Howard P. Isermann Department of Chemical and Biological Engineering
Ask Prof. Bungay a question
A different equation is more likely to describe adsorption
where the adsorbate exceeds a monolayer. The Brunauer-Emmett-Teller (BET)equation is:
where Cs = concentration at which all layers are filled
Kb = a coefficient
Its assumptions are:
It is important that many unusual adsorption isotherms are fitted well bythe BET equation. This is to be expected when there are three coefficients to manipulate.
The maximum loading, Qm, just multipiles to move the entire curve up and down.
The coefficient, Kb, has a major effect on shape. The concentration at which
all sites are saturated (maybe several layers) can be adjusted to get a portion of the
isotherm. In other words, you can look at just part of the curve. As C approaches
Cs, the denomenator of the equation becomes small, and the curve shoots up.
- Adsorbed molecules stay put
- Enthalpy of adsorption is the same for any layer
- Energy of adsorption is the same for layers other than the first
- A new layer can start before another is finished.
Plotting the B.E.T. Equation with Various Coefficients
"Go with the flow"