A Molecular Model for Adsorption of Water on Activated Carbon: Comparison of Simulation and Experiment

Abstract

Experimental and molecular simulation results are presented for the adsorption of water onto activated carbons. The pore size distribution for the carbon studied was determined from nitrogen adsorption data using density functional theory, and the density of acidic and basic surface sites was found using Boehm and potentiometric titration. The total surface site density was 0.675 site/nm2. Water adsorption was measured for relative pressures P/P0 down to 10-3. A new molecular model for the water/activated carbon system is presented, which we term the effective single group model, and grand canonical Monte Carlo simulations are reported for the range of pressures covered in the experiments. A comparison of these simulations with the experiments show generally good agreement, although some discrepancies are noted at very low pressures and also at high relative pressures. The differences at low pressure are attributed to the simplification of using a single surface group species, while those at high pressure are believed to arise from uncertainties in the pore size distribution. The simulation results throw new light on the adsorption mechanism for water at low pressures. The influence of varying both the density of surface sites and the size of the graphite microcrystals is studied using molecular simulation.