Enhanced diffusion in self-nanoconfined water channels between periodically modulated surfaces: insights from molecular dynamics simulations

TL;DR

This work addresses how nanoconfinement between periodically modulated nanotube-bundle surfaces influences water structure and transport. It employs all-atom molecular dynamics with TIP4P/2005 water to explore multiple nanotube diameters and inter-surface gaps under hydrophobic and hydrophilic surface interactions, analyzing channel morphology, hydrogen bonding, and axial diffusion. The results show a transition from two-dimensional to one-dimensional confinement as the gap widens, with diffusion greatly enhanced in the one-dimensional regime; diffusion exceeds bulk values and exhibits dramatic increases when reducing diameter from 10 to 5 nm, particularly for hydrophilic (about tenfold) and hydrophobic (about sixfold) surfaces. Interstitial voids exhibit symmetry breaking toward a groove, yielding behavior similar to surface channels. These findings shed light on humidity-dependent conduction in nanoporous materials and offer design principles for tailoring water transport through self-nanoconfined channels in carbon-based nanostructures.

Abstract

Water nanoconfinement is known to occur inside material void spaces, such as 2D confinement between surfaces, 1D confinement inside nanotubes, and variable-dimension confinement inside nanoporous materials. In the present work we investigate, through molecular dynamics simulations, the morphologies and self-diffusion coefficient of water channels that are nanoconfined in the void space between adjacent surfaces of nanotube bundles - an existing class of materials. In our simulations, we begin with water filling completely the void space, and then we progressively increase the inter-surface separation, maintaining the water content. We find that, as the inter-surface separation progresses, the dimensionality of the water channel decreases from 2D to 1D, the latter consisting of self-confined water channels along surface grooves. The morphologies and self-diffusion coefficients of these 1D water nanochannels are strongly dependent on the nature of the water-surface interaction and on the diameter of the nanotubes. Interestingly, as we decrease the nanotube diameter from 10 to 5 nm, the self-diffusion coefficients of the 1D channels increase by tenfold for hydrophilic surfaces and by sixfold for hydrophobic surfaces, surpassing, in both cases, the bulk water values. We also investigated the water channels at the interstitial voids of the bulk bundle material, finding 1D water channels that are similar to the surface ones.

Figures (10)

• Figure 1: Model of nanoconfined water between adjacent surfaces of nanotube bundles. The water molecules are confined to the nanochannel formed by regions A and B, with periodicity defined by the lattice vectors L$_{X}$ (horizontal in the figure) and L$_{Z}$ (perpendicular to the figure). The considered nanotube diameters D are 5, 10, 20, 40 nm, and the distance between the surfaces ( X$_{c}$, the channel width) varies from 1.0 to 3.0 nm.
• Figure 2: Atomic scale snapshot of regions A and B of Figure \ref{['fig_xc']}. The minimum vertical distance between the rows of carbon nanotubes is X$_{c}$, as indicated. This creates a two-dimensional void region between the rows of carbon nanotubes, where water molecules are inserted at a density of 0.310 molecules/nm$^{3}$.
• Figure 3: Two infinite parallel rows of (a)-(e) hydrophobic and (f)-(j) hydrophilic carbon nanotubes (CNT) with a diameter of 5 nm are placed at a distance from each other such that the minimum vertical distance between the rows X$_{c}$ varies from 1.0 nm to 3.0 nm, as indicated. This establishes a 2D nanochannel between the CNT rows where water molecules are inserted at a density of 0.310 molecules/nm$^{3}$.
• Figure 4: Same as Fig. \ref{['fig_xc5nm']}, for nanotubes with diameter of 10 nm.
• Figure 5: Axial diffusion coefficient as a function of the minimum vertical distance between rows X$_{c}$ ranging from 1.0 nm to 3.0 nm. The reported values cover nanosheets of 5 nm and 10 nm in diameter for both hydrophobic and hydrophilic interactions. We also present the values for the experimental harris1980pressure diffusion coefficient and for the theoretical model TIP4P/2005 abascal2005general at 298.15 K.