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

TL;DR

This study uses molecular dynamics simulations to explore how water diffuses in nanoconfined channels between modulated surfaces, revealing enhanced diffusion in 1D channels that surpass bulk water, depending on surface interactions and nanotube size.

Contribution

It provides new insights into the morphology and diffusion behavior of water in self-nanoconfined channels within nanotube bundles, highlighting the effects of surface nature and nanotube diameter.

Findings

Self-diffusion coefficients increase tenfold for hydrophilic surfaces as nanotube diameter decreases.

Water channel morphology transitions from 2D to 1D with increasing inter-surface separation.

Enhanced diffusion in 1D channels exceeds bulk water values, influenced by surface interaction and nanotube size.

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.