Gibbs Energy of Liquid Water and the Liquid–Liquid Critical Point Hypothesis

To study the relationship between liquid water and two amorphous ices in terms of energy, the Gibbs energy of liquid water was constructed as a function of pressure and temperature using available experimental thermodynamic data of stable liquid water, metastable supercooled water, and amorphous ices. It is examined whether the energy surface of low-temperature liquid water is consistent with the liquid–liquid critical point hypothesis. Even though the data of the bulk pure low-density liquid water at low pressures was missing, most of the data suggested indirectly that the low-density liquid water has low density and low entropy. In this case, the Gibbs energy surface became consistent with the phase separation of liquid water into two liquid phases. From the definition of the thermodynamic properties, all the complex properties of water would be derived in a unified manner from the Gibbs energy surface.