Adsorption Isotherms: Freundlich and Langmuir Adsorption isotherms describe the relationship between the amount of adsorbate adsorbed onto a surface and the...
Adsorption isotherms describe the relationship between the amount of adsorbate adsorbed onto a surface and the partial pressure or concentration of the adsorbate gas. The two most commonly used isotherms are the Freundlich isotherm and the Langmuir isotherm.
Freundlich isotherm:
The Freundlich isotherm is an empirical equation that describes the adsorption of a gas onto a solid surface:
θ = (k₁P)/(K) + (k₂)P
where:
θ: The fractional coverage of the surface occupied by the adsorbate
P: The partial pressure of the adsorbate
k₁: The Freundlich constant for the first stage of adsorption
K: The Freundlich constant for the second stage of adsorption
P: The total pressure of the gas
The Freundlich isotherm assumes that the adsorption process follows a multilayer mechanism, with the adsorbate molecules physically adsorbing onto the surface.
Langmuir isotherm:
The Langmuir isotherm is another empirical equation that describes the adsorption of a gas onto a solid surface:
θ = k₃P
where:
θ: The fractional coverage of the surface occupied by the adsorbate
P: The partial pressure of the adsorbate
k₃: The Langmuir constant
The Langmuir isotherm assumes that the adsorption process follows a monolayer mechanism, with the adsorbate molecules adsorbing onto the surface in a monolayer.
The Langmuir isotherm is applicable when the adsorption process is complete and when the surface is fully covered with adsorbate molecules.
Differences between Freundlich and Langmuir isotherms:
The Freundlich isotherm has two constants, k₁ and k₂, while the Langmuir isotherm has only one constant, k₃.
The Freundlich isotherm assumes a multilayer mechanism, while the Langmuir isotherm assumes a monolayer mechanism.
The Freundlich isotherm is applicable when the surface is completely covered, while the Langmuir isotherm is applicable when the surface is fully covered.
In conclusion, adsorption isotherms provide valuable insights into the mechanisms and rates of gas adsorption onto solid surfaces. By understanding the shape of the isotherm curve, we can predict the amount of adsorbate that will be adsorbed at different partial pressures and determine the nature of the adsorption process