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Entropy of single-file water in (6,6) carbon nanotubes
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Image of FIG. 1.
FIG. 1.

Simulation system. (a) Slab view of the nanotube filled with water and surrounded by water reservoir. (b) Close-up of the single file of H-bonded water molecules inside the nanotube.

Image of FIG. 2.
FIG. 2.

Free energies of transfer of water into a (6,6) nanotube open to a reservoir as a function of the occupancy number N for temperatures ranging from 280 K to 320 K for λ = 0.752 and 0.785.

Image of FIG. 3.
FIG. 3.

Sensitivity of water occupancy in the open (6,6) nanotubes to temperature. The left panels show the water occupancy N as a function of time from MD simulations at T = 280, 300, and 320 K (bottom to top) with nanotube-water attractive interactions scaled by λ = 0.785. The right panels show the corresponding normalized occupancy histograms P(N). Dashed vertical lines indicate the probability P(N = 5) of the filled state at the lowest and highest temperature simulated.

Image of FIG. 4.
FIG. 4.

Transfer free energy per water molecule in units of k B T as a function of the inverse temperature for open (6,6) nanotubes immersed in TIP3P water. The energy of transfer (ΔU N /N) and entropy of transfer (−ΔS N /Nk B ) are obtained from the slope and intercept, respectively, of lines fitted to the MD data obtained with the Andersen thermostat (middle, bottom) and weak-coupling thermostat (top). Results are shown for λ = 0.752 (top), λ = 0.785 (middle), and λ = 0.8 (bottom) between 280 and 320 K (filled symbols: Andersen26 thermostat; open symbols: weak-coupling25 thermostat).

Image of FIG. 5.
FIG. 5.

Entropy (top) and energy (bottom) of transfer per water molecule in infinite periodic tubes with λ = 1 and different repeat lengths L and diameters D as a function of the average spacing per particle Δz = L/N along the tube axis at 300 K. Wide and narrow pores of diameters 0.835 nm and 0.777 nm were obtained by scaling the carbon-carbon bond lengths of the original pore (D = 0.806 nm) from 0.14 nm to 0.145 and 0.135 nm, respectively. The vertical line indicates the equilibrium spacing of Δz ≈ 0.26 nm in the open nanotube.1 Lines are linear and quadratic fits to the entropy and energy, respectively. The bottom plot shows ΔU N /N (symbols with error bars: from Monte Carlo simulations), and ΔA N /N (magenta line: obtained by combining the fits to the energy and entropy; symbols: from Eqs. (2) and (4)) for the original pore diameter (D = 0.806 nm).

Image of FIG. 6.
FIG. 6.

Comparison of transfer free energies ΔA N –ΔA 5 for different values of λ calculated directly from the logarithm of the occupancy probabilities (open symbols: 280 K; filled symbols: 320 K), and from global fits to the free energies and free energy derivatives with respect to λ (upper and lower lines for N<5 are at 280 and 320 K, respectively). In the fits, the entropy and enthalpy were assumed to be constant and quadratic polynomials in λ, respectively.

Image of FIG. 7.
FIG. 7.

Thermodynamic driving force and entropy-enthalpy compensation for water filling of the (6,6) nanotube at 300 K. Free energy (black triangles), energy (blue circles), and entropy of transfer (red squares) as a function of the occupancy for λ = 0.752 (open symbols) and 0.785 (filled symbols).


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Table I.

Lennard-Jones parameters of the λ-dependent carbon-water interactions.

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Table II.

Thermodynamics of transferring TIP3P water into an open and solvated nanotube at T = 300 K in units of kJ/mol for λ = 0.752 and 0.785. Results for the completely filled N = 5 state are in italics. The first and second line for each N list the results obtained from fits of the temperature dependence; the third line lists the results obtained directly from differences of the system enthalpies and free energies. Results in line one for each N are for weak-coupling thermostat simulations,25 and in lines two and three for Andersen26 thermostat simulations. Numbers in parentheses indicate estimated statistical errors in the last digits.

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Table III.

Thermodynamic properties of bulk TIP3P water and real water near ambient conditions from MD simulations in an NVT ensemble. The excess entropies (last column) are per particle. Numbers in parentheses indicate estimated statistical errors in the last digits.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Entropy of single-file water in (6,6) carbon nanotubes