Water diffusion in carbon nanotubes for rigid and flexible models

TL;DR

This study investigates how water diffusion in carbon nanotubes varies with nanotube structure and water models, revealing complex interactions that influence water mobility in confined spaces.

Contribution

It compares water diffusion in different nanotube structures using both rigid and flexible water models, highlighting the impact of water force field degrees of freedom on diffusion.

Findings

Water forms fewer hydrogen bonds in narrow nanotubes.

Water molecules tend to avoid the nanotube surface due to hydrophobicity.

Greater flexibility in water models reduces water mobility in confinement.

Abstract

We compared the diffusion of water confined in armchair and zigzag carbon nanotubes for rigid and flexible water models. Using one rigid model, TIP4P/2005, and two flexible models, SPC/Fw and SPC/FH, we found that the number of the number of hydrogen bonds that water forms depends on the structure of the nanotube, directly affecting the diffusion of water. The simulation results reveal that due to the hydrophobic nature of carbon nanotubes and the degrees of freedom imposed by the water force fields, water molecules tend to avoid the surface of the carbon nanotube. This junction of variables plays a central role in the diffusion of water, mainly in narrow and/or deformed nanotubes, governing the mobility of confined water in a non-trivial way, where the greater the degree of freedom of the water force field, the smaller it will be mobility in confinement, as we limit the competition between area/volume, and it no longer plays the unique role in changing water diffusivity.