BET theory

BET theory models gas molecules adsorbing in multiple layers on a solid surface. Its equation is commonly used to estimate specific surface area from gas-adsorption data.

BET equation: A linearized relation between relative gas pressure and adsorbed amount used to estimate monolayer capacity and surface area. It is the calculation at the center of the theory’s practical use.
Specific surface area: Surface area per unit mass or volume of a material. This is the principal material property BET analysis is used to estimate.
Langmuir adsorption model: A model of adsorption onto a uniform surface that assumes a single layer of adsorbed molecules. Its monolayer assumption contrasts with BET’s allowance for multilayer adsorption.
Stephen Brunauer: A physical chemist whose work on gas adsorption helped establish the BET theory. He co-developed the theory that bears his initials.
Monolayer capacity: The amount of adsorbate required to cover a surface with one molecular layer. The BET fit estimates this quantity before converting it into surface area.
Brunauer–Emmett–Teller method: A procedure for estimating specific surface area by fitting gas-adsorption data to the BET equation. It is the practical measurement method built around the theory.
Dubinin–Radushkevich equation: An adsorption equation used chiefly to describe micropore filling in porous materials. It treats micropore filling differently from BET’s layer-by-layer surface adsorption.
Paul Hugh Emmett: An American chemist known for research on catalysis and gas adsorption. His adsorption research contributed to the theory’s development.
Adsorption isotherm: A relation between the amount adsorbed and the equilibrium pressure or concentration at constant temperature. BET analysis interprets one such relation to infer surface properties.
IUPAC adsorption isotherm classification: A classification system describing adsorption isotherm shapes and their associated surface and pore characteristics. Classification helps assess whether an observed isotherm suits BET analysis.

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Freundlich equationCompared with: BET theory addresses multilayer adsorption, while Freundlich is an empirical concentration relation.