How water adsorbs in hydrophobic nanospaces ?

Abstract

The entry of water into hydrophobic nanospaces is critical to a variety of biological processes and in nanotechnologies for desalination and separation by nanofluidic devices. Idealized models of hydrophobic carbons have hitherto been used in simulations to investigate the anomalous adsorption of water, but the answer to the difficult question of how water enters such spaces has remained elusive. Here we show that while water entry does not occur in idealized independent carbon slit pores it is observed in realistic models of carbons having connected pore spaces. Good agreement with experimental water adsorption data is obtained for a realistic atomistic model of a disordered hydrophobic activated carbon fiber. Upon analyzing the adsorption in this atomistic model, and in another comprising adjacent slit pores connected by a small window in the separating wall, we find that the critical factor governing the behavior is the formation of sufficiently large and stable water clusters at windows or nanospaces connecting small and large pores. When this window size is large enough for the formation of a stable water cluster, condensation in a small pore induces the filling of empty large connected pores. This unique feature is not observed for nonpolar or weak polar gases (e.g., Ar or N₂) at subcritical conditions and explains why the Kelvin equation fails to estimate the condensation pressure for water.