(574d) A Two-Structure Equation of State for Super Cooled and Stretched Light and Heavy Water

Numerous thermodynamic models have been developed to explain the anomalous behavior of supercooled water. One of them, two-structure thermodynamics, is based on a concept of two interconvertible states or structures, A and B, with a fraction of conversion controlled by thermodynamic equilibrium. It has been successfully applied to explain the anomalies in supercooled water and atomistic water-like models. However a disadvantage of the previous works was that the state A (responsible for the vapor-liquid transition and serving as a “background” for the global equation of state) was introduced in the thermodynamics empirically. In this study, we have introduced state A through the Gibbs energy based on a Taylor expansion of pressure around the liquid branch of the vapor-liquid spinodal (the absolute stability limit of liquid with respect to vapor). We apply the new approach to study the difference between ordinary water (H₂O) and heavy water (D₂O), both exhibiting similar thermodynamic anomalies under supercooled conditions. However, heavy water has stronger hydrogen bonds, with a more pronounced tetrahedral structure, than ordinary water. We have found a significant difference between these two substances in the supercooled region. In particular, our model predicts that the Widom line in heavy water crosses the homogeneous ice nucleation line at positive pressure. We discuss the experimental consequences of this difference between water isotopes, especially for the detection of the anomalies in stretched water (at negative pressures).