Journal of Colloid and Interface Science 282 (2005) 249–260 www.elsevier.com/locate/jcis Does the Dubinin–Serpinsky theory adequately describe water adsorption on adsorbents with high-energy centers? Piotr A. Gauden ∗ Physicochemistry of Carbon Materials Research Group, Department of Chemistry, N. Copernicus University, Gagarin St. 7, 87-100 Toru´ n, Poland Received 4 March 2004; accepted 11 August 2004 Available online 11 November 2004 Abstract The paper is an attempt to explain the mechanism of water adsorption on carbonaceous adsorbents with high-energy centers (surface or cation-exchange groups). The equations formulated previously by Dubinin et al. and Barton et al. are analyzed. Thus, four types of empirical factors defining the decrease in adsorption site concentration are taken into account. The isotherms of water adsorption measured on two microporous activated carbons containing various densities of hydrophilic sites are described by the above-mentioned theoretical isotherm equations. It is shown that none of these models provides an adequate description of the experimental data, especially at low relative pressures. This leads to the conclusion that with regard to the Dubinin and Serpinsky theory, the chemisorption of water at very high-energy adsorption sites needs to be taken into account. In view of the above, a new theoretical relationship is proposed which includes a correct description of the decreasing number of active sites. As a consequence, a good correlation between the new theoretical model and experimental data is observed over the whole range of relative pressures.  2004 Elsevier Inc. All rights reserved. Keywords: Carbon; Water adsorption; Chemisorption; Microporosity; Hydrophilic centers; Primary adsorption sites; Dubinin–Serpinsky theory 1. Introduction Water adsorption on adsorbents has been an important subject of investigation in both experimental and theoretical studies. Although adsorption of water has been studied for a long time, there is no consensus among the scientific com- munity regarding the adsorption mechanism and the shape of the resulting isotherms [1–12]. Nevertheless, there is general agreement that adsorption initially occurs (at low relative pressures) via the formation of 3-D water clusters and is then followed by pore filling at high relative pressures. However, there are significant discrepancies in the details of the role of the surface site type and the pore-filling mechanism. It is well known that activated carbons prepared by chemical modification of a surface have a variety of hydrophilic sites, i.e., surface groups such as hydroxyl, carboxyl, quinine, per- * Fax: +48-056-654-2477. E-mail address: gaudi@uni.torun.pl . oxide, aldehyde, and nitrogen-containing or surface cations, in addition to hydrophobic sites comprising the defects of the graphene basal plane layers [3,6,7,9,13–16]. Thus, the dependence of the concentration of centers at the carbon surface (so-called primary adsorption sites) on water adsorp- tion at low relative pressures cannot be neglected in studies of initial adsorption data. On the other hand, the distribu- tion and distance between the primary adsorption centers are of critical importance. If their location is appropriate, water molecules may bridge adsorbed water molecules: this represents a fluid–fluid cooperative effect. Analysis of the experimental data and theoretical considerations show that significant discrepancies exist between the part played by specific surface groups and their energy of interaction with water molecules. It should be pointed out that chemisorp- tion of water molecules is observed at high-energy centers, a process that has already been reported [3,7,8,17–24]. The uptake of water vapor is thus enhanced by functional groups, their density at the surface, the volume available for adsorp- tion, and the vapor pressure [5,6,9,25]. New investigations 0021-9797/$ – see front matter  2004 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2004.08.091