Researchers at the Institute of Industrial Sciences at the University of Tokyo are sifting through experimental data to study the possibility of supercooled water having a liquid-to-liquid phase transition between disordered and tetrahedrally structured forms. They found evidence of a critical point based on the co-formation of tetrahedra and showed its insignificant role in water anomalies. This work shows that the special qualities of water ̵
Liquid water is indispensable for life as we know it, but many of its properties do not match the way other liquids behave. Some of these anomalies, such as the maximum density of water at 4 ° C and its high heat capacity, have important consequences for living organisms. The origin of these characteristics has provoked fierce debate in the scientific community since the time of Röntgen.
Researchers at the University of Tokyo have now used a two-state model that represents the dynamic coexistence of two types of molecular structures in liquid water. These are the known disordered normal-liquid structures and the locally favored tetrahedral structure. As with many other phase transitions, there may be a “critical point” at which the correlation between the tetrahedra assumes a power degree, which means that there will no longer be a “typical” length scale.
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 — the researchers were able to narrow down where the critical point, if any, should be.
“If the formation of tetrahedral structures in liquid water is joint under these conditions, then a liquid-liquid phase transition with a critical point is possible,” says lead author Rui Shi.
The team showed that this happens around a temperature of -90 ° C and a pressure of about 1700 atmospheres. Experiments in this range are extremely difficult: because the water is so below its normal freezing, ice crystals can form quickly. However, samples may remain liquid in a metastable “supercooled” state at these very high pressures.
“We have seen evidence that the critical point is real, but its effect is almost negligible in the experimentally accessible area of liquid water because it is too far from the critical point. This means that water anomalies come from the two-state characteristic, not criticality. “, says senior author Hajime Tanaka. Scientists expect that this project will lead to the convergence of the long debate on the origin of water anomalies and more experimental research on access to the second critical point of water.
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Anomalies and criticality of liquid water, PNAS (2020). DOI: 10.1073 / pnas.2008426117
Provided by the University of Tokyo
Quote: Tetrahedra can explain the uniqueness of the water (2020, October 12), extracted on October 12, 2020 from https://phys.org/news/2020-10-tetrahedra-uniqueness.html
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