Adsorption

The term Adsorption was first coined in 1881 by a German physicist named Heinrich Kayser. Adsorption is often described as a surface phenomenon where particles are attached to the top layer of material. It normally involves the molecules, atoms or even ions of a gas, liquid or a solid in a dissolved state that are attached to the surface.

Adsorption is mainly a consequence of surface energy. Generally, the surface particles which can be exposed partially tend to attract other particles to their site. Interestingly, adsorption is present in many physical, natural, biological and chemical systems and finds its use in many industrial applications. We will learn about the concept in detail below.

Table of Content

What is Adsorption?

Adsorption is a process which involves the accumulation of a substance in molecular species in higher concentration on the surface. If we look at Hydrogen, Nitrogen and Oxygen, these gases adsorb on activated charcoal. Meanwhile, we have to note that adsorption is different from absorption. The two processes involve totally different mechanisms.

For the adsorption process, two components are required,

  • Adsorbate: Substance which is deposited on the surface of another substance. For example, H2, N2 and O2 gases.
  • Adsorbent: Surface of a substance on which adsorbate adsorbs. For example, Charcoal, Silica gel, Alumina.

Also Read: Surface Chemistry

Types of Adsorption

On the basis of interaction forces between adsorbate and adsorbent, adsorption is of two types. 

  1. Physical adsorption:

This type of adsorption is also known as physisorption. It is due to weak Van der Waals forces between adsorbate and adsorbent. 

For example, H2 and N2 gases adsorb on coconut charcoal.

  1. Chemical adsorption:

This type of adsorption is also known as chemisorption. It is due to strong chemical forces of bonding type between adsorbate and adsorbent. We can take the example involving the formation of iron nitride on the surface when iron is heated in N2 gas at 623 K. 

Adsorption of gas on a solid is a spontaneous exothermic reaction. Amount of heat liberated when a unit mass of a gas is adsorbed on the surface is called heat of adsorption.

Physisorption and Chemisorption Adsorption Characteristics

Characteristics of physical adsorption:

  1. This type of adsorption is caused by physical forces.
  2. Physisorption is a weak phenomenon.
  3. This adsorption is a multi-layered process.
  4. Physical adsorption is not specific and takes place all over the adsorbant.
  5. Surface area, temperature, pressure, nature of adsorbate effects physisorption.
  6. Energy for activation is low (20 – 40 kg/mol).

Characteristics of chemical adsorption:

  1. This type of adsorption is caused by chemical forces.
  2. It is a very strong process.
  3. This type of adsorption is almost a single-layered phenomenon.
  4. Chemisorption is highly specific and takes place at reaction centres on the adsorbant.
  5. Surface area, temperature, nature of adsorbate effects chemisorption.
  6. Energy of activation is very high 40 – 400 kJ/mol.

Adsorption Isotherm

Adsorption is usually described by isotherms. It is due to the fact that temperature plays an important role or that it has a great effect on the whole process. Moreover, there are several isotherm models that are used to describe the adsorption technique. These include;

Freundlich Theory

Freundlich adsorption isotherm is obeyed by the adsorption where the adsorbate forms a monomolecular layer on the surface of the adsorbent.

\(\frac{x}{m}=K.{{p}^{\frac{1}{n}}};\,\,\log \frac{x}{m}=\frac{1}{n}\log p+\log k\)

x represents the amount of gas adsorbed on the m gram of adsorbent, K and n are adsorption constants, ‘p’ is pressure n always greater than one. 

A major drawback of Freundlich adsorption isotherm it fails at high pressure. It could not explain the multi-layered adsorption process.

Langmuir Theory

In 1916 Langmuir proposed theory of adsorption of a gas on the surface of the solid to be made up of elementary sites each of which would adsorb one gas. It is assumed that all adsorption sites are equivalent and the ability of a gas molecule to get bound to any one site is independent whether or not the neighbouring sites are occupied. Additionally, it is also assumed that dynamic equilibrium exists between adsorbed and non adsorbed gas molecule.

Following principles can be obtained from Langmuir adsorption isotherm 

  • The gas adsorbed behaves ideally in a vapour phase.
  • Only monolayer adsorption takes place.
  • The surface of the solid is homogeneous.
  • There are no lateral interactive force between the adsorbate molecule.
  • The adsorbed gas molecules are localized.

BET theory (after Brunauer, Emmett, and Teller)

The BET theory was proposed by Brunauer, Emmett and Teller in the year 1938. This theory explains the formation of multilayer adsorption during physisorption. This theory also talks about the uniformity in the sites of adsorption of solid surfaces. It assumes that when adsorption occurs at one site it will not affect adsorption at neighbouring sites.

Differences Between Absorption and Adsorption

Absorption Adsorption
Complete deposition of a substance in another substance is absorption Deposition of a substance on the surface is known as adsorption.
It is not a surface phenomenon. It is a surface process.
It is not spontaneous. Adsorption of gas on solid is spontaneous.
It takes place uniformly throughout. It does not take place uniformly.
Greater molecular interaction. Less molecular interaction.
It involves the application of potential in the absorption of water by root hairs. There is no involvement of potentials during adsorption.
It is not subdivided. Surface absorption is adsorption it is subdivided.

Applications of Adsorption

1) Air pollution masks:

These consists of silica gel or activated charcoal powder, when dust or smoke are paused through them, those particles get adsorbed on the surface of these materials.

2) Separation of noble gases by Dewar’s flask process:

A mixture of noble gases of Ne, Ar, Kr is passed through Dewar’s flask in presence of heated coconut charcoal. Argon and Krypton gels adsorbed leaving Neon.

3) Purification of water:

By the addition of alum stone to the water, impurities get adsorbed on the alum and water gets purified.

4) Removal of moisture and humidity:

Moisture in the air is removed by placing silica gel on which water molecular gets adsorbed.

5) Adsorption chromatography:

It is used to separate pigments and hormone.

6) Ion exchange method:

In this method of removing the hardness of water, calcium and magnesium ions get adsorbed on the surface of ion exchange resin

7) In metallurgy:

In froth floatation process of concentration of ore, the particle gets adsorbed on the froth.

Adsorption Questions

1. Adsorption of gas on a solid is always exothermic explain?

Solution:

Adsorption of gas on a solid is a spontaneous process. When a gas adsorbs on the solid, due to molecular interaction process, the entropy of gas molecules decreases. To make the process spontaneous, adsorption must be exothermic.

2. Among SO2, CH4, H2 gases which gas adsorbs more on the charcoal and why?

Solution:

Sulphur dioxide adsorbs more than methane and hydrogen since the critical temperature of sulphur dioxide is higher than methane and hydrogen.

3. What do you mean by critical temperature? What is the relation of critical temperature and the gas adsorbed?

Solution:

The temperature above which a gas cannot be liquefied even by the application of high pressure is known as critical temperature. Greater is the critical temperature (To) more in the gas adsorbed on the surface.

4. What are the factors effective adsorption of gases on solids?

Solution:

  1. Nature of gases: Ease is liquefication more is gas adsorbed.
  2. The surface area of adsorbent: More the surface area more gas is adsorbed.
  3. Temperature: Physisorption increases with a decrease in temperature while chemisorption increases with an increase in temperature.
  4. Pressure: It affects only physisorption if pressure increase amount of gas adsorbs also increase.

5. Define activation.

Solution:

When adsorbents are heated in a vacuum from 573 – 623 K, the surface area is increased and it is known as activation.

6. Write about adsorption from solutions?

Solution:

Charcoal pieces are added to the acetic acid solution. Some of the acetic acid molecules adsorb on charcoal. This depends upon,

  1. Nature of adsorbate
  2. The surface area of the adsorbent 
  3. Temperature 
  4. Concentration