CHEMISTRY OF CARBON SURFACE AND MECHANISM OF WATER MOLECULE ADSORPTION

The experimental adsorption isotherms of water vapour and nitrogen on nonporous carbon adsorbents with various specific surfaces and concent rations of primary adsorption centres (PACs) have been measured. The a dsorption of water molecules ends with the formation of either individ ual isolated clusters in equilibrium with saturated vapour or a contin uous adsorption film depending on the concentration of the PACs. In th e former case, the pure differential heats of adsorption are negative over the whole range of adsorption values (differential heat is lower than the heat of condensation), and the threshold adsorption value of water vapour (a(s)) is proportional to the concentration of PACs and c orresponds to the formation of fractions of a dense monomolecular laye r on the adsorbent surface. In the second case, the value of the pure differential heat of adsorption is positive over the entire range of c overage and decreases continuously to zero at saturation, and the a(s) value depends on the specific surface area of the carbon adsorbents, and corresponds to the formation of a 1.7 +/- 0.3 dense monomolecular layer. A method of evaluating the surface of nonporous carbon adsorben ts and the mesopore surface of active carbons based on this experiment al finding is proposed. The mesopore surfaces determined from the wate r and nitrogen (benzene) vapour adsorption isotherms are in good agree ment.