Experimental evidence of two liquid waters

The National Institute for Materials Science (NIMS) has succeeded in direct observation of reversible transition in a trehalose aqueous solution between two liquid states by measuring the volume changes at high pressures at low temperatures. This result represents experimental proof that there exist two liquid waters at low temperatures and may help explain anomalous behavior of low-temperature liquid water, for example including the density maximum of water at 4°C.

General substances decrease in volume as they are cooled. Although the volume of water also decreases until temperature reaches approximately 4°C, further cooling causes it to expand. This anomalous behavior of water, which has been known for more than 400 years, has yet to be explained scientifically. Recent researches on supercooled water and glassy water (i.e., amorphous ice) have found that water may exist in two liquid states with different densities at low temperatures (Figure)—a possibility relates to the anomaly of liquid water. Theoretical studies show that water undergoes a reversible, discontinuous transition between the two liquid states. However, experiments with liquid water at low temperatures are difficult because of the rapid crystallization, and there is as yet no experimental evidence of the liquid-liquid transition.

Yoshiharu Suzuki (Principal Researcher, Research Center for Advanced Measurement and Characterization, NIMS) recently developed a technique capable of observing a glassy trehalose aqueous solution with a concentration of 0.020 in molar fraction (i.e., a ratio of one trehalose molecule to 49 water molecules) transforming between low-density and high-density liquid states over wide temperature-pressure ranges. Using this technique, he succeeded in directly observing the reversible transition between them (i.e., the polyamorphic transition indicated by the two-way arrow in the Figure) and proving the existence of the two liquid waters. Suzuki then investigated the relationship between the polyamorphic transition and glass transition temperatures of the trehalose aqueous solution by measuring the change in the solution’s volume with the change in pressure. Based on this relationship, he determined the temperature-pressure region in which both a high- or low-density viscous liquids exist. These results indicated that the reversible polyamorphic transition observed above 140 K (-133°C) corresponds to the liquid–liquid phase transition. This is the first confirmation that a low-density liquid exists stably at temperatures as low as approximately 160 K (-113°C) and the first direct observation of the liquid-liquid transition from a low-density to a high-density state.

The two liquid states confirmed to exist at low temperatures under high pressure are thought to influence the physical properties and structures of both pure water and aqueous solutions at room temperature at ambient pressure. Control of these two liquid states may potentially enable modification of the structures and functions of aqueous solutions and biomolecules. In future research, Suzuki plans to investigate the relationship between water in the two liquid states and materials and apply water polyamorphism to scientific fields related to low-temperature water, such as solution chemistry, cryobiology, meteorology and food engineering.

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This project was supported by the JSPS Grant-in-Aid for Scientific Research (project number: 20K03888).

 This research was published in the online version of the Proceedings of the National Academy of Sciences of the United States of America (PNAS) on January 25, 2022（https://www.pnas.org/doi/full/10.1073/pnas.2113411119）.