Supercooled water

Supercooled water remains liquid below its melting point.

Liquid water

Supercooled water

Cold metastable glassy water

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 melting point, is thermo-dynamically less stable than ice but typically 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). Simple molecules, like water, usually crystallize easily. Water is an exception to this generalization. If it is very pure and cooled very 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 (H₂O), ≈ -40 °C (D₂O), ≈ ms, [2706 , 3134 , 3837 ]). At about this temperature (≈ -45 °C), the density of supercooled water equals that of hexagonal ice, (see left).

Supercooling (as low as -20 °C) of large volumes of water (up to 100 ml) for extended periods (as long as 100 days) simultaneously can be achieved by the surface sealing of the water by an oil phase [3233 ]. Many factors can cause supercooled water to crystallize, including vibration, particulate initiators, and neutrons but not electromagnetic irradiation [3402 ]. The freezing process, for supercooled water, takes place in two stages. The first stage produces a distinct peak of infrared emission (heat release) with the rapid but short-lived appearance of dendritic needles inside a spongy ice-water mixture [3638 ].. This is followed by the slower freezing of the bulk.

Water activity of supercooled water inequilibrium with ice Ih, from [3434] — Water activity of supercooled water in equilibrium with ice Ih

from [3434]

A model for the thermodynamic properties of supercooled water has been developed that gives its heat capacity, vapor pressure, density, thermal expansion and water activity (see right) [3434 ]. As water is (super)cooled the number of tetrahedrally hydrogen-bonded water clusters reduces, but they are larger in size [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 readily 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. A two-structure approach to the description of the thermodynamic properties of supercooled and stretched water, below 300 K and up to 400 MPa and down to −140 MPa has been described. This work also clarified the concept of the fast interconversion of alternative states in supercooled water as a phenomenological representation of a distribution of short-ranged local structures. It concluded that they "hope that the two-state approach to supercooled water is more than a grossly simplified phenomenology" [3660 ].

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 [3420 ]. The cause/effect of the nearby second critical point (see the diagram) causing the 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 reach No man's land 150 K< T < 227 K). However, a concentrated solution of the ionic liquid salt hydrazinium trifluoroacetate (N₂H₅ ⁺.TFA^-, x_w = 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-I_Q 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]). Recently, research has explored further into the edges of No man's land giving improved estimates of the possible location of water’s second critical point [3337]. Wide angle x-ray scattering of supercooled water down to 234.8 K shows there are 5-member pentamer rings in low density liquid-like structures [3625]. Using simulations, the size of correlated molecular fluctuations increases on (super)cooling reaching a value corresponding to a few thousand molecules, indicating the growth of spatially localized dynamic fluctuations [3782].

Supercooled water may be formed by the fast evaporative cooling of micrometer-sized water droplets in a vacuum, where a fraction of the droplets remain liquid down to 230.6 K [3298].

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