Molecular simulation study of hydrophilic and hydrophobic behavior of activated carbon surfaces

We report a molecular simulation study of the adsorption of methane/water vapor mixtures on porous activated carbons, which are doped by placing associating oxygenated sites on the pore surfaces. Simulation results show that the adsorption of water does not occur via the formation of a monolayer, followed by further layers (as is common for simple non-associating fluids such as methane), but rather through the formation of three-dimensional clusters centered on the active sites on the surfaces. Sharp capillary condensation transitions are no longer observed due to the change in the filling mechanism. The adsorption depends strongly on the location of the sites; by placing the sites at suitable separations, cooperative "bridging" effects are brought into play and greatly enhance the amount of water adsorbed. The transition from hydrophobic to hydrophilic behavior due to the addition of such sites is demonstrated. A series of snapshots of the equilibrium configurations showing this transition is presented. (C) 1998 Elsevier Science Ltd. All rights reserved.