Updates to water’s equation of state hold the key to describing its anomalous behaviors

Despite its ubiquity and necessity for all known forms of life, water exhibits a large number of anomalies that have been difficult to understand. For example, water expands upon freezing and melts under pressure, both of which are unusual. To help explain its anomalous behaviors, Michal Duška considered an update to water’s equation of state.

“A lot of theoretical concepts have been tested on molecular or statistical physics models, so we know which phenomena could be behind water anomalies,” said Duška. “But for real water, a definite proof is still out of reach.”

Duška explored the relationship between two thermodynamic properties at play in describing water’s characteristics. One is the liquid-vapor spinodal, the limit of stability beyond which only vapor can exist, and the other is the hypothesized second critical point in water’s liquid-liquid phase transition.

To implement this second critical point, water must be treated as separately existing in two different forms, a low-density form and a high-density form differing both in density and in the arrangement of the hydrogen bond network. The cooperativity of hydrogen bonds ensures the two forms of water separate in a phase transition, controlling their ratio.

After implementing the second critical point into water’s equation of state, Duška evaluated the capacity

of describing the liquid-vapor spinodal. He shows his equation can describe correctly

the spinodal in the presence of a second critical point. The new equation of state can accurately describe water’s thermodynamic properties, including its many anomalies.

Extending this representation to amorphous ice will be the focus of Duška’s future efforts, allowing for the development of a complete equation of state of polymorphous water.

Source: “Water above the spinodal,” by Michal Duška, Journal of Chemical Physics (2020). The article can be accessed at https://doi.org/10.1063/5.0006431.