Effect of pressure on the anomalous response functions of a confined water monolayer at low temperature

TL;DR

This study uses simulations and mean field calculations to analyze how pressure affects the response functions of a confined water monolayer at low temperatures, revealing two maxima linked to hydrogen bonding and cooperativity, supporting the existence of a liquid-liquid critical point.

Contribution

The paper introduces a combined simulation and mean field approach to identify two distinct maxima in response functions of confined water, providing evidence for a liquid-liquid critical point at positive pressure.

Findings

Two maxima in response functions converge at the LLCP with increasing pressure.

The higher-temperature maximum is due to hydrogen bond fluctuations.

The lower-temperature maximum arises from hydrogen bond cooperativity.

Abstract

We study a coarse-grained model for a water monolayer that cannot crystallize due to the presence of confining interfaces, such as protein powders or inorganic surfaces. Using both Monte Carlo simulations and mean field calculations, we calculate three response functions: the isobaric specific heat $C_P$, the isothermal compressibility $K_T$, and the isobaric thermal expansivity $\alpha_P$. At low temperature $T$, we find two distinct maxima in $C_P$, $K_T$ and $|\alpha_P|$, all converging toward a liquid-liquid critical point (LLCP) with increasing pressure $P$. We show that the maximum in $C_P$ at higher $T$ is due to the fluctuations of hydrogen (H) bond formation and that the second maximum at lower $T$ is due to the cooperativity among the H bonds. We discuss a similar effect in $K_T$ and $|\alpha_P|$. If this cooperativity were not taken into account, both the lower-$T$ maximum and the LLCP would disappear. However, comparison with recent experiments on water hydrating protein powders provides evidence for the existence of the lower-$T$ maximum, supporting the hypothesized LLCP at positive $P$ and finite $T$. The model also predicts that when $P$ moves closer to the critical $P$ the $C_P$ maxima move closer in $T$ until they merge at the LLCP. Considering that other scenarios for water are thermodynamically possible, we discuss how an experimental measurement of the changing separation in $T$ between the two maxima of $C_P$ as $P$ increases could determine the best scenario for describing water.