Strong Correlations and Fickian Water Diffusion in Narrow Carbon Nanotubes

TL;DR

This study uses molecular dynamics simulations to show that water molecules inside narrow carbon nanotubes exhibit solid-like ordering and undergo Fickian diffusion, with collective motion driven by strong positional correlations.

Contribution

The paper demonstrates that water in narrow nanotubes diffuses normally and moves collectively, supported by a simple random walk model and detailed MD simulation analysis.

Findings

Water molecules form solid-like structures inside nanotubes.

Diffusion remains Fickian over long timescales.

Residence time scales quadratically with nanotube length.

Abstract

We have used atomistic molecular dynamics (MD) simulations to study the structure and dynamics of water molecules inside an open ended carbon nanotube placed in a bath of water molecules. The size of the nanotube allows only a single file of water molecules inside the nanotube. The water molecules inside the nanotube show solid-like ordering at room temperature, which we quantify by calculating the pair correlation function. It is shown that even for the longest observation times, the mode of diffusion of the water molecules inside the nanotube is Fickian and not sub-diffusive. We also propose a one-dimensional random walk model for the diffusion of the water molecules inside the nanotube. We find good agreement between the mean-square displacements calculated from the random walk model and from MD simulations, thereby confirming that the water molecules undergo normal-mode diffusion inside the nanotube. We attribute this behavior to strong positional correlations that cause all the water molecules inside the nanotube to move collectively as a single object. The average residence time of the water molecules inside the nanotube is shown to scale quadratically with the nanotube length.