Effects of surface chemistry on water adsorption on carbons revealed by theoretical isotherm models

Two spherical resorcinol-formaldehyde-based carbons differing in average particle size and external surface area were precursors of two series of modified adsorbents. The modifying agents and modification conditions were chosen to introduce oxygen and/or nitrogen of different content and not to significantly change the porous structure of the samples. The adsorption isotherms of water from the gaseous phase on 16 carbons were the basis for systematic investigations. The differences between them were analyzed in four relative pressure ranges corresponding to subsequent adsorption process stages. The presence of heteroatoms affected the adsorption amount in the low-pressure range, when water molecules directly interact with the oxygen and nitrogen surface groups. However, it also affected the next stage of adsorption: the formation of multimolecular clusters and pore filling. Adsorption in the third range, corresponding to the final plateau, was similar for all the adsorbents and limited by the micropore volume. Differences between the series were revealed in the fourth range (relative pressure close to unity). Adsorption amounts for carbons with more developed external surface area and interparticle porosity were significantly higher, and the isotherms increased more strongly in this part compared to the other series. Qualitative differences between isotherms were also quantified by fitting the data using generalized Barton and Buttersack models. Some of their best-fit parameters were correlated with the content of heteroatoms. Finally, the water adsorption amount for the relative pressure of ca. 0.25 was found as a simple estimate of the heteroatom content in the carbon.