ADSORPTION OF WATER ON ACTIVATED CARBONS - A MOLECULAR SIMULATION STUDY

We report a molecular simulation study for a model of water adsorption on nonporous and porous activated carbons. The grand canonical Monte Carlo method is used, and the temperature is fixed at 300 K. Water mol ecules are modeled as a Lennard-Jones sphere with four square-well sit es to account for the hydrogen bonding. The carbon surfaces consist of planar graphite sheets, with active chemical sites oh the surface mod eled as square-well sites. The effect of the density and geometric arr angement of the active sites on the surface is studied. Both macroscop ic properties (particularly adsorption isotherms) and molecular config urations are obtained. The adsorption mechanism for water on such surf aces is markedly different from that of simple nonassociating molecule s such as hydrocarbons or nitrogen. In contrast to the usual buildup o f adsorbed layers on the surface, water adsorption is characterized by the formation of peculiar three-dimensional water clusters and networ ks, whose formation relies on a cooperative effect involving both flui d-fluid interactions and fluid-solid ones with suitably placed active sites. Both the density and arrangement of the sites on the surface ha ve a pronounced effect on the adsorption. Capillary condensation is ob served only for low densities of active sites; for higher densities, c ontinuous filling occurs.