Brunauer, Emmett and Teller (BET) derived the first isotherm for multilayer adsorption. It assumes a random distribution of sites that are empty or that are covered with by one monolayer, two layers and so on, as illustrated alongside. The main equation of this model is
where
and [A] is the total concentration of molecules on the surface, given by
where
in which [A] is the number of bare sites, and [A] is the number of surface sites covered by i molecules.