Pascal, Tod A. and Goddard, William A., III and Jung, Yousung (2011) Entropy and the driving force for the filling of carbon nanotubes with water. Proceedings of the National Academy of Sciences of the United States of America, 108 (29). pp. 11794-11798. ISSN 0027-8424 http://resolver.caltech.edu/CaltechAUTHORS:20110803-120608933
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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.
|Additional Information:||© 2011 National Academy of Sciences. Contributed by William A. Goddard, May 25, 2011 (sent for review April 14, 2011). Published online before print June 27, 2011. Thanks to Prof. Hyung Gyu Park (Eidgenössiche Technische Hochschule, Zurich) and Dr. Hyungjun Kim (Korea Advanced Institute of Science and Technology, KAIST) for helpful discussions. This work is supported by the World Class University program (R31-2008-000-10055-0) of Korea; Energy, Environment, Water, and Sustainability Initiative funding from KAIST; and the generous allocation of computing time from the Korea Institute of Science and Technology Information supercomputing center (KSC-2009-S01-0012). Y.J. acknowledges the support from the Korean-Swiss Cooperative Program (2009-00535). 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.|
|Subject Keywords:||wettability; porous media; capillary action|
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|Deposited By:||Jason Perez|
|Deposited On:||03 Aug 2011 22:51|
|Last Modified:||26 Dec 2012 13:26|
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