Tetrahedra May Explain Water 's Uniqueness and Life Giving Ability

Credit: Institute of Industrial Science, the University of Tokyo

Using computer simulations of water molecules, along with a comprehensive analysis of experimental structural, thermodynamic, and dynamic data–including X-ray scattering, density, compressibility, and viscosity measurements–the researchers were able to narrow down where a critical point should be, if it exists.

“If the formation of tetrahedral structures in liquid water is cooperative under these conditions, then a liquid-liquid phase transition with a critical point is possible,” lead author Rui Shi says.

The team showed that this occurs around a temperature of ?90°C and a pressure of about 1,700 atmospheres. Experiments in this range are exceedingly difficult: because the water is so far below its normal freezing, ice crystals can quickly form. However, samples can remain liquid in a metastable “supercooled” state at these very high pressures.

“We saw evidence that the critical point is real, but its effect is almost negligible in the experimentally accessible region of liquid water because it is too far from the critical point. This means that water’s anomalies come from the two-state feature and not from criticality,” senior author Hajime Tanaka says. The scientists anticipate that this project will lead to the convergence of the long debate on the origin of water’s anomalies and more experimental research to access the second critical point of water.
 
The work is published in PNAS as “The anomalies and criticality of liquid water” (DOI: 10.1073/pnas.2008426117).
 
Institute of Industrial Science (IIS), the University of Tokyo is one of the largest university-attached research institutes in Japan.