The key difference between Langmuir and Bet isotherm is that Langmuir isotherm describes the monolayer molecular adsorption, whereas BET isotherm describes the multilayer molecular adsorption.
An isotherm, in chemistry, is the curve on a volume versus temperature versus pressure diagram, which indicates a single temperature level. The term “isotherm” is derived from “iso”, referring to single-phase and “therm”, referring to temperature.
CONTENTS
1. Overview and Key Difference
2. What is Langmuir Isotherm
3. What is Bet Isotherm
4. Langmuir vs Bet Isotherm in Tabular Form
5. Summary – Langmuir vs Bet Isotherm
What is Langmuir Isotherm?
Langmuir adsorption isotherm is the method used to predict linear adsorption at low adsorption densities and a maximum surface coverage at higher solute metal concentrations. It is a theoretical expression, and the chemical equation for this term is as follows:
X/M = abc(1 + ac)
Where X is the weight of solute absorbed, M is the mass of the adsorbent, c is the equilibrium concentration of the solute, and a and b are constants. Moreover, Langmuir adsorption isotherm is applicable for monolayer adsorption onto a homogeneous surface. However, there should not be any interaction between adsorbed species.
What is Bet Isotherm?
BET isotherm describes the adsorption of gas molecules on a solid surface. The term BET stands for Brunauer-Emmett-Teller isotherm. This technique serves as the basis for an important analysis method, which is important for the measurement of a specific surface area of materials. We can observe it as physical adsorption or physisorption. This theory was introduced by Stephen Brunauer, Paul Hugh Emmett, and Edward Teller in 1938.
This theory can be applied for systems of multilayer adsorption, and it usually uses probing gases (named as the adsorbent) to quantify the specific surface area. Nitrogen gas is a common absorbate gas used in surface probing through the BET method.
Figure 01: BET Model
We can observe that the BET isotherm is an extension of the theory of the Langmuir isotherm. This extension takes place from monolayer adsorption to multilayer adsorption. However, there are some hypotheses that we need to consider when using this isotherm:
- Gas molecules physically get adsorbed infinitely on a solid in layers
- Gas molecules react only with the adjacent layers
- We can apply Langmuir theory for each layer
- Enthalpy for adsorption by the first layer is constant, and it is greater than that of the second layer
- Enthalpy of adsorption of the second layer is equal to the enthalpy of liquefaction
What is the Difference Between Langmuir and Bet Isotherm?
An isotherm, in chemistry, is the curve on a volume versus temperature versus pressure diagram, which indicates a single temperature level. Langmuir adsorption isotherm is the method used to predict linear adsorption at low adsorption densities and a maximum surface coverage at higher solute metal concentrations. BET isotherm describes the adsorption of gas molecules on a solid surface. The key difference between Langmuir and Bet isotherm is that Langmuir isotherm describes monolayer molecular adsorption, whereas BET isotherm describes multilayer molecular adsorption.
Summary – Langmuir vs Bet Isotherm
An isotherm is a curve on a volume versus temperature versus pressure diagram, which indicates a single temperature level. The key difference between Langmuir and Bet isotherm is that Langmuir isotherm describes monolayer molecular adsorption, whereas BET isotherm describes multilayer molecular adsorption. We can define Langmuir adsorption isotherm as the method to predict linear adsorption at low adsorption densities and a maximum surface coverage at higher solute metal concentration and BET isotherm as the adsorption of gas molecules on a solid surface.
Reference:
1. “Supplemental Modules (Analytical Chemistry).” Chemistry LibreTexts, 15 Aug. 2020.
Image Courtesy:
1. “BET Multilayer Adsorption” By Life of Riley – Own work (CC BY-SA 3.0) via Commons Wikimedia