Adsorption isotherms have been of great importance for research related to environmental protection and adsorption techniques. The two primary methods used to estimate the adsorption capacity of a given substance are known as the Freundlich and Langmuir isotherms.
An adsorption isotherm is a graph that represents the variation in the amount of adsorbent (x) adsorbed on the surface of an adsorbent with a change in pressure at a constant temperature.
As we know from Le Chatelier’s principle, the equilibrium in the reaction shifts to the direction in which the stress is relieved. So, here we can see that on applying additional pressure on the system, the equilibrium shifts in the direction where the number of molecules is reduced thereby reducing the pressure in the system.
From the graph, we also see that after achieving a pressure PS, that is, the saturation pressure, the variation in the amount of adsorbent being absorbed becomes zero. This is because the surface area available for adsorption is limited and since all sites are occupied, a further increase in pressure does not make any difference.
Various adsorption isotherms have been proposed by different scientists, namely,
- Langmuir Isotherm
- Freundlich isotherm
- BET theory
Freundlich Adsorption Isotherm
Freundlich adsorption gives the change in the amount of gas adsorbed by the unit mass of the solid adsorbent with the change in pressure of the system for a given temperature.
Langmuir Adsorption Isotherms
The Freundlich adsorption isotherm is followed by another two isotherms, the Langmuir adsorption isotherm and the BET principle. Langmuir adsorption isotherms predict linear adsorption at low adsorption densities and maximum surface coverage at high solute metal concentrations.
BET adsorption isotherm
The theory of multilayer adsorption, proposed by Bruner, Emmett and Taylor in 1938 (BET theory), assumes that physical adsorption results in the formation of multilayer adsorption. The theory also assumes that the solid surface has a similar number of adsorption sites and that adsorption at one site does not affect adsorption at neighbouring sites.
After the formation of the monolayer, the adsorption process can continue with the formation of a multilayer incorporating a second layer, a third layer, etc.
Applications of adsorption
Following are the applications of adsorption:
Gas Mask: Toxic gases are adsorbed on the surface of the mask and are not encountered when used by coal miners.
Vacuum production: The air fractions are adsorbed on the charcoal and removed from the equipment going through the extraction process.
Moisture removal: Silica gel pellets are used to absorb moisture in medicines and newer plastic bottles to control moisture.
Decolourization: The juice extracted from the cane is treated with animal charcoal to remove the colouring agent to obtain a clear liquid solution.
As Catalyst: Suitable material is used as a catalyst such that the reactants stick to its surface, thus enabling the reaction to proceed at a faster rate and increasing the rate of the reaction.
What is the purpose of Adsorption?
Adsorption is the adhesion to a layer of atoms, ions or molecules from a gas, liquid or dissolved material. This process forms an adsorbent film on the surface of the adsorbent.
What is the purpose of Samatapi?
Isotherm, a line drawn on a map or graph connecting points at the same temperature. Isotherms are commonly used in meteorology to view the temperature distribution over the Earth's surface or on a chart showing constant or constant pressure.