Is BET equation applicable to only monolayer adsorption range?

One of my senior colleagues is stating that although BET adsorption isotherm equation hypothesises about multilayer adsorption, the P/P0 values (usually from 0.05 to 0.3) accountable for BET surface area measurement only correspond to monolayer adsorption - as multilayer adsorption doesn't occur at that P/P0 range.


I am confused about that as I used to know that in BET the total adsorption volume of a gas (i.e., V) at different P/P0 values are taken (it may be monolayer or multilayer adsorption) and we only get the monolayer adsorption volume (Vm) after calculation of the BET equation.




In general, BET is applicable to low pressure range, not necessarily the monolayer adsorption range.


According to BET theory, it is not necessary to have a fully formed first layer for the second to start forming; this is a good assumption and it is indeed what occurs in real cases. Although most of the adsorption will occur on the solid surface before monolayer formation, there is a statistical probability of molecules adsorbing on top of the incomplete monolayer. This is especially true when the C value of the BET equation is less than 2 (where it starts to look like a type III isotherm).


However, the BET derivation does have assumptions that occur at low pressures: the most critical one being that there is no lateral interaction between adsorbed molecules. If you plot the isotherms in the BET-linearized fashion (that is, $\frac{p/p^0}{n(1-p/p^0)}$ as a function of $p/p^0$) you will see that the values deviate from linearity at pressures above $0.1-0.3$.


The best way of selecting the correct range for fitting the BET plot, is the classic paper by Rouquerol:


Rouquerol, J.; Llewellyn, P. L.; Rouquerol, F. *Is the BET Equation Applicable to Microporous Adsorbents?* In Characterization of porous solids VII; Studies in surface science and catalysis; Elsevier, 2007; pp 49–56.


 1