All solid substances have the capabilities to attract to their surfaces molecules of solutions or gases with which they are contacted is called adsorption. Here in this article, we will understand the adsorption meaning, types of adsorption like chemical adsorption, physical adsorption, isothermal adsorption. Also, we will discuss different adsorbate and adsorbent. So, let’s discuss adsorbate and adsorbent before any discussion. Well, the solids that are used to adsorb dissolved substances or gases are known as adsorbents and the molecules that are adsorbed by solids are collectively known as adsorbate. Let’s take an example of charcoal. Charcoal is an adsorbent as it removes impurities and poisons from air or stream in gas masks.
Define Adsorption
Adsorption is the process of collecting molecules by the internal surface or external surface of solids or by liquid surface. Adsorption refers to the collecting of molecules by the external surface or internal surface (walls of capillaries or crevices) of solids or by the surface of liquids. You may have confusion with the word adsorption and absorption. Absorption is the process of penetrating the substance of blocks of amorphous liquids or solids, or into the actual interior crystals. Sometimes a solid takes up the liquid or gas without specifying a particular process of absorption or absorption, such process is called sorption.
Difference Between Adsorption and Absorption
Most people have confusion between these two terms as they sound similar but both the processes are different. Adsorption is the surface phenomenon and is an exothermic process. It depends on temperature and initially, the process will increase and will decrease gradually. Whereas, absorption is an endothermic process. Absorption is not affected by temperature and occurs at uniform rates. Substances penetrate the surface of the material. Absorption is spontaneous whereas adsorption is not spontaneous. Absorption is unique throughout the surface area and adsorption is not unique on the surface area. Molecule interaction is higher in absorption than adsorption.
Types of Adsorption
Adsorption is classified into two types further, physical adsorption and chemical adsorption. Adsorption, an exothermic process is an attractive force that takes place in-between the adsorbent and adsorbate, where heat is released. Moreover, the chemical and physical adsorption also depends upon the forces between adsorbent and adsorbate. In physical adsorption, gases get condensed to liquids depending on Van der Waals, or forces exist between adsorbates and solid absorbents. If the temperature is low and gas pressure is quite high, then solid can adsorb any gas without concerning chemical specifications. Whereas in chemical adsorption, the process depends upon the chemical forces acting upon the solid surfaces and gas absorbates. Chemical adsorption needs a higher temperature as compared to physical adsorption. Chemical adsorption involves energy activation, hence, it takes time and it is a slower process.
Physical Adsorption and Chemical Adsorption: Difference
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Physical Adsorption |
Chemical Adsorption |
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The process involves weak or Van Der Waals forces. |
The process involves covalent or ionic chemical bonds. |
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By decreasing pressure and increasing temperature, physical adsorption can be reversed. |
Irreversible process and makes compound when forces applied to free the adsorbed gas. |
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No activation energy is needed. |
Activation energy is needed. |
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A multimolecular layer is formed. |
The unimolecular layer is formed. |
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Low heat adsorption |
High heat adsorption |
Adsorption Isotherm
Research on environmental protection and techniques of adsorption involves adsorption isotherm as it helps in the prediction of the adsorption capacity of the solid material. Furthermore, an adsorption isotherm is a graph that represents the amount of adsorbed adsorbate on the adsorbent surface keeping a constant temperature and changing pressure.
Application of Adsorption
Charcoal adsorbs the colouring molecules from the coloured sugar solution and decolorizes it.
Moisture is adsorbed by silica gel from the desiccators Alumina and silica gels remove moisture and control the humidity of offices and rooms because they act as adsorbents. In gas masks, activated charcoal is used as it adsorbs vapours, toxic gases to purify the breathing air. Heterogeneous catalysis is carried out through the adsorption process. Charcoal can be used as an absorbent to separate noble gases. The chromatographic analysis is based on the adsorption phenomenon. In syrups and cosmetics, stable emulsions are formed through adsorption. Drug adsorption kills germs.