Absence of a Diffusion Anomaly in Water Perpendicular to Hydrophobic Nanoconfining Walls

TL;DR

This study uses molecular dynamics simulations to explore water diffusion in nanoconfined spaces, revealing that the diffusion anomaly present in bulk water is absent perpendicular to hydrophobic walls, likely due to confinement effects.

Contribution

It demonstrates that water's diffusion anomaly is absent in the direction perpendicular to hydrophobic nanoconfining walls, highlighting the impact of confinement on water dynamics.

Findings

Diffusion anomaly is absent perpendicular to the walls.

Diffusion anomaly persists parallel to the walls.

Nanoconfinement likely causes the absence of the anomaly.

Abstract

We perform molecular dynamics simulations to investigate the diffusive motion of TIP5P water in the direction perpendicular to the two hydrophobic confining walls. To calculate the diffusion coefficient, we use the concept of the characteristic residence time which is calculated from the exponential decay of the residence time probability distribution function. We find that a diffusion anomaly of water, increase of diffusion upon compression, is absent in the direction perpendicular to the confining walls down to the lowest temperature we simulate, 220K, whereas there is a diffusion anomaly, similar to that in bulk water, in the direction parallel to the walls. The absence of a diffusion anomaly may arise mainly due to nanoconfinement, rather than due to the hydrophobic property of the confining walls.