286116 Mechanism of Water Adsorption in Hydrophobic Nanospaces of Disordered Carbons
The understanding of the mechanism of water adsorption in the hydrophobic nanospaces of carbons is critical to many industrial processes for gas separation and water purification, and to emerging nanotechnologies for desalination, CO2 capture from flue gas, and separation by nanofluidic devices. While there have been numerous attempts at simulating water adsorption in hydrophobic carbons using idealized models of independent slit pores, only qualitative agreement with experiment has been achieved, and the answer to the difficult question of how water enters such spaces has remained elusive. Using grand canonical Monte Carlo (GCMC) simulations with a realistic model of a disordered hydrophobic carbon, we show that the key to the puzzle is the connectivity of the structure – overlooked by independent slit pore models. Our simulations and data confirm that significant amount of water adsorbs below the saturation pressure in purely hydrophobic nanopores, and it is demonstrated that this occurs only when pore entries are sufficiently large to allow the passage of stable hydrogen-bonded water clusters. We investigate the effect of pore connectivity through synthetic models of connected and unconnected slit pores, and show that the connectivity to narrow water-filled pores mediates the adsorption of water in large hydrophobic nanospaces. This unique feature is not observed for nonpolar or weakly polar gases (e.g. Ar or N2) at subcritical conditions, and explains why the Kelvin equation fails to estimate the condensation pressure for water. The results open the door for the design and tailoring of efficient adsorbents for CO2 capture, in which the co-adsorption of water vapor which saturates flue gas is inhibited.
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