The Role of Surface Heterogeneity in Multilayer Adsorption

Very little work has been published on this subject, although it is well known that multilayer adsorption may begin at low adsorbate surface coverages. The majority of treatments have adopted the patchwise heterogeneity model or have not accounted for adsorbate self-interactions and extended the analysis by using a multilayer adsorption isotherm as the local isotherm in the integral equation. The effects of multilayer adsorption in the patchwise heterogeneity analysis with a monolayer local isotherm have been demonstrated by van 

A similar approach to Halsey s was also devised by D Arcy and Watt by extending Langmuir s equation [i.e. equation (3)] but including an extra term to account for multilayer formation (assumed to occur at a constant adsorption energy independent of the surface heterogeneity), i.e. 

The results of Dormant and Adamson have also been discussed by Jaroniec and Rudzinski. They present an elegant procedure for determining the adsorption energy distribution by employing a modified BET equation (Anderson s equation ) in which a finite number of adsorbed layers are allowed  

The method is essentially numerical and leads to a series form for F U) in terms of orthogonal Legendre polynomials and coefficients obtained by a least-squares fit to the experimental data. Hsu et have also adopted the modified BET equation (111) and by combining it with a Maxwell-Boltzmann 

Jaroniec and W. Rudzinski, Acta Chim. Acad. Sci. Hung., 1976, 88, 351. 

---