Characterization of Single Wall Carbon Nanotubes and Activated Carbon with Water Adsorption in Finite-Length Pore Models
A Grand Canonical Monte Carlo simulation method is used to study the adsorption of water in activated carbon and carbon nanotubes, and the simulation results are compared with the experimental data obtained gravimetrically. To investigate the role of functional groups on water adsorption, the hydroxyl group is chosen and grafted at various locations on the surface. The spacing between adjacent nanotubes is varied. For small spacings, it has no effect on the isotherms if the functional group is located outside the central tube. This is due to the geometrical constraint that hinders the hydrogen bonding for water molecules to form clusters. However, the onset of adsorption occurs at lower pressures for both inside and outside the tubes when the functional group is positioned near the interstice. For larger separation, clusters are readily formed and the functional group enhances adsorption isotherms both inside and outside the tubes, and capillary condensation occurs in the interstices. For slits, the position and concentration of functional group affect adsorption. At low temperatures, water isotherms have an unusual behavior in that the 10°C isotherm is less than that at 15°C, which is in opposite to what thermodynamics suggests. However this behavior can be explained with our computer simulation, suggesting that water adsorption in carbon could be affected strongly by the metastable state and the slow kinetics of cluster formation at low temperatures.