WATER-ADSORPTION IN MICROPOROUS GRAPHITIC CARBONS

We report grand canonical Monte Carlo simulations at 298 K for a model of water in graphitic carbon pores of silt shape, for a range of pore widths from 5 to 27 angstrom. Large systems (typically 500-1000 water molecules and 2016 carbon atoms) and long runs (30-50 million configurations) were used, and the structure of the carbon wall was included. Results for water are compared with those for methane in the same pores and with the same reduced temperature. For water the adsorption isotherms are of type V, in agreement with laboratory experiments. For the larger pores, the water film consists of an adsorbed layer near the wall, in which H-bonding is considerably weaker than for bulk liquid water, and an interlayer in the interior of the pore that retains a strong H-bonded structure. The solvation force is positive (hydrophobic) in this interlayer, and shows almost no oscillatory character. The water molecules in the contact layer prefer an orientation with their plane parallel to the wall; this orientational preferences extends weakly to the molecules in the interlayer for the higher densities.