Supercooling of water

The fact that surfaces disrupt the natural order of a bulk phase serves as basis for Stillinger s attention to supercooling of water in small droplets or clusters [1]. His statistical mechanical approach to the structure of ice Ih and water as hydrogen-bonded in ordered and disordered polygonal structures, respectively, results in a qualitative estimate of the depression of temperature of maximum density. His approach also explains the behavior of supercooled water in terms of structural fluctuations in the bonded bicyclic octameric water network that represents ice Ih. 

Physico-chemical ways of achieving metastability of the initial system are usually related to changes in temperature, pressure (less often), and composition of solvent [10]. The supersaturation (supercooling) of water vapor is the reason for certain meteorological phenomena (cloud formation). The formation of disperse systems upon changes in temperature is the key for the preparation of all polycrystalline materials in metallurgy. Here control of... 

Figure 10.46 The supercooling of water. The extent of supercooling is given by S.

A large number of studies have dealt with the behavior of water below 0°C (e.g., supercooling of water-in-oil emulsions) and determinations of free and bound water, around OX [20-25]. The crystalUzation enthalpy of water depends on temperature (see Ref. 19), which may be important in supercooled emulsions. Moreover, the difference in the specific heats of ice [2.05 J/(g K)] and water [4.18 J/(g K)] may introduce some error. 

It should be mentioned that in the last few years super-cooled water has attracted the interest of many scientists because of its exceeding properties and life at temperatures below 0 °C 1819). Speedy recently published a model which allows for the interpretation of the thermodynamic anomalies of supercooled water 20). According to this model there are hydrogen bonded pentagonal rings of water molecules which have the quality of self-replication and association with cavities. 

To survive freezing, a cell must be cooled in such a way that it contains little or no freezable water by the time it reaches the temperature at which internal ice formation becomes possible. Above that temperature, the plasma membrane is a barrier to the movement of ice crystals into the cytoplasm. The critical factor is the cooling rate. Even in the presence of external ice, most cells remain unfrozen, and hence, supercooled, 10 to 30 degrees below their actual freezing point (-0.5 °C in mammalian cells). Supercooled cell water has a higher chemical potential than that of the water and ice in the external medium, and as a consequence, it tends to flow out of the cells osmotically and freeze externally (Figure 1). 

Dried product resistance normally increases with increasing solute concentration and frequently decreases as the temperature of the frozen product approaches the eutectic temperature or T., . Production of larger ice crystals by a lower degree of water supercooling and/or an annealing process during freezing may also decrease the resistance. 

FIG. 5 The density of liquid and supercooled water as a function of temperature, illustrating the anomalous liquid phase density maximum of water (data from Lide, 2002-2003). 

Water is a very structurally versatile molecule. Water exists in all three physical states solid, liquid, and gas. Under extremely high temperature and pressure conditions, water can also become a supercritical fluid. Liquid water can be cooled carefully to below its freezing point without solidifying to ice, resulting in two possible forms of supercooled water. In the solid state, 13 different crystalline phases (polymorphous) and 3 amorphous forms (polyamorphous) of water are currently known. These fascinating faces of water are explored in detail in this section. 

Ludwig s (2001) review discusses water clusters and water cluster models. One of the water clusters discussed by Ludwig is the icosahedral cluster developed by Chaplin (1999). A fluctuating network of water molecules, with local icosahedral symmetry, was proposed by Chaplin (1999) it contains, when complete, 280 fully hydrogen-bonded water molecules. This structure allows explanation of a number of the anomalous properties of water, including its temperature-density and pressure-viscosity behaviors, the radial distribution pattern, the change in water properties on supercooling, and the solvation properties of ions, hydrophobic molecules, carbohydrates, and macromolecules (Chaplin, 1999, 2001, 2004). 

The cooled reaction product is treated with 200 cc. of water, the layer of oil separated, washed once with a second portion of water, and subjected to distillation in vacuo. The first fraction of the distillate contains benzyl alcohol together with unchanged aldehyde, as well as a small quantity of water. The temperature then rises rapidly to the boiling-point of benzyl benzoate, when the receivers are changed. The product boils at 184-185°/15 mm., and analysis by saponification shows it to consist of 99 per cent ester. A yield of 410-420 g. is obtained, which corresponds to 90-93 per cent of the theoretical amount. This benzyl benzoate supercools readily, but after solidifying... 

The preceding observations stimulated Olander and Rice 4> to search for a substance that is simultaneously simpler" than water yet a "good model of it. They suggested that amorphous solid water [H O/as)], first reported by Burton and Oliver 5> in 1935, satisfied these two requirements. Unlike the liquid, amorphous solid water can be studied at low temperature where the effects of thermal excitation and positional and orientational disorder can be separated. Moreover, it is plausible to accept as a working hypothesis that the amorphous solid is, essentially, extensively supercooled liquid water if so, the properties of the amorphous solid should be directly related to those of the liquid. 

While it has been known for years that water droplets in the micrometer size range can supercool down to -40°C (Fletcher, 1962 Rasmusse et al, 1973), very few attempts have been carried out on water droplets in the nanometer range, which are obtained with micromicellar solutions of water in a number of nonpolar solvents of very low freezing point. Such solutions are homogeneous and of low viscosity they can remain perfectly colorless and therefore optically transparent at very low temperature (s-60 C) and can be used as media to investigate enzyme-catalyzed reactions. 

DL-threonine and L-threonine crystals were supplied from Ajinomoto Co. Inc. and were used without further purification. Excess amounts of DL-threonine crystalline particles were dissolved in water kept at 55, 57, 58 or 60 C. After decantation and filtration each saturated solution was placed in the crystallizer maintained at 50 C. The difference between the saturation temperature and the crystallization temperature was defined as the initial supersaturation in terms of supercooling of the solution and was the driving force for the crystallization. 

Fourier transform infrared reflection-absorption spectroscopy studies (FTIR-RAS) by Tolbert and coworkers (Zondlo et al., 1998) of the uptake of HNO, on ice at 185 K have shown that a supercooled liquid forms on the surface upon evaporation of water, the ice film becomes more concentrated in HN03 and at stoichiometries of 3 1 and 2 1 H20 HN03, respectively, NAT and NAD crystallize out. The reactions of C10N02 and N2Os with the ice also led to the formation of supercooled H20-HN03 liquid layers on the ice surface. 

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