Specific Surface Area, Langmuir, BET

by Amalia Aventurin (Author)

Seminar Paper 2013 21 Pages

Geography / Earth Science - Miscellaneous



I. Specific Surface Area, Langmuir, BET
I.1. Introduction
I.2. Theoretical background
I.2.1. Langmuir
I.2.2. Specific Surface Area and BET
I.3. Experimental methods and Results
I.3.1. Langmuir
I.3.2. Specific Surface Area and BET - "Shale 1"
I.3.3. Specific Surface Area and BET - "Norit AC"
I.4. Evaluation
I.5. References

I. Specific Surface Area, Langmuir, BET

I.1. Introduction

The theme of this report is the evaluation of gas adsorption isotherms using the Langmuir, BET methods to assess the specific surface area. Gas adsorption experiments on porous materials yield different isotherm curves. These curves have been classified into six types by the International Union of Pure and Applied Chemistry (IUPAC) (Figure 1).

Adsorption is in general an enrichment of gas or fluid at a surface/solid, in more detail, at the border of two phases. Because of the complexity and difficulty of adsorption and desorption, process of leaving atoms or molecules from the surface of a solid, the data get analyzed by the following Isothermal models. These models assume dynamic equilibrium at constant temperature between adsorption and desorption, and are described in more detail in the following topic “1.2 Theoretical Background”.

illustration not visible in this excerpt

Fig 1: The six different types of adsorption-isotherms according to the IUPAC classification.

The type I isotherm in Figure 1 is the Langmuir-isotherm. This isotherm type assumes monomolecular sorption and approaches limiting value at high pressures as shown in the sketch.

Type II is the BET-isotherm. This situation appears after the development of the monomolecular cover furthermore a cover of fluids, so that there is at the end a multi-layer system.

Type III is a curve which appears if there is only a small force between the first layer and the solid. Because of this reason, the development of the multi-layer system is finished first before the monomolecular layer is completed.

Type IV and V are special curves which occurs only at capillary condensation. Type V has the same phenomena like type III, at both types are small forces at the beginning. The types III and V are very rare because of the small interaction force at the adsorption [4].

Type VI is a very special process; it shows a stepwise multi-layer adsorption. This case is the rarest and because of this neglected in the report.

The most important types are I and II and on these two types we take a closer look. At first we go into detail to the Langmuir-isotherm and after that to the BET-isotherm and the specific surface area.

I.2. Theoretical background

I.2.1. Langmuir isotherm

The Langmuir isotherm (see Fig. 1 type I) is a mathematical way to describe monomolecular adsorption. This model shows the variation of adsorption with pressure. It gets proposed by Irving Langmuir in 1916 [5]. This adsorption isotherm is based on the following assumptions:

- There is a dynamic equilibrium between adsorption (adsorbed gaseous molecules) and desorption (the free gaseous molecules).
- The adsorptive built on the solid a monomolecular layer (Adsorption is monolayer or unilayer [5]).
- The surface has the same energy level, which can bond one gas molecule. This means that adsorption is localized and the adsorbed molecules can’t move on the solid (All the vacant sites are of equal size and shape on the surface of adsorbent [5]).
- No interaction between the adsorbed molecules (Each site can hold of one gas molecule and a constant amount of heat energy is released during this process [5]).
- A fixed number of vacant or adsorption sites are available on the surface of solid [5].

illustration not visible in this excerpt

Fig. 2: Langmuir isotherm with different Langmuir constants and identical maximum sorption capacities (coverage) [6].



ISBN (eBook)
ISBN (Book)
File size
924 KB
Catalog Number
Institution / College
RWTH Aachen University – Lehr- und Forschungsgebiet Geologie - Endogene Dynamik
specific surface area langmuir


  • Depending on your browser, you might see this message in place of the failed image.

    Amalia Aventurin (Author)


Title: Specific Surface Area, Langmuir, BET