Surprising properties of water on its binodal as the reflection of the specificity of intermolecular interactions

TL;DR

This paper investigates the unique thermodynamic properties of water on its binodal, highlighting how molecular rotation and dimerization influence its behavior, and compares it with argon and hydrogen sulfide.

Contribution

It reveals the role of molecular rotation in water's similarity to argon and introduces modified equations of state to describe its binodal behavior and entropy peculiarities.

Findings

Water's density and heat of evaporation show argon-like behavior due to molecular rotation.

The entropy diameter of water exhibits nontrivial temperature dependence linked to molecular rotation.

Strong dimerization and inner rotation of water molecules near the critical point are observed.

Abstract

In the paper the behavior of density (or specific volume), the heat of evaporation and entropy per molecule for normal and heavy water on their coexistence curves is discussed. The special attention is paid on the physical nature of the similarity in the behavior of density and the heat of evaporation for water and argon as well as the nearest water homolog $H_{2} S$. It is shown that the appearance of this similarity is a consequence of the rotational motion of water molecules, which averages the inter-particle potential in water and leads it to the argon-like form. To describe the fine distinctions in the behavior of the binodals for water and argon the dependence of the proper molecular volume on pressure is taken into account. In accordance with this often used the van der Waals and Carnahan-Starling equations of states are modified. The very surprising behavior of the entropy diameter for water is analyzed. It is shown that the nontrivial details of the temperature dependence for the entropy diameter is directly connected with the peculiarities of the rotational motion of molecules in water. The effect of strong dimerization of water molecules in the fluctuation region near the critical point is studied in details. The inner rotation of monomers forming the dimer $(D_{2} O)_{2} $ near the critical point is discovered.