Vapor-solid Adsorption, BET Theory

The true curve must pass through the origin, so the leftmost part of the curve cannot be completely correct. I have drawn it the way it appears in the original papers. 

FIGURE 11.24 Experimental measurements of the adsorption of nitrogen on soUd silica gel at 77 K, the NBP of nitrogen [25]. The curve is a simple smooth interpolation. 

The above explanation is simple and intuitively satisfying, but is certainly a great simplification of what nature is really doing. 

Faced with this type of adsorption, Brunauer, Emmett and Teller (BET) [26] worked out the mathematics of multiple layers of adsorbed molecules, subject to some simplifying assumptions, including that there were multiple one-molecule-thick layers, and that each layer covers a smaller part of the surface than the one below it Their resulting equation (rewritten to use the notation in fliis chapter) is 

The calculation in this example, based on measured adsorption of nitrogen at its NBP, is the standard BET Method of determining surface area of porous solids. If the BET theory were perfect then the areas determined for a single solid would be independent of which gas were used. Alas, the values are similar but not identical the 

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