Journal of Colloid and Interface Science 286 (2005) 425–432 www.elsevier.com/locate/jcis Characteristic adsorption functions and the surface structure of solid adsorbents Krisztina László a , József Tóth b,∗ a Department of Physical Chemistry, University of Technology and Economics, H-1521 Budapest, Hungary b Research Institute of Applied Chemistry, University of Miskolc, H-3515 Miskolc-Egyetemváros, P.O. Box 2, Hungary Received 15 July 2004; accepted 15 September 2004 Available online 5 April 2005 Abstract A thermodynamic model of gas/solid adsorption has been constructed from two elements. The first is the original Gibbs equation. The second consists of functions ψ(Θ) or ψ(P) that are calculable from measured isotherms. Based on this model the characteristic adsorption functions (CAFs) were defined and calculated. The CAFs, which concentrate into one function all measured isotherms having the same change in relative free energy of the surface, are very sensitive to the structure of the adsorbents. This statement was tested with nitrogen isotherms measured at 77 K on well-characterized chemically/physically treated activated carbons prepared from poly(ethylene terephtha- late). Changes in the surface structure were followed by small angle X-ray scattering (SAXS). This experimental approach made it possible to observe the correspondence between structural changes and the CAF.  2004 Elsevier Inc. All rights reserved. Keywords: Characteristic adsorption functions (CAFs); Sensitivity of CAFs to structure of adsorbents; Nitrogen isotherms measured on virgin and treated activated carbons; Surface structure measured by SAXS; Correlations between CAF and SAXS measurements 1. Introduction In the past hundred years more than 30,000 publica- tions and 1000 books dealing with gas/solid adsorption have been published. Most of them proposed models to provide a physical (thermodynamic) explanation of the adsorption processes and allowed conclusions to be drawn about the surface structure of the adsorbents. The first of these mod- els was due to Langmuir, who assumed homogeneous sur- faces [1]. Brunauer, Emmett, and Teller’s work also assumed homogeneous surfaces, but in their model the adsorption was of multilayer type [2]. The pore-filling model, introduced by Dubinin and Radushkevich [3], gave a good approximation to the structure of microporous adsorbents (activated car- bons). * Corresponding author. E-mail address: tothig@me.akki.hu (J. Tóth). Up-to-date models are partially based on these old mod- els, and the new ones may be regarded as developments of the classical considerations. A collection of these recent models was published in 2002 [4]. Recently, new physi- cal methods have been applied to determine the structure of the adsorbents, both for the morphology of the surface itself and for the adsorbed particles or complexes. These methods comprise atomic force microscopy (AFM), small angle X-ray scattering (SAXS), and scanning electron mi- croscopy (SEM). The important new information that can be obtained by these techniques has been described in the re- cent literature [5–9]. The common advantage of these phys- ical methods is that they provide visible information about the concrete structure of the solid surface and so they com- plete the data obtained by adsorption–desorption measure- ments. However, for these complex investigations, models or theories of adsorption are required that also reflect the structure of adsorbents. Fortunately, there are many methods for realizing this aim, especially those from which pore-size 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.09.026