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FIZIKA A 3 (1994) 3, 177-192
 
THEORETICAL ISOTHERMS FOR MONOLAYER ADSORPTION ON HETEROGENEOUS
SURFACES
IVICA RUŽIĆ
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
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