BDDT equation

An extension to the BET model was put forward by Brunauer, Deming, Deming and Teller (BDDT) in 1940. The BDDT equation contains four adjustable parameters and was designed to fit the isotherm Types I-V. From a theoretical standpoint, the BDDT treatment appears to offer very little more than the original BET theory and the cumbersome equation has very rarely been applied to experimental data. 

The n-layer treatment mentioned above takes an account of the forces of capillary condensation. In 1940, Brunauer, Deming, Doming and Teller [132] were able to extend the BET theory by introducing an additional contribution to the energy of adsorption resulting from these forces. The so-called BDDT equation, contrary to the BET isotherm, can be applied over a wider range of relative pressures. This equation, however, is rather complex and contains four adjustable parameters which cannot be assessed independently. For this reason, this is seldom applied and only a few attempts have been made to use the equation under consideration [95,118],... 

Another possibility in analyzing for micropores is to modify the BET equation to allow for only a certain number of monolayers to adsorb. This introduces another parameter, i.e. the number of allowed monolayers. This equation is not widely used and has not proven to be successful. With N being the number of allowed layers, the modified equation, called the BDDT [21], is... 

Fig. 99. The BDDT equation for various values of N. The c constant used for this was 20.

P) = correction for molecular flow (a function of pressure) nip = mass recording of the trial for mass flow correction Mp = the molar mass of the buoyancy probe gas N = number of adsorbate molecules N = being the number of allowed layers in BDDT equation = the number of moles adsorbed in the localized layer = Avogadro s number (6.022X 10 mor ... 

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