Brunauer-Emmett-Teller Isotherm BET

Extending the classical Langmuir adsorption isotherm (7.1) from monolayer to ideal multilayer adsorption and considering the limiting case of infinite many layers, Brunauer, Emmett, and Teller derived in 1938 the AI [7.1-7.5, 7.42] 

The BET-isotherm has been derived by statistical mechanical methods by Hill and later again by W. Steele (1974) [7.5], [7.43], It can be shown that the infinitive number of adsorbate layers assumption used in deriving (7.70), is a reasonable approximation for multilayer adsorbates with more than 4 layers. Though admolecular interactions are not explicitly taken into account in (7.70), today this isotherm provides the basis for a standardized method to determine the mesoporous surface of porous sorbent materials by N2-adsorption at T = 77 K or Ar-adsorption at T = 87 K, i. e. ps = 0.1 MPa. For this normally only the initial portion of a measured isotherm (0 p/ps 0.35) is used. Details are given in [7.1, 7.5, 7.44-7.47] and in the respective ISO-documents published meanwhile [7.14]. 

A thermodynamic consistent extension of the BET-isotherm to multicomponent systems (N 1) and to real gas adsorptives will be presented in Sect. 4 of this Chapter. 

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Multilayer adsorption and other complications are accounted for in the more elaborate Brunauer-Emmett-Teller isotherm (BET isotherm) [28], which involves the heats of adsorption of the first and subsequent layers and with which not only Type I behavior can be approximated. In its simplest form, for an unlimited number of adsorption layers and partial pressures well below saturation, the BET equation can be written... 

Brunauer-Emmett-Teller Isotherm (BET Isotherm) An adsorption isotherm equation that accounts for the possibility of multilayer adsorption and different enthalpy of adsorption between the first and subsequent layers. Five types of adsorption isotherm are usually distinguished. These are denoted by roman numerals and refer to different characteristic shapes. See Adsorption Isotherm. 

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