Thermodynamics, Structure, and Dynamics of Water Confined between Hydrophobic Plates

TL;DR

This study uses molecular dynamics simulations to explore how water behaves when confined between hydrophobic plates, revealing shifted thermodynamic anomalies and unique crystalline structures not seen in bulk water.

Contribution

It demonstrates that confinement shifts water's thermodynamic anomalies to lower temperatures and uncovers novel crystalline structures specific to nanoscale confinement.

Findings

Thermodynamic anomalies are shifted by approximately 40 K to lower temperatures.

Confined water exhibits crystalline structures absent in bulk water.

No liquid-liquid phase transition observed down to 220 K.

Abstract

We perform molecular dynamics simulations of 512 water-like molecules that interact via the TIP5P potential and are confined between two smooth hydrophobic plates that are separated by 1.10 nm. We find that the anomalous thermodynamic properties of water are shifted to lower temperatures relative to the bulk by $\approx 40$ K. The dynamics and structure of the confined water resemble bulk water at higher temperatures, consistent with the shift of thermodynamic anomalies to lower temperature. Due to this $T$ shift, our confined water simulations (down to $T = 220$ K) do not reach sufficiently low temperature to observe a liquid-liquid phase transition found for bulk water at $T\approx 215$ K using the TIP5P potential. We find that the different crystalline structures that can form for two different separations of the plates, 0.7 nm and 1.10 nm, have no counterparts in the bulk system, and discuss the relevance to experiments on confined water.