Adsorption Isotherms II

Unlike the Langmuir theory of monolayer adsorption, the Brunauer-Emmett-Teller or BET theory considers multilayer adsorption of gas molecules on a homogeneous solid surface with localized, non-interacting sites.

Here, each subsequent layer is held by Van der Waals forces from the layer beneath it. Each new layer can form only after the previous layer has already covered the surface.

The adsorption energy for the first layer is assumed to be constant, and the energy for subsequent layers is equal to the energy of gas liquefaction, which is the energy released when a gas condenses into a liquid.

The amount of adsorption increases with rising pressure due to the formation of additional layers.

The BET equation quantifies the relationship between the volume of gas adsorbed and pressure.

A BET plot is a straight line. It helps estimate the monolayer adsorption volume and the BET constant c. These values are used to find out the number of molecules required to form a monolayer and the surface area per unit mass of the sample.

This plot is accurate for pressures up to one-third of the saturation pressure, with deviations at higher pressures.