Simulated water adsorption isotherms in carbon nanopores

Abstract

Water adsorption isotherms are calculated by grand canonical Monte Carlo simulations for the SPC/E water model in carbon nanopores at 298 K. The pores are of slit or cylindrical morphology. Carbon-slit pores are of widths 0.8, 1.0 and 1.6 nm. The simulated single-walled carbon nanotubes are of 1.4 and 2.7 nm diameter ((10:10) and (20:20) respectively). In all cases considered, the adsorption isotherms are characterized by negligible adsorption at low pressures, pore filling by a capillary-condensation-like mechanism and adsorption–desorption hysteresis loops. For both pore morphologies considered, the relative pressures at which pore filling occurs, and the width of the adsorption–desorption hysteresis loop decrease with decreasing pore size. Adsorption isotherms simulated for water in carbon nanotubes show pore filling at lower relative pressures and narrower adsorption–desorption hysteresis loops when compared to adsorption isotherms simulated in carbon-slit pores of similar sizes. By using representative simulation snapshots, the mechanisms of pore filling and pore emptying are discussed. Pore filling happens by growth of hydrogen-bonded clusters of adsorbed water molecules, without the formation of monolayers as observed in the adsorption of simple fluids. Pore emptying occurs by the formation of bubbles, often in contact with the hydrophobic surface, followed by the coalescence and growth of these bubbles.