BETyhtälö

BETyhtälö, or the BET equation, is a theory used to determine the surface area and pore size distribution of porous materials. It was developed by Stephen Brunauer, Paul Emmett, and Edward Teller in 1938. The equation is based on the multilayer adsorption of gas molecules onto a solid surface. It extends the Langmuir theory of monolayer adsorption to account for the formation of multiple adsorbed layers.

The BET equation relates the amount of gas adsorbed by a solid material to the pressure of the gas at a given temperature. It is derived by assuming that gas molecules adsorb onto the surface in layers. The first layer is assumed to adsorb with the same heat of adsorption as the heat of liquefaction of the adsorbate. Subsequent layers are assumed to adsorb with the heat of adsorption equal to the heat of liquefaction. This leads to an equation that can be linearized for analysis.

By measuring the volume of gas adsorbed at various relative pressures (the ratio of the gas pressure to the saturation vapor pressure of the adsorbate at that temperature), one can plot the data according to the BET equation. The slope and intercept of this linear plot allow for the calculation of the BET surface area and the BET constant. The BET constant is related to the energy of adsorption.

The BET method is widely used in fields such as materials science, chemistry, and geology to characterize porous materials like zeolites, activated carbons, and catalysts. It is particularly useful for determining the specific surface area of these materials, which is a critical parameter influencing their properties and applications, such as adsorption capacity, catalytic activity, and reactivity. Limitations of the BET equation include its assumptions about adsorption mechanisms and its applicability to certain types of pore structures.