Water Adsorption in Carbon-Slit Nanopores

Abstract

With the use of the grand canonical Monte Carlo simulation, water adsorption isotherms were determined for SPC/E water in slit-shaped graphitic nanopores at 298 K. The pore widths considered were 0.6, 0.625, 0.7, 0.8, 1.0, 1.6, and 2.0 nm. The resulting adsorption isotherms indicated negligible adsorption at low pressures, pore-filling by a capillary-condensation-like mechanism, and adsorption/desorption hysteresis loops. For pore widths equal to or larger than 0.7 nm, the relative pressure at which pore filling occurs and the size of the hysteresis loop decrease with decreasing pore width. For 0.6-nm pores, pore filling occurs at pressures approaching saturation. Upon decreasing the pore width from 2.0 to 0.7 nm, the zero-coverage isosteric heat of adsorption increases from 6 to 14 kJ/mol. The limit at high coverage converges to the enthalpy of condensation for SPC/E water as the pore width increases. From the simulated adsorbed-water densities and distributions across the pores and the use of a mean-field approximation, the solvation force was computed between two flat hydrophobic walls. The force profile oscillates as a function of the wall-to-wall separation. Our results show that attractive peaks of the force correspond to wall-to-wall separations at which confined water molecules cannot establish a finite number of layers, while repulsive peaks correspond to separations at which confined water molecules form well-defined layers of molecules, often stabilized by intralayer hydrogen bonds.