The Langmuir Isotherm and Its Limitations

Physical Bectrochemistry Fundamentals, Techniques and Applications. Eliezer Gileadi Copyright 2011 WILEY-VCH Verlag GmbH Co. KGaA. Weinheim ISBN 978-3-S27-31970-1 

The Langmuir isotherm is only applicable if the following conditions are satisfied  

2) The surface is assumed to have well-defined sites, with each molecule of the adsorbate occupying a single site, and each site able to accommodate only a single adsorbate molecule. For example, the adsorption of Ag on Au satisfies this condition, since the crystal radii of the two elements are almost equal (Ag 0.1445 nm Au.0.1442 nm). On the other hand, the adsorption of Pb on Au cannot be fitted rigorously to the Langmuir isotherm, since the atomic radius of Pb is 0.175 nm. 

3) Equilibrium between the species in the bulk and on the surface is assumed. 

4) The isotherm is applicable only to monolayer adsorption. The last point may seem obvious when an isotherm is discussed in the context of adsorption of intermediates in electrode kinetics, but it should be emphasized here that this does not apply to the discussion of surface excess and the Gibbs adsorption isotherm, which were treated in Section 9.1. 

---