Molecular dynamics study of nanoconfined TIP4P/2005 water: how confinement and temperature affect diffusion and viscosity

TL;DR

This study uses molecular dynamics simulations to explore how nanoscale confinement and temperature influence the structural, diffusive, and viscous properties of TIP4P/2005 water, revealing enhanced diffusion in nanotubes and discrepancies in viscosity estimation methods.

Contribution

It provides new insights into water behavior under hydrophobic confinement, proposing a confined Stokes-Einstein relation and analyzing differences with traditional viscosity calculations.

Findings

Water becomes more structured near hydrophobic walls.

Diffusion is enhanced in hydrophobic nanotubes due to curvature effects.

Confined Stokes-Einstein relation yields different viscosity estimates than Green-Kubo.

Abstract

In the last decades a large effort has been devoted to the study of water confined in hydrophobic geometries at the nanoscale (tubes, slit pores), because of the multiple technological applications of such systems, ranging from drugs delivery to water desalinization devices. To our knowledge, neither numerical/theoretical nor experimental approaches have so far reached a consensual understanding of structural and transport properties of water under these conditions. In this work, we present molecular dynamics simulations of TIP4P/2005 water under different hydrophobic nano-confinements (slit pores or nanotubes, with two degrees of hydrophobicity) within a wide temperature range. On the one side, water is more structured near the hydrophobic walls, independently on the confining geometries. On the other side, we show that the combined effect of confinement and curvature leads to an enhanced diffusion coefficient of water in hydrophobic nanotubes. Finally, we propose a confined Stokes-Einstein relation to extract viscosity from diffusivity, whose result strongly differs from the Green-Kubo expression that has been used in previous work. We discuss the shortcomings of both approaches, which could explain this discrepancy.