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Research Article

Entropy and the driving force for the filling of carbon nanotubes with water

Tod A. Pascal, William A. Goddard, and Yousung Jung
  1. aGraduate School of Energy, Environment, Water, and Sustainability, Korea Advanced Institute of Science and Technology, Daejeon 305-701, Korea; and
  2. bMaterials and Process Simulation Center, California Institute of Technology, Pasadena, CA 91125

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PNAS July 19, 2011 108 (29) 11794-11798; https://doi.org/10.1073/pnas.1108073108
Tod A. Pascal
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William A. Goddard
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  • For correspondence: ysjn@kaist.ac.kr wag@wag.caltech.edu
Yousung Jung
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  • For correspondence: ysjn@kaist.ac.kr wag@wag.caltech.edu
  1. Contributed by William A. Goddard, May 25, 2011 (sent for review April 14, 2011)

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Abstract

The spontaneous filling of hydrophobic carbon nanotubes (CNTs) by water observed both experimentally and from simulations is counterintuitive because confinement is generally expected to decrease both entropy and bonding, and remains largely unexplained. Here we report the entropy, enthalpy, and free energy extracted from molecular dynamics simulations of water confined in CNTs from 0.8 to 2.7-nm diameters. We find for all sizes that water inside the CNTs is more stable than in the bulk, but the nature of the favorable confinement of water changes dramatically with CNT diameter. Thus we find (i) an entropy (both rotational and translational) stabilized, vapor-like phase of water for small CNTs (0.8–1.0 nm), (ii) an enthalpy stabilized, ice-like phase for medium-sized CNTs (1.1–1.2 nm), and (iii) a bulk-like liquid phase for tubes larger than 1.4 nm, stabilized by the increased translational entropy as the waters sample a larger configurational space. Simulations with structureless coarse-grained water models further reveal that the observed free energies and sequence of transitions arise from the tetrahedral structure of liquid water. These results offer a broad theoretical basis for understanding water transport through CNTs and other nanostructures important in nanofluidics, nanofiltrations, and desalination.

  • wettability
  • porous media
  • capillary action

Footnotes

  • 1To whom correspondence may be addressed. E-mail: ysjn{at}kaist.ac.kr or wag{at}wag.caltech.edu.
  • Author contributions: T.A.P., W.A.G., and Y.J. designed research; T.A.P. and Y.J. performed research; T.A.P. and Y.J. analyzed data; and T.A.P., W.A.G., and Y.J. wrote the paper.

  • The authors declare no conflict of interest.

  • This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1108073108/-/DCSupplemental.

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Entropy and the driving force for the filling of carbon nanotubes with water
Tod A. Pascal, William A. Goddard, Yousung Jung
Proceedings of the National Academy of Sciences Jul 2011, 108 (29) 11794-11798; DOI: 10.1073/pnas.1108073108

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Entropy and the driving force for the filling of carbon nanotubes with water
Tod A. Pascal, William A. Goddard, Yousung Jung
Proceedings of the National Academy of Sciences Jul 2011, 108 (29) 11794-11798; DOI: 10.1073/pnas.1108073108
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Proceedings of the National Academy of Sciences: 108 (29)
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