Water above the spinodal

Abstract

The liquid spinodal 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 doubting several of those scenarios, we lacked a tool to localize the spinodal and the liquid-liquid critical point. We constructed a unique equation of state combining Speedy's well known expansion 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 (reentrance of the spinodal) was predicted because the density surface is curved by the presence 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 the recent discovery of a compressibility maximum behind the homogeneous nucleation limit at positive pressure, the findings argue in favor of excluding 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.