Researchers probe the notion that supercooled water undergoes a liquid-to-liquid phase transition between its disordered and tetrahedrally structured forms - finding evidence of a critical point in this transition - and proposing a two-state model that explains water's unique properties.
Liquid water has been an important component in sustaining life as we know it. However, unlike most fluids, water exhibits unusual behaviors not found in other substances. Among water's "anomalous qualities" include having a maximum density at 4 degrees Celsius and an unusually large heat capacity - characteristics that are related to its ability to sustain life.
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Explaining Water With a Two-State Model
Researchers from the Institute of Industrial Science at the University of Tokyo used a two-state model to describe the coexistence of two different molecular structures within liquid water. The dynamically coexisting structure involves the known disordered structure, plus a "locally favored" tetrahedral structure.
Like most phase transitions - or the change between solid, liquid, gaseous, and even plasma states of matter - there is also a "critical point" where the tetrahedral structured part undergoes a power-law form. At this point, the structure no longer follows conventional length scales.
Researchers then conducted a computer simulation of water molecules and conducted a comprehensive analysis of its structural and thermodynamic data. They also took viscosity, density, compressibility, and X-ray scattering observations for the simulated molecules - closing into where a phase transition critical point must be located - if it actually exists.
Researchers found that this possible critical point happens around -90 degrees Celsius, with a pressure of almost 1,700 atmospheres. Generally, water begins to freeze and turn solid below 0 degrees Celsius, but scientists managed to maintain water in liquid form - although it is in a "metastable supercooled state" thanks to the extremely 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," explained Hajime Tanaka, senior author of the study. He adds that their work illustrates that water's anomalous qualities come from this two-state feature, not from criticality.
Special Even Among Tetrahedral Liquids
In 2018, Hajime Tanaka from the University of Tokyo also conducted a study on observing the anomalies in water as a function of its tetrahedrality. By experimenting with the degree of tetrahedrality, they were able to interpolate their sample to exhibit behavior commonly observed with liquids and water-like behavior. Through computer simulation, researchers observe the phase diagram for different tetrahedral liquids.
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With a parameter designated as lambda, used to describe the volume of tetrahedral structure in the model liquids, researchers were able to find that liquids with greater values of lambda exhibited more anomalies like expansion at low temperatures. They also found out that water maximizes this relationship, making it "so anomalous and special even compared with other tetrahedral liquids."
This tetrahedrality is created by the hydrogen bonds among water molecules, whose structures are always formed in specific directions. Tetrahedral liquid models used in the model follow the Stillinger-Weber potential in characterizing the strength of bonds in the tetrahedral structures.
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