorption rate because the number of adsorption sites is fixed, and no further adsorption can occur in those sites. Therefore, the extent of adsorption becomes independent of pressure in this situation. It is seen that when the gas is adsorbed on the surface of the solid, the volume of the solid decreases. So, according to Le Chatelier’s principle, the quantity of gas adsorbed by the surface increases with the increase in pressure at a given temperature. Therefore, adsorption is directly proportional to the pressure of a gas. Several other factors which influence the adsorption of a gas on a solid.

Types of adsorption

Adsorption can be further divided into two main types- physical adsorption and chemical adsorption.

Physical adsorption

In physical adsorption, the adsorbate molecules are held by weak van der Walls’s force on the surface of the adsorbent. This type of force generally occurs between particles of every matter. Being universal in nature, these forces operate between all adsorbent and adsorbate pairs. This is why physical adsorption is observed on the surface of all solids. The physical adsorption of a gas by a solid is also reversible, and an equilibrium is formed between the adsorbent and the adsorbate. The rate of adsorption increases with the increase in pressure, and desorption occurs when the pressure is released. Physical adsorption decreases with the increase in temperature and increases with the decrease in temperature. In this process, many adsorbate layers are layered one over the other. The enthalpy of adsorption varies from 10 to 40kJ/mol.

Chemisorption or Chemical Adsorption

It has been seen that some unsaturated vacancies exist on the surface of solids. The adsorption becomes very strong when a chemical combination occurs between the adsorbent and the adsorbate. This process is known as chemical adsorption or chemisorption. The enthalpy of chemical adsorption varies in the range of 40-400 kJ/mol.  As chemical adsorption is highly specific, it is only possible between a specific pair of adsorbents and adsorbates and is irreversible as most chemical changes occur. Attempts to release the adsorbed gas provide the gas with some definite compound. For instance, oxygen is chemically adsorbed on tungsten and gets released from the tungsten surface as a combination of oxygen and tungsten oxide. With the rise in temperature, chemical adsorption first increases and then decreases. This, in turn, shows that chemical adsorption has activation energy.  During this process, only one layer of the molecule is adsorbed.

Examples of adsorption:

Some examples of adsorption that we notice on a day-to-day basis are listed below: Pollution masks – In this pandemic, masks are extended body parts that prevent dust particles and viruses from entering our bodies through our mouth and nose. Masks consist of two or more layers of fabric that absorbs the dust and viruses from the air and allow clean air to enter our nostrils. Curing diseases – The process of adsorption is used in curing many diseases.  The disease-causing germs or viruses get adsorbed on the surface of the medicine taken against it and get ejected from our body via natural processes. Purification of water – Water is purified using alum. A piece of alum is dropped into the dirty water. All the impurities get adsorbed to the surface and can be easily removed by separation methods such as sieving, sedimentation, etc.

Conclusion

Adsorption is the surface phenomenon that we notice on a day-to-day basis. Solids more easily absorb liquids and gases. Many factors affect the adsorption of gases on solids; some major factors have been explained above. This process helps in many ways, such as water purification, air, curing diseases, etc. The adsorption of gases on solids depends upon many other factors, including temperature and pressure.  This often forms a layer on the surface of the solid. It has been used for a long time for various purposes.