Types of Adsorption: Heinrich Kayser, a German scientist, initially invented the word adsorption in 1881. Adsorption is commonly referred to as a surface phenomenon in which particles adhere to the top layer of a substance. It usually involves the molecules, atoms, or even ions of dissolved gas, liquid, or solid adhered to the surface. Surface energy is the primary cause of adsorption. Surface particles that can be partially exposed tend to attract additional particles to their location. Adsorption is found in a variety of physical, natural, biological, and chemical systems, and it is used in a variety of industrial applications.

When charcoal is dipped in the coloured solution, the solution decolourises due to the adsorption of coloured particles by the charcoal. The adsorption is classified into two types based on the interaction between adsorbate and adsorbent. In this article, you will learn about different types of adsorption, adsorption isotherm, types of adsorption chromatography, types of adsorption indicator, etc.

What is Adsorption?

The process in which molecular, atomic or ionic species of one substance get accumulated at the surface of another is called adsorption. Adsorption is also defined as the phenomenon of attracting and retaining the molecules of a substance on the surface of a liquid or a solid, resulting in a higher concentration of the molecules on the surface.

1. Adsorbate: The substance which gets adsorbed on any surface is called adsorbate. For example, if gas gets adsorbed on a solid surface, the gas is termed adsorbate.

2. Adsorbent: The substance on the surface of which adsorption takes place is called adsorbent. Adsorbents may be a solid or a liquid. Metal powders, powdered charcoal, and silica powder are the commonly used adsorbents. Activated charcoal, silica, and alumina are very effective adsorbents due to their large surface area.

Types of Adsorptions

Depending on the nature of the force existing between the adsorbate and adsorbent molecules, the absorption is classified into two types, i.e., physisorption and chemisorption.

1. Physical Adsorption or Physisorption: If the adsorbate is held on the surface of the adsorbent by weak forces such as van der Waals’ force, then the absorption is called van der Waals’ adsorption or physical adsorption or physisorption.
   Example 1: Adsorption of gases on animal charcoal.
   Example 2: Adsorption of water vapour on silica gel.
2. Chemical Adsorption or Chemisorption: If the adsorbate is held on the surface of the absorbent by the strong chemical bond, the adsorption is called chemical adsorption or chemisorption.
   Example: Rusting of iron.

Physical Adsorption or Physisorption

The characteristics of physical adsorption are as follows:

1. Physical change: In this, there is no chemical bond formation. It is simply a physical change. Therefore, it is called physical adsorption.
2. Non-specific nature: As van der Waals forces are universal and not associated with a particular type of substance, physisorption is non-specific.
3. Nature of adsorbate: Easily liquefiable gases have stronger van der Waals forces of attraction.
4. Reversible nature: Physisorption is generally reversible. It can reverse by decreasing the pressure or increasing the temperature.
5. Enthalpy of adsorption: Physisorption is an exothermic process, but the heat of adsorption involved is quite low, about \(20 – 40\;{\rm{kJ}}/{\rm{mol}}\).
6. Multi-layered nature: Due to the non-specific nature and van der Waals forces, physisorption is multi-layered.
7. Effect of temperature: Physisorption decreases with an increase in temperature. The energy of activation of physical adsorption is almost equal to zero. Therefore, the rate of adsorption remains unaffected at a very low temperature.
8. Effect of pressure: Physisorption increases with an increase in pressure.

Chemical Adsorption or Chemisorption

The characteristics of chemical adsorption are as follows:

1. Chemical change: It involves the formation of a chemical bond between the adsorbate and the adsorbent.
2. Highly specific nature: Chemisorption is highly specific because it occurs only if chemical bond formation is possible between the adsorbate and the adsorbent.
3. Nature of adsorbate: Adsorbate should have the ability to form a chemical bond with the adsorbent.
4. Irreversible nature: Due to irreversible chemical change in most cases, chemisorption is generally irreversible.
5. Enthalpy of adsorption: Chemisorption is an exothermic process, and the heat of adsorption involved is about \(80 – 240\;{\rm{kJ}}/{\rm{mol}}.\)
6. Unimolecular layers of adsorption: Chemisorption forms a unimolecular layer. The chemical bond is formed only with the layer of molecules coming in direct contact with the surface of the adsorbent.
7. Effect of temperature: Chemisorption is exothermic. It is favoured at low temperature. The extent of adsorption first increases and then decreases with a temperature rise.
8. Effect of pressure: Chemisorption increases with an increase in pressure. At higher pressure, the gas molecules come closer to the surface of the adsorbent and help in the formation of chemical bonds.

Difference between Physisorption and Chemisorption

	Physisorption		Chemisorption
1.	The force which binds the adsorbate to the adsorbent by weak Van der Waals forces.	1.	The force which binds the adsorbate to the adsorbent is as strong as a chemical bond.
2.	Low heat of adsorption: Usually in the range of \(20 – 40\;{\rm{kJ}}/{\rm{mol}}\).	2.	High heat of adsorption: Usually in the range of \(80 – 240\;{\rm{kJ}}/{\rm{mol}}\).
3.	Usually occurs at low temperature and decrease with increasing temperature.	3.	Usually occurs at higher temperatures.
4.	It is reversible.	4.	It is irreversible.
5.	It is not specific.	5.	It is specific.
6.	It forms multimolecular layers on the surface of the adsorbent.	6.	It forms monomolecular layers on the surface of the adsorbent.

What is Positive and Negative Adsorption?

Positive Adsorption: The adsorption in which adsorbate concentration is more on the surface of the absorbent than its concentration in bulk.

Example: Shaking concentrated solution of KCl with blood charcoal shows positive adsorption.

Positive Adsorption: The adsorption in which adsorbate concentration is less on the surface of the absorbent than its concentration in bulk.

Example: Shaking the dilute solution of KCl with blood charcoal shows negative adsorption.

Types of Adsorption Isotherms Curves

The types of adsorption isotherms curves are:

Adsorption Isotherm

A graph between the amount of the gas adsorbed per gram of the adsorbent \({\rm{(x\backslash m)}}\) and the equilibrium pressure of the adsorbate at constant temperature is called the adsorption isotherm. Let us study the types of adsorption curves in detail.

1. Freundlich Adsorption Isotherm:
The relation between the amount absorbed \(\left( {\frac{{\rm{x}}}{{\rm{m}}}} \right)\) with equilibrium pressure \(\left( {\rm{P}} \right)\) is given by the equation, \(\frac{{\rm{x}}}{{\rm{m}}}{\rm{ = k}}{{\rm{P}}^{\frac{{\rm{1}}}{{\rm{n}}}}}\), where \({\rm{n}}\) is a positive integer, and \({\rm{n}}\) and \({\rm{k}}\) are constants depending upon the nature of the adsorbate and adsorbent at a particular temperature. The factor \(\frac{{\rm{1}}}{{\rm{n}}}\) has values between \({\rm{0}}\) and \({\rm{1}}\).  This equation is applicable only for physisorption.
Freundlich puts forward this relationship and hence is known as Freundlich adsorption isotherm.

2. Langmuir Adsorption Isotherm:
The Langmuir Adsorption Isotherm equation is \(\frac{{\rm{x}}}{{\rm{m}}}\,\,{\rm{ = }}\,\,\frac{{{\rm{aP}}}}{{{\rm{1 + bP}}}}{\rm{,}}\) \({\rm{a}}\) and \({\rm{b}}\) are constants that depend upon the nature of the gas adsorbate, the solid adsorbent and the temperature. \(\frac{{\rm{x}}}{{\rm{m}}}\) is the amount absorbed, and \({\rm{P}}\) is the pressure. This equation is applicable for physisorption as well as chemisorption.
Langmuir puts forward this relationship and hence is known as Langmuir adsorption isotherm.

Adsorption Isobar

Adsorption is an exothermic process, and it decreases with an increase in temperature. The effect of temperature on adsorption is expressed in terms of a graph known as adsorption isobar. For example, a graph drawn between the amount of the gas adsorbed per gram of the adsorbent \(({\rm{x}}/{\rm{m}})\) and temperature ‘\({\rm{t}}\)’ at a constant equilibrium pressure of adsorbate gas is known as adsorption isobar.

The physical adsorption decreases continuously as temperature increases. In chemical adsorption, the adsorption first increases and then decreases with an increase in temperature.

Types of Adsorption Indicators

Adsorption is used in precipitation titration. In precipitation titration, at the endpoint, the indicator is adsorbed by the precipitate, leading to a change in the colour of the indicator. Thus, these indicators are called adsorption indicators.

Various dyes such as fluorescein, eosin, and rhodamine are used as adsorption indicators in the precipitation titrations.

For example, \({\rm{KBr}}\) can be titrated against \({\rm{AgN}}{{\rm{O}}_{\rm{3}}}\) using eosin as an indicator. After the completion of the titration, the ions produced by the dye in the solution get adsorbed onto the precipitates of silver bromide and undergo a colour change.

Types of Adsorption Chromatography

The selective adsorption of certain substances from a solution by a particular solid adsorbent is used to separate the component of the mixture in chromatographic analysis. Chromatography is based on the general principle of distributing components of a mixture of organic compounds between two phases, i.e., the stationary phase and the mobile phase.

Depending upon the principles, chromatography is classified into two categories; adsorption chromatography and partition chromatography.

Adsorption chromatography is based on the principle that different components are adsorbed to different extents. The two important types of adsorption chromatography are column chromatography and thin-layer chromatography (TLC).

Column (adsorption) Chromatography

Column chromatography was developed by D.T. Day, American chemistry. The column chromatography is based on the principle of selective adsorption and desorption.
Procedure:

1. The stationary phase consists of a solid packed in a glass tube. For example, alumina, activated charcoal, silica gel, magnesia, calcium carbonate, and starch can be used as a stationary phase.
2.  The mixture to be separated is dissolved in a suitable solvent like petroleum ether. Then, the mixture to be separated is allowed to run through the stationary phase.
3. The constituents of the moving phase all adsorbed at different parts of the adsorbent column depending upon their nature. It is called selective or differential adsorption.
4. The adsorbed components are recovered by dissolving the components in suitable solvents, and this process is called elution. The commonly used eluent are petroleum, ether, benzene, and chloroform. The eluent acts as a moving phase, dissolves the components, and takes them down the column. The weekly adsorb component is eluted more rapidly than the more strongly absorbed components. The components thus get separated from one another and come out of column one by one. Finally, they are collected in separate receivers.
5. The components from the eluent are recovered by evaporation or distillation.

Thin Layer Chromatography (TLC)

Adsorption chromatography, or thin-layer chromatography, is a form of adsorption chromatography. With the use of an applicator, a thin layer of an adsorbent, such as silica gel formed from a slurry in a suitable solvent, is spread across a glass plate of size \(5 \times 2\;{\rm{cm}}\).

The separated mixture is dissolved in a suitable solvent and placed on a glass plate using a fine capillary at a distance of about \(2\;{\rm{cm}}\) from the bottom. The dry plate is then placed in a development chamber containing a suitable solvent or a mixture of solvents in a vertical position.

The spot marked on it should not be dipped in the solvent during the operation. The solvent will progressively increase due to capillary action once the compartment is closed.

A number of fine spots form when the mixture separates. Finally, the plate is removed and dried. Coloured components can be found using the naked eye, while colourless components can be found using UV light, iodine, sulfuric acid mixed with an oxidising agent such as potassium permanganate, nitric acid, and other methods. The components can be identified using the \({\rm{Rf}}\) value or using a suitable procedure to elute each component separately.

The relative adsorption of each component of a mixture is expressed by a retention factor known as the \({{\rm{R}}_{\rm{f}}}\) value. \({{\rm{R}}_{\rm{f}}}\) value is defined as the ratio of the rate of movement of a substance under the investigation to that of the solvent.

\({{\rm{R}}_{\rm{f}}}\,\,{\rm{ = }}\,\,\frac{{{\rm{Distance}}{\mkern 1mu} \,{\rm{moved}}\,{\rm{by}}\,{\rm{the}}{\mkern 1mu} {\rm{substance}}{\mkern 1mu} {\rm{from}}{\mkern 1mu} {\rm{base}}{\mkern 1mu} {\rm{line}}}}{{{\rm{Distance}}{\mkern 1mu} {\rm{moved}}{\mkern 1mu} {\rm{by}}{\mkern 1mu} {\rm{solvent}}{\mkern 1mu} {\rm{from}}{\mkern 1mu} {\rm{base}}{\mkern 1mu} {\rm{line}}}}\)

The \({{\rm{R}}_{\rm{f}}}\) value of a substance depends on the substance’s nature, solvent, adsorbents, and temperature.

Summary

Adsorption is the process of a material accumulating in molecular forms at increasing concentrations on the surface. Hydrogen, Nitrogen, and Oxygen are all gases that adsorb on activated charcoal. Meanwhile, we must keep in mind that adsorption is not the same as absorption. The mechanisms involved in the two processes are completely different.

In this article, you learnt types of adsorptions, i.e., physisorption and chemisorption, their properties and differences. You have also understood types of adsorption isotherm, i.e., Freundlich adsorption isotherm and Langmuir Adsorption Isotherm. Adsorption has various applications in our daily life. With this article, you can recall adsorption in column chromatography and thin-layer chromatography and its uses in adsorption indicators.

FAQs on Type of Adsorption

Q.1. What are the two common types of adsorptions?
Ans: Physical adsorption and chemical adsorption are two common types of adsorption.

Q.2. What are the different types of adsorption isotherms?
Ans: The different types of adsorption isotherm are

1. Freundlich adsorption isotherm: It is the relation between the amount absorbed \(\left( {\frac{{\rm{x}}}{{\rm{m}}}} \right)\) with equilibrium pressure \({\rm{(P)}}\), is given by the equation, \(\frac{{\rm{x}}}{{\rm{m}}}{\rm{ = k}}{{\rm{P}}^{\frac{{\rm{1}}}{{\rm{n}}}}}\), where n is a positive integer and \({\rm{n}}\) and \({\rm{k}}\) are constant.
2. The Langmuir Adsorption Isotherm: It equation is \(\frac{{\rm{x}}}{{\rm{m}}}\,\,{\rm{ = }}\,\,\frac{{{\rm{aP}}}}{{{\rm{1 + bP}}}}{\rm{,}}\) where \({\rm{a}}\) and \({\rm{b}}\) are constants. \(\frac{{\rm{x}}}{{\rm{m}}}\) is the amount absorbed, and \({\rm{P}}\) is the pressure.

Q.3. What is physical adsorption? Give an example.
Ans:  If the adsorbate is held on the surface of the adsorbent by weak forces such as van der Waals’ force, then the adsorption is called physical adsorption or physisorption.
Example: Adsorption of water vapour on silica gel.

Q.4. Where is adsorption used for?
Ans: Adsorption is used to separate noble gases, precipitation titration, chromatographic techniques, froth floatation, softening hard water, etc.

Q.5. What are the different types of adsorptions? Give any four differences.
Ans: The different types of adsorptions are physisorption and chemisorption. The four differences between them are as follows.

	Physisorption		Chemisorption
1.	The force which binds the adsorbate to the adsorbent is the weak van der Waal’s forces.	1.	The force which binds the adsorbate to the adsorbent is as strong as a chemical bond.
2.	Low heat of adsorption: usually in the range of \(20 – 40\;{\rm{kJ}}/{\rm{mol}}\).	2.	High heat of adsorption: usually in the range of \(80 – 240\;{\rm{kJ}}/{\rm{mol}}\).
3.	It is reversible.	3.	It is irreversible.
4.	It is not specific.	4.	It is specific.

Q,6. What is the principle of adsorption?
Ans: Adsorption is based on the principle of attraction and retaining of molecules on the surface of a liquid or a solid substance.

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