BETteori

BETteori, short for Brunauer–Emmett–Teller theory, is a framework that describes the physical adsorption of gas molecules on solid surfaces and the formation of multiple adsorbed layers. It extends the Langmuir model from monolayer adsorption to multilayer adsorption and provides a practical method to determine the specific surface area of a material from gas adsorption isotherms. Developed in 1938 by Stephen Brunauer, Paul Emmett, and Edward Teller, BET theory assumes that adsorption occurs in distinct layers, with the first layer binding more strongly than subsequent layers, and that the heat of adsorption for all layers above the first approximates the heat of condensation of the adsorbate.

The BET equation expressing the isotherm is P/(V(P0 - P)) = 1/(Vm C) + (C - 1)/(Vm C) · (P/P0), where P is the equilibrium partial pressure of the adsorbate, P0 is its saturation vapor pressure, V is the volume of gas adsorbed, Vm is the monolayer adsorbed volume, and C is a constant related to the interaction energies. A linear plot of P/(V(P0 - P)) versus P/P0 in a suitable range yields Vm and C, from which the specific surface area (S BET) is calculated using the monolayer volume and the molar volume of the adsorbate.

BET theory is widely used with nitrogen adsorption at 77 K to characterize powders, catalysts, and porous materials. It requires sample degassing and is most reliable for non-chemically activated adsorption; microporous materials may require alternative analyses or models.