Water above spinodal

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.