BDDT theory

First, allow me to impart to you some of my present thoughts on the BET theory (3, 4) for whatever they are worth. [Reference (3) is a continuation and extension of the BET theory. Some authors refer to it as the BDDT theory others call both papers together the BET theory.] Probably there are very few among you who have never made a BET plot, and many of you have made quite a few. I suppose, it will surprise no one among you if I say that I made the first BET plot. I did not know at that time that it was a BET plot, because that name did not exist yet. I tried to give a name to the theory that the three of us had developed, and I called it the multimolecular adsorption theory, which was probably not a very good name, but it was the best I could think of. Fortunately, somewhat later Professor Harkins invented the colorful name BET theory, and that name has stuck. 

Isotherms of the type now characterized as Type IV have played an essential part in the development of adsorption theory and practice, as being the first kind of isotherm to be studied in detail. Already in 1888, half a century before the BDDT classification had appeared, van Bemmelen had... 

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. 

In the present theory of van der Waals adsorption, the multilayer starts below the pressure P and builds up with increasing pressure, and the capillaries are not filled up even for pressures greater than this condensation pressure P, which contradicts the capillary condensation. This means that the multilayer theory based on van der Waals adsorption has an upper limit in the pressure range. Despite this limitation, the van der Waals theory puts all five types of adsorption isotherm shape into one framework, that is it can deal with unimolecular adsorption (Langmuir), multilayer adsorption (BET) and enhanced adsorption in capillaries (BDDT). 

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],... 

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