Relations between the diffusion anomaly and cooperative rearranging regions in a hydrophobically nanoconfined water monolayer

TL;DR

This study investigates how the microscopic structure of hydrophobically confined water influences its diffusion behavior, revealing the role of hydrogen bond defects and cooperative regions in diffusion anomalies.

Contribution

It provides a quantitative relation linking diffusion extrema to hydrogen bond dynamics and cooperative rearranging regions in nanoconfined water.

Findings

Diffusion maxima and minima are explained by the interplay of hydrogen bond breaking and cooperative regions.

A quantitative relation reproduces the diffusion behavior in the high-temperature regime.

Cooperative rearranging regions of about 1 nm size are key to understanding diffusion anomalies.

Abstract

We simulate liquid water between hydrophobic walls separated by 0.5 nm, to study how the diffusion constant D_\parallel parallel to the walls depends on the microscopic structure of water. At low temperature T, water diffusion can be associated with the number of defects in the hydrogen bonds network. However, the number of defects solely does not account for the peculiar diffusion of water, with maxima and minima along isotherms. Here, we calculate a relation that quantitatively reproduces the behavior of D_\parallel, focusing on the high-T regime. We clarify how the interplay between breaking of hydrogen bonds and cooperative rearranging regions of 1-nm size gives rise to the diffusion extrema, possibly relevant for both bulk and nanoconfined water.