ARTICLE pubs.acs.org/JPCC
How Water Adsorbs in Hydrophobic Nanospaces Thanh X. Nguyen and Suresh K. Bhatia* School of Chemical Engineering, The University of Queensland, Brisbane, QLD 4072 Australia
bS Supporting Information 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.
’ INTRODUCTION The molecular level understanding of the infiltration of water, and its equilibrium and dynamics, in the hydrophobic nanospaces of carbons has long intrigued researchers.16 Water has a very strong electrostatic intermolecular interaction compared to dispersive interactions and forms hydrogen-bonded clusters in either the bulk or the confined state.7 Its adsorption in hydrophobic nanospaces, where the formation of such clusters is restricted, is therefore unexpected2 but has been observed in both laboratory experiments4 and in molecular dynamics simulation studies.1 While H2O adsorption occurs in coals, activated carbons, and other carbonaceous materials from natural sources even at low pressures,8,9 it is believed to be due to the presence of numerous polar carboxyl, OH, or epoxide-like groupterminated surface sites, which have high affinity for H2O. The initial H2O molecules adsorbed on such sites serve as nuclei for the growth of H2O clusters due to electrostatically mediated H-bonding,10 leading to the onset of water filling in the entire structure. Its adsorption in the nanospaces of covalently bonded carbons devoid of such polar sites, such as carbon nanotubes4 (CNTs) or other carbons that have been stripped of such sites by heat treatment under H23 or Ar,11 is, however, much less understood. Besides its scientific interest, the understanding of the mechanisms of water infiltration into these hydrophobic confined spaces is critical to such diverse processes as desalination, separation using nanofluidic devices,12 activity and folding of proteins, exclusion from lipid bilayers, and transport in aquaporins and biological protein channels.1,2,7 It is also known that the presence of coadsorbed water in porous media leads to complex adsorption behavior of other sorbates6 and can affect the efficiency of CO2 capture and storage by adsorption not only r 2011 American Chemical Society
in carbons but also in other materials such as metal organic frameworks.13 Experimental studies have revealed that strong water adsorption commences at moderate relative pressures (P/Po < 0.5) and is rapidly followed by water condensation below saturation pressure, in hydrophobic micro- or mesoporous carbons having insignificant amounts of polar functional groups. These include CNTs,4,14 activated carbon fibers3,15 (ACFs), carbide-derived carbons16,17 (CDCs), and some activated carbons.18 The experimental heat of water adsorption in such hydrophobic carbons generally increases monotonically with coverage,3 quite opposite to the behavior observed for hydrophilic carbonaceous adsorbents and coals where the heat of adsorption decreases with increase in coverage.9 In the latter case, the decrease is due to the strong interaction of the water in the first hydration shell around the polar surface groups, which is followed by weaker interaction of water molecules in the second and subsequent shells at higher coverage. While such behavior is commonly observed in adsorption, the anomalous behavior in hydrophobic carbons is yet to be explained. Several simulation studies6,15,19 of water adsorption in single graphitic slit-like pores with various pore sizes have shown the onset of the water filling in narrow pores (carbon centercenter pore width 16 Å). However, activated carbons including ACFs are generally disordered materials, whose nanostructure comprises a wide range of pore sizes up to 20 Å or larger. Received: June 7, 2011 Revised: July 18, 2011 Published: July 19, 2011 16606
dx.doi.org/10.1021/jp2053162 | J. Phys. Chem. C 2011, 115, 16606–16612
The Journal of Physical Chemistry C Recent experiments with a variety of hydrophobic micro- and mesoporous carbons have shown that adsorption isotherms of both nitrogen at 77 K and water at 298 K yield similar specific pore volumes,18,20,21 indicating water filling of both micro- and mesopores at moderate pressures (P/Po < 0.8). This would appear to be in contradiction with observations of negligible uptake of water on nonporous graphitic carbon black below the saturation pressure6,22 since the dispersive interaction energy of water with carbon walls of mesopores (>40 Å) is equivalent to that of hydrophobic open carbon surfaces.23 This leads to two important questions: (i) how water fills wide hydrophobic pores (>16 Å) below the saturated pressure, and (ii) does the water filling in ultramicropores (widths
