The specific surface area is usually calculated by using the BET theory from the gas adsorption isotherm data. The BET theory stipulates that the adsorptive first starts to adsorb on the strongest energy spots on the surface, and then, with increasing pressure, adsorption can also be observed on the spots with the next lower energy levels. The model also takes account of 2nd, 3rd and higher layer adsorption.
BET Theory
This BET theory for multi-layer adsorption is the expansion of Langmuir equation, which is for the single layer adsorption.
This theory well fits to type II and IV isotherms at the relative pressure between 0.05 and 0.35. The monolayer adsorption amount (Vm) and C parameter are calculated from the slope and intercept of BET-plot by using the least square fitting. C parameter represents the surface interaction energy, so it must take a positive value.
Type IV isotherm and BET-plot
The specific surface area is calculated from this Vm and cross sectional area (σ).
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