Entropy of single-file water in (6,6) carbon nanotubes

Department of Chemistry, University of Maine, Orono, Maine 04469-5706, USA.
The Journal of Chemical Physics (Impact Factor: 2.95). 07/2012; 137(4):044709. DOI: 10.1063/1.4737842
Source: PubMed

ABSTRACT We used molecular dynamics simulations to investigate the thermodynamics of filling of a (6,6) open carbon nanotube (diameter D = 0.806 nm) solvated in TIP3P water over a temperature range from 280 K to 320 K at atmospheric pressure. In simulations of tubes with slightly weakened carbon-water attractive interactions, we observed multiple filling and emptying events. From the water occupancy statistics, we directly obtained the free energy of filling, and from its temperature dependence the entropy of filling. We found a negative entropy of about -1.3 k(B) per molecule for filling the nanotube with a hydrogen-bonded single-file chain of water molecules. The entropy of filling is nearly independent of the strength of the attractive carbon-water interactions over the range studied. In contrast, the energy of transfer depends strongly on the carbon-water attraction strength. These results are in good agreement with entropies of about -0.5 k(B) per water molecule obtained from grand-canonical Monte Carlo calculations of water in quasi-infinite tubes in vacuum under periodic boundary conditions. Overall, for realistic carbon-water interactions we expect that at ambient conditions filling of a (6,6) carbon nanotube open to a water reservoir is driven by a favorable decrease in energy, and opposed by a small loss of water entropy.

Download full-text


Available from: Jayendran C. Rasaiah, Sep 27, 2015
26 Reads
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Mechanism of ion permeation through an anion-doped carbon nanotube (ANT), a model of ion channel, is investigated. Using this model system, many trajectory calculations are performed to obtain the potential energy profile, in addition to the free energy profile, that enables to separate the energy and the entropic contributions, along the ion permeation. It is found that the mechanism of the transport is governed by the interplay between the energetic and the entropic forces. The rate of the ion permeation can be controlled by changing the balance between these contributions with altering, for example, the charge and/or the length of ANT, which increases the rate of the ion permeation by nearly two orders of magnitude. The dominant free energy barrier at the entrance of ANT is found to be caused by the entropy bottleneck due to the narrow phase space for the exchange of a water molecule and an incoming ion.
    The Journal of Chemical Physics 10/2013; 139(16):165106. DOI:10.1063/1.4827088 · 2.95 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: Single-file water chains confined in carbon nanotubes have been extensively studied using molecular dynamics simulations. Specifically, the pore loading process of periodic (6,6) and (5,5) single-walled carbon nanotubes was thermodynamically characterized by means of free-energy calculations at every loading state and compared to bulk water employing thermodynamic cycles. Long simulations of each end-state allowed for the partitioning of the free energy into its energetic and entropic components. The calculations revealed that the initial loading states are dominated by entropic (both translational and rotational) components, whereas the latter stages are energetically driven by strong dipolar interactions among the water molecules in the file.
    Physical Chemistry Chemical Physics 01/2014; 16(11). DOI:10.1039/c3cp54554g · 4.49 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: We have carried out classical molecular dynamics simulations on the formation of extended water chains inside single-walled carbon nanotubes (SWCNTs) in water in the presence of selected functional groups covalently attached to the inner wall of the tube. Analogues of polar amino acid sidechains have been chosen to carry out the endohedral functionalization of SWCNTs. Our results show a spontaneous and asymmetric filling of the nanotube with dynamical water chains in all the cases studied. The presence of Asp- and Glu-like sidechains is found to result in the formation of well-ordered water chains across the tube having the maximum number of water molecules being retained within the core with the largest residence times. The presence of methyl or methylene groups along the suspended chain is observed to disrupt the formation of water chains with higher length and/or longer residence times. The importance of hydrogen bonding in forming these water chains is assessed in terms of the relaxations of different hydrogen bond correlation functions. For a given dimension of the hydrophobic nanopore, we thus obtain a scale comparing the ability of carboxylic, alcohol, and imidazole groups in controlling the structure and dynamics of water in it. Our results also suggest that SWCNTs of varying lengths, endohedrally functionalized with Asp- and Glu-like sidechains, may be used as design templates in CNT-based water storage devices.
    The Journal of Chemical Physics 05/2014; 140(18):184511. DOI:10.1063/1.4873695 · 2.95 Impact Factor
Show more