FIZIKA A 3 (1994) 3, 177-192


Center for Marine Research Zagreb, Ruđer Bošković Institute, Zagreb, Croatia

A part of this work was made during a visit to the Department of Chemistry at the University of Aarhus, Denmark.
Received 14 April 1994
Revised manuscript received 4 November 1994

Theoretical isotherms for monolayer adsorption on heterogeneous surfaces have been obtained on the basis of the Langmuirian local isotherm and three different distribution functions of adsorption energies (Boltzmann, quasi-Gaussian and shifted quasi-Gaussian distribution functions). The isotherm obtained with the Boltzmann distribution function proposed 40 years ago by Glueckauf predicts well the maximum surface coverage, Freundlich region, as well as Henry behaviour at extremely low surface coverages. It has been demonstrated that the so-called Dubinin - Radushkevich isotherm appears in the transition between the Freundlich and Henry regions if the upper limit to the energy of adsorption is high enough. The isotherm obtained with the quasi-Gaussian distribution function also predicts well the maximum surface coverage and reduces to the Henry region at low surface coverages. Shifted quasi-Gaussian adsorption energy distribution function suggested by Cerofolini et al. results in mixed isotherms between these two extreme cases. Transition between these two types of adsorption energy distributions can also result in the appearance of the so-called Dubinin - Radushkevich behaviour in a certain range of adsorbate concentrations . The characteristics of the corresponding isotherms are discussed in detail and advantages of Glueckauf's suggestions are demonstrated.

UDC 536.712, 539.233
PACS 68.45.Da, 68.90.+g, 68.35.Md, 82.65.Dp



Copyright by The Croatian Physical Society
For problems or questions please contact