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Formation of supercooled water

Formation of supercooled water

Supercooled water

Supercooled water remains liquid below its melting point.

 

V Supercooled water

V Cold metastable glassy water

V Ultra-viscous water and the glass transition temperature


supercooling of water was discovered by
D. G. Fahrenheit (1686-1736)

Supercooled water

Liquid water cooled below its normal melting point normally remains liquid (in a metastable state) for a few degrees below 0 °C [1573] and then forms solid hexagonal ice if shaken or after seemingly random periods of time (see above right). If it is very pure and cooled quickly or carefully without vibration, the liquid water may supercool further and occasionally to a minimum temperature of about -42 °C, or even lower temperatures at higher pressures or shorter times (~-46 °C, [2706]). As water is (super)cooled the number of tetrahedrally hydrogen-bonded water clusters reduce but they grow much larger [1850]. Supercooled water occurs naturally in high altitude clouds. It possesses many properties that differ anomalously from warm water (see water anomalies) [2580]. It has been proposed that this difference in properties is due to the large proportion of the expanded ES-related clusters in the two-state mixture model of water. In brief, supercooled water contains a greater number of strong tetrahedral hydrogen-bonded water molecules [2501] and pentameric water clusters [2439], and these structures increase as the temperature is lowered. Such structuring does not easily form crystalline hexagonal ice. There is also a significant positive deviation from the extrapolated surface tension behavior below 235 K consistent with the tail of an exponential growth in surface tension as temperature decreases [2737]. This is thought due to support the coexistence of two liquid forms in pure water of macroscopic size at these low temperatures. Interestingly, molecular dynamics simulations indicate that homochiral domains appear as major constituents in supercooled liquid water [2738] . There are earlier [569] and more recent [1794, 1860] comprehensive reviews of the properties of supercooled water.

 

Cluster of five water molecules

Cluster of five water molecules

Deeply supercooled water has large density fluctuations. It has been proposed that, if water could be supercooled to lower temperatures, without crystallization, then it may reach a liquid-liquid transition between low-density and high- density water that is the cause/effect of the nearby second critical point (see diagram) at which two liquid water phases differing only in density merge with divergent behavior of many of pure water's thermodynamic properties [871]. Although this phase change has many supporters, it is impossible to investigate as the water crystallizes before it can be reached no man's land, 150 K< T < 235 K). However, a concentrated solution of the ionic liquid salt hydrazinium trifluoroacetate (N2H5 +.TFA-, xw = 0.84) allowed this phase change to be reversibly crossed [2602], although at a lower temperature (~188 K) than that predicted (~225 K) for pure water. Remarkably, the molecular ratio of the water to salt (~5.94) that is required is very similar to that found in the clathrate hydrates (Clathrate I, CS-I 5.75; Clathrate II, CS-II 5.67; Clathrate H, HS-III 5.67; Clathrate HS-IQ 5.71). This indicates that the ionic liquid salt groups are separated by only a single water molecule layer, and that the structure of LDA may include many clathrate-like structures (as this liquid ionic salt solution is proposed to be thermodynamically very similar to supercooled water and LDA [2602]).

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This page was established in 2004 and transferred in 2016. It was last updated by Martin Chaplin on 8 November, 2017


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