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Water in carbon nanotubes: Adsorption isotherms and thermodynamic properties from molecular simulation
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10.1063/1.1924697
/content/aip/journal/jcp/122/23/10.1063/1.1924697
http://aip.metastore.ingenta.com/content/aip/journal/jcp/122/23/10.1063/1.1924697

Figures

Image of FIG. 1.
FIG. 1.

Schematic view of (6:6) (left), (12:12) (center), and (20:20) (right) single-walled carbon nanotubes (SWCN) considered in this work.

Image of FIG. 2.
FIG. 2.

Simulated adsorption isotherms computed at 298 K. The coverage of the porous surface, (number of adsorbed water molecules per square nanometer of porous surface) is reported as a function of the bulk relative pressure. Triangles are for water adsorption isotherm in (20:20) SWCNs; circles are for water in (12:12) SWCNs; diamonds are for water in (10:10) SWCNs; squares are for water in (8:8) SWCNs; gray triangles are for water in (6:6) SWCNs. Solid symbols are for simulation results along the adsorption path of the isotherm and open symbols are for desorption. Lines are guides for the eye, symbols are larger than statistical uncertainty.

Image of FIG. 3.
FIG. 3.

Isosteric heat of adsorption at low coverage for water adsorbed in SWCNs at 298 K. Symbols have the same meaning as in Fig. 2. Lines are guides for the eye.

Image of FIG. 4.
FIG. 4.

Energy profiles for a water molecule within a (6:6) (black dot-dash line), (8:8) (gray dotted line), (10:10) (black dotted line), (12:12) (gray continuous line), and (20:20) (black continuous line) SWCNs. The pore-water potentials are computed at 298 K.

Image of FIG. 5.
FIG. 5.

Oxygen and hydrogen density profiles for water confined in (8:8) [(a) and (b)], (10:10) [(c) and (d)], (12:12) [(e) and (f)], and (20:20) [(g) and (h)] SWCNs. Results are shown before (left-hand figures) and after (right-hand figures) pore filling at 298 K. Gray lines are for oxygen-atom density profiles and black lines are for hydrogen-atom density profiles.

Image of FIG. 6.
FIG. 6.

Front and lateral views of representative simulation snapshots for water adsorbed in SWCNs at 298 K. From top to bottom figures are for water confined in (6:6), (8:8), (10:10), and (12:12) SWCNs. Dark gray spheres represent oxygen atoms and light gray spheres represent hydrogen atoms. Representative snapshots for water confined in (20:20) SWCNs can be found in Ref. 44.

Image of FIG. 7.
FIG. 7.

Order parameters (a) and (b) along the pore radii for water confined in (6:6), (8:8), (10:10), (12:12), and (20:20) SWCNs at 298 K.

Image of FIG. 8.
FIG. 8.

Atomic density profiles across the pore diameter and along the pore axis for water confined in (8:8) SWCNs at 273 K [(a) and (b)], (10:10) SWCNs at 248 K [(c) and (d)], and (12:12) SWCNs at 248 K [(e) and (f)]. Results are shown after pore filling. Gray lines are for oxygen-atom density profiles and black lines are for hydrogen-atom density profiles.

Image of FIG. 9.
FIG. 9.

Front and lateral views of representative simulation snapshots for water adsorbed in SWCNs. From top to bottom figures are for water confined in (8:8) at 273 K, (10:10) at 248 K, and (12:12) SWCNs at 248 K. Dark gray spheres represent oxygen atoms and light gray spheres represent hydrogen atoms.

Image of FIG. 10.
FIG. 10.

Dipole-dipole correlation function for water confined in (6:6) (light gray), (8:8) (gray), and (10:10) (black) SWCNs. Results were obtained after pore filling at 298 K (continuous lines) and at 248 K (dotted lines).

Image of FIG. 11.
FIG. 11.

Simulated diffractograms for water confined in (12:12) (a), (10:10) (b), and (8:8) (c) SWCNs at different temperatures. Black solid, black dotted, and gray solid lines in (a) and (b) are for results obtained at 298, 273, and 248 K, respectively. Black solid line and gray solid line in (c) are for results obtained at 348 and 273 K, respectively. Results at 273 and at 298 K are indistinguishable in (c).

Image of FIG. 12.
FIG. 12.

Comparison between simulated x-ray diffraction patterns. The diffractograms are translated along the axis to facilitate reading. From top to bottom, the lines represent results for water confined in (10:10) SWCNs at 248 K (see Fig. 11); simulated Debye-functional analysis of a perfect octagonal nanotube; water confined in (8:8) SWCNs at 273 K (see Fig. 11); simulated Debye-functional analysis for a perfect cubic lattice; and simulated Debye-functional analysis for a perfect closed-packed hexagonal lattice. The vertical gray dashed line represents the location of the peak observed by Maniwa et al. (Ref. 34) for water confined within (10:10) SWCNs at temperatures below 235 K.

Image of FIG. 13.
FIG. 13.

Simulated water adsorption isotherms in (12:12) (a) and (20:20) (b) SWCNs at different temperatures. Circles are for results obtained at 298 K, diamonds are obtained at 373 K, triangles are obtained at 423 K, and squares are obtained at 498 K. Symbols are larger than computational uncertainty and lines are guides to the eye.

Tables

Generic image for table
Table I.

Lennard-Jones parameters for the water-water and carbon-carbon dispersive interactions. is the minimum depth of the interaction potential and is the center-to-center distance when the potential passes through zero. is Boltzmann’s constant.

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/content/aip/journal/jcp/122/23/10.1063/1.1924697
2005-06-22
2014-04-18
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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Water in carbon nanotubes: Adsorption isotherms and thermodynamic properties from molecular simulation
http://aip.metastore.ingenta.com/content/aip/journal/jcp/122/23/10.1063/1.1924697
10.1063/1.1924697
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