BET Method and Physical Surface Area

The surface area of solids has long been important in physical sciences, applied sciences, and environmental sciences, because this feature is capable of affecting the quality and utility of many materials. For this reason it is important to determine and control surface areas accurately. Likewise, knowledge of porosity and surface area are frequently important keys in understanding the structure, formation, and potential applications of different natural materials. 

The most widely used and studied technique for estimating surface area is the so-called BET method, which stands for Brunauer, Emmett, and Teller, who published a paper in the Journal of the American Chemical Society in 1938 [54]. Their work allowed others to finally make sense of the adsorption of gases by a solid material with a complicated shape, and helped us interpret the relationship between surface area and adsorbed gas molecules. This method may seem complicated however, its application is in fact quite straightforward and very easy to comprehend. 

The BET method basically uses the surface of adsorbed gas molecules as a ruler. For example, take a nitrogen molecule. We know from other experiments that the area covered by a nitrogen molecule at liquid nitrogen temperature is 16.2 A. Next, we need to figure out how many of these nitrogen molecules are required to completely cover the surface of the solid, so we expose the solid in a vacuum chamber to different amounts of a gas and measure the pressure in that chamber. We accept the important hypothesis that no more than one atom can occupy one site and the atoms do not interact with each other. The pressure will decrease as more and more gas molecules adsorb on the surface. 

For different amounts of gas that have been added to the vacuum chamber, we find a certain amount of molecules that are in equilibrium and adsorbed on the surface. The result of the so-called adsorption and desorption isotherms is used to determine the amount of gas molecules adsorbed to a surface, according to the BET equation  

Nitrogen is used most often to measure BET surface, but if the surface area is very low, argon or krypton may be used, as both give a more sensitive measurement because of their lower saturation vapor pressures at liquid nitrogen temperature. 

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