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

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 carbonwater 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.

Glass-Forming Tendency of Molecular Liquids and the Strength of the Intermolecular Attractions

“…Similar procedure was also utilized by Hummer and co-workers to model carbon-water interactions in the nanotubes5152. We have considered three cases with the parameter λ  = 0.8, 1.0 and 1.2, where the increase of λ signifies the increase of the attraction strength.…”

On the role of water density fluctuations in the inhibition of a proton channel

“…Consistent with this view, previous studies by Hummer et al (64) and Hummer and coworkers (66) have shown that slight changes in the interactions between water and the confining medium crucially affect water occupancy in the context of small cavities. In particular, interactions control the amplitude of density fluctuations that, on a nanosecond scale, induce transitions between empty and filled states, so-called "wet/ dry transitions" (64,(97)(98)(99). The latter were also observed in biological systems as a consequence of minor changes of the hydrophobic environment, as in the case of the melittin tetramer (100).…”

Multilayer Two-Dimensional Water Structure Confined in MoS2

“…Also, a delicate balance between entropy and enthalpy can render these confined water thermodynamically more stable than the bulk water. 3,[14][15] Theoretical investigations of the structures of the two-dimensional (2D) water confined in between the flat walls have suggested puckered rhombic monolayer ice, planar hexagonal, or amorphous phases depending on the conditions and models employed in the simulations. [16][17][18][19][20][21][22][23][24][25][26][27][28][29] Although the structures of confined water have been predicted for a variety of dimensions and materials using molecular dynamics (MD) simulations, the first experimental observation of the 2D water in between the two graphene sheets was obtained very recently using high-resolution transmission electron microscopy measurements (TEM).…”