Defining the structural transformation in the liquid-liquid transition of water

For decades, the scientific community has been exploring the possibility of the existence of a liquid-liquid critical point in supercooled water. Recent research and experiments have corroborated the idea of this potential phase transition between two distinct liquid states -- one with low density and one with high density.

Foffi et al investigated the molecular structure of these two differing liquid states.

“We confirmed the presence of a liquid-liquid transition and then investigated the differences in the molecular arrangements of the two coexisting liquids,” said author Riccardo Foffi.

In the high-density liquid versus the low-density liquid, the researchers found distinct “ring” structures, or paths of consecutive hydrogen bonds. Their main discovery was in the high-density liquid, where some molecules were separated by many hydrogen bonds but close together in space.

“It was not easy at first to understand what structural features could be considered characteristic for the two states, which quantities were relevant, and which weren’t,” said Foffi. “But when we found the right point of view, the results just followed naturally.”

The team used computer simulations to generate the trajectories of molecules under extreme temperature and pressure.

“If our description of the structure of water holds consistent over a wide thermodynamic range -- and preliminary results hint at the fact that it does -- much insight could be gained on the origin of the anomalous properties of water,” said Foffi.

The team’s research could be used to study other complex liquids, where the structural characteristics are still not identified. Another potential application is in soft-matter research.

Source: “Structural and topological changes across the liquid-liquid transition in water,” by Riccardo Foffi, John Russo, and Francesco Sciortino, Journal of Chemical Physics (2021). The article can be accessed at https://doi.org/10.1063/5.0049299 .