(162a) 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.