Hi,
to measure surface areas by means of adsorption what we usually do is: determine the amount of matter adsorbed in the surface as a monolayer and the, multiply the number of particles that form this first monolayer by the area of the substance used as a probe (usually N2, CO2...).
Langmuir determines this monolayer coverage by considering a flat surface, an interaction between the surface and the probe that is near a chemical adsorption (the probe molecule is supposed to stay steady).The graph obtained by the theory is a type I, and that is the reason people still uses this isotherm when calculating apparent surface area for microporous materials (similar shape of experimental data and fitting graph). However, you have to be cautious on it, as the micropores filling kills one of the considerations of the theory: all the surface has to adsorb in an uniform way, an micropores usually fill their bodies completely before monolayer covering is complete.
BET on the other hand considers that you can have the adsorption in multilayers at the surface, even though the monolayer is not well defined at the isotherm, but you can determine the statistical monolayer coverage and them proceed with the calculations, and here you have the reason of the smaller value. BET still fails to determine the real value if microporosity is present, instead you should report as apparent specific surface area.
(When I say real value I mean a more accurate value as real surface is a concept that should be discussed with more precaution)
Mostafa M. Hamed
Langmuir isotherms (1 type) differ from BET isotherms (type 2). In order to compare the surfaces of the same dispersed material, as you do, you need to use the same equation and the isotherms must match. If you use the Langmuir equation for the isotherm of type 1 for one sample, and BET (type 2) for the other, then you will get any results.
Langmuir uses the assumption that a monolayer of gas is formed. BET calculation uses an assumption with multiple layers of gas. With the same surface area of a single N2 molecule the Langmuire surface area is much higher.
