Initial filling mechanism of predominant water adsorption on hydrophobic slit-shaped carbon nanopores

Predominant water adsorption in hydrophobic slit-shaped carbon nanopores begins at a threshold pressure of water vapor. Adsorption of water vapor was faster for pores of 0.7 nm in width than for pores of 1.1 nm in width. The formation of water dimers is the rate-determining step of the water adsorption from analysis of the adsorption kinetics. The adsorption amount from grand canonical Monte Carlo simulation oscillated with the calculation cycle up to an adsorption amount of 30 mg ml-1 and then steadily increased to an equilibrium adsorption amount. The Gibbs free energy change with the cluster size of water molecules has a barrier for the cluster growth below the critical adsorption density. The snapshot analysis of the grand canonical Monte Carlo simulation showed that formation of nanoscale clusters of water molecules begins just at the critical density of 30 mg ml-1. Then the nanoscale cluster formation should be essential to induce the predominant adsorption at the threshold pressure corresponding to the critical adsorption density.