Water above spinodal

TL;DR

This paper develops a new equation of state for water that clarifies the roles of the spinodal and liquid-liquid critical point, challenging previous explanations of water anomalies and emphasizing the importance of the critical point.

Contribution

The authors constructed a novel equation of state combining Speedy's model with the liquid-liquid critical point to independently depict the spinodal and analyze water anomalies.

Findings

Reentrance of the spinodal is not valid as an explanation for water anomalies.

The liquid-liquid critical point is essential to explain water's density surface.

Heat capacity data shows significant discrepancies, favoring the more accurate dataset.

Abstract

The liquid spinodal, which is the bedrock of water thermodynamics, has long been discussed alongside the elusive liquid-liquid critical point hidden behind the limit of homogeneous nucleation. This has inspired numerous scenarios that attempt to explain water anomalies. Despite recent breakthrough experiments eliminating several of thous scenarios, we lacked a tool to localize the spinodal and the liquid-liquid critical point. We constructed a unique equation of state combining the famous Speedy's equation and the liquid-liquid critical point to remove that deficit and to review these explanations. For the first time, the proposed equation of state independently depicts the spinodal in the presence of the liquid-liquid critical point and demonstrates that the explanation for water anomalies based on the reentrance of the spinodal is not valid; this feature (reentering the spinodal) was solely predicted based on the curved density surface caused by the existence of the second critical point. However, the critical point alone is not sufficient to explain the shape of the density surface of water. In the new equation, hydrogen bond cooperativity is important to force the critical point to exist outside of zero temperature. Together with mounting evidence for the existence of a compressibility maximum behind the homogeneous nucleation limit at positive pressure, the findings practically exclude all explanations for water anomalies except for the existence of the liquid-liquid critical point at positive pressure. Finally, an extensive study of heat capacity demonstrated profound disagreement between the two major experimental heat capacity datasets and identified the more accurate dataset.