Supercooling anomalies

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. 

In a supercooled liquid near the glass transition temperature, the self-consistent calculation is the only way to explain the anomalies in different dynamical quantities. As mentioned before, the first such self-consistent calculation was done by Geszti to explain the behavior of the viscosity near the glass transition temperature. He had argued that an increase in the viscosity slows down the structural relaxation and thus the relaxation of the density mode. This in turn increases the density mode contribution to the viscosity, t]spp... 

Sorensen, C.M., Clathrate Structures and the Anomalies of Supercooled Water, in Proc. 12th Int. Conf. on Prop, of Water and Steam, Orlando, FL, September (1994). 

The anomalies of liquid water become more pronounced when it is supercooled. For example, the volume and entropy fluctuations of liquid water become more pronounced as the temperature decreases. This is in contrast to most other liquids, in which the volume and entropy fluctuations become smaller as the temperature is lowered. Furthermore, the volume and entropy fluctuations in water at less than 4°C are anticorrelated, that is, the increase in volume which occurs when water is cooled results in a decrease in entropy (Debenedetti, 2003). 

Tmskett and Dill (2002) proposed a two-dimensional water-like model to interpret the thermodynamics of supercooled water. This model is consistent with model (1) for liquid water. Cage-like and dense fluid configurations correspond to transient structured and unstructured regions, observed in molecular simulations of water (Errington and Debenedetti, 2001). Truskett and Dill s model provides a microscopic theory for the global phase behavior of water, which predicts the liquid-phase anomalies and expansion upon freezing. 

The question of whether there is a tme glassy nature of amorphous ices is of interest when speculating about possible liquid-liquid transitions in (deeply) supercooled water. For true glasses, the amorphous-amorphous transitions described here can be viewed as the low-temperature extension of liquid-liquid transitions among LDL, HDL, and possibly VHDL. That is, the first-order like LDA <-> HDA transition may map into a first-order LDL HDL transition, and the continuous HDA <-> VHDA transition may map into a smeared HDL VHDL transition. Many possible scenarios are used how to explain water s anomalies [40], which share the feature of a liquid-liquid transition [202, 207-212]. They differ, however, in the details of the nature of the liquid-liquid transition Is it continuous or discontinuous Does it end in a liquid-liquid critical point or at the reentrant gas-liquid spinodal ... 

Water is well known for its unusual properties, which are the so-called "anomalies" of the pure liquid, as well as for its special behavior as solvent, such as the hydrophobic hydration effects. During the past few years, a wealth of new insights into the origin of these features has been obtained by various experimental approaches and from computer simulation studies. In this review, we discuss points of special interest in the current water research. These points comprise the unusual properties of supercooled water, including the occurrence of liquid-liquid phase transitions, the related structural changes, and the onset of the unusual temperature dependence of the dynamics of the water molecules. The problem of the hydrogen-bond network in the pure liquid, in aqueous mixtures and in solutions, can be approached by percolation theory. The properties of ionic and hydrophobic solvation are discussed in detail. 

As we have mentioned in the Introduction, the location of the critical point of the lowest density liquid-liquid transition of real water is unknown and both scenarios (critical point at positive or at negative pressure) can qualitatively explain water anomalies. Recent simulation studies of confined water show the way, how to locate the liquid-liquid critical point of water. Confinement in hydrophobic pores shifts the temperature of the liquid-liquid transition to lower temperatures (at the same pressure), whereas effect of confinement in hydrophilic pores is opposite. If the liquid-liquid critical point in real water is located at positive pressure, in hydrophobic pores it may be shifted to negative pressures. Alternatively, if the liquid-liquid critical point in real water is located at negative pressure, it may be shifted to positive pressures by confinement in hydrophilic pores. Interestingly, that it may be possible in both cases to place the liquid-liquid critical point at the liquid-vapour coexistence curve by tuning the pore hydrophilicity. We expect, that the experiments with confined supercooled water should finally answer the questions, concerning existence of the liquid-liquid phase transition in supercoleed water and its location. 

He shows that pure water at normal pressures cannot be supercooled below -40°C and that virtually all physical properties of water point to a "lambda anomaly" at -45 C [1]. 

The possible existence of an endpoint for the supercooled liquid locus is particularly Interesting in view of the experiments of Angell and coworkers (7,8,9,10). They find that pure water at ordinary pressures (even very finely dispersed) cannot apparently be supercooled below about —40 "C, and that virtually all physical properties manifest an impending "lambda anomaly at T, = —45 . The most striking features of this anomaly are the apparent divergences to infinity of isothermal compressibility, constant-pressure heat capacity, thermal expansion, and viscosity. We now seem to have in hand a qualitative basis for explaining these observations. 

B. Jana, R. S. Singh and B. Bagchi, String-Uke propagation of the 5-coordinated defect state in supercooled water molecular origin of dynamic and thermodynamic anomalies. Phys. Chem. Chem. Phys. 13 (2011), 16220-16226. 

Several explanations have been proposed to account for the anomalies of water (i) the stability limit conjecture [18], (ii) the metastable liquid-liquid critical point (LLCP) hypothesis [19], and (iii) the singularity free (SF) scenario [20]. An excellent review of the properties of supercooled water and the explanations proposed can be found in Ref. [16]. We just give here a brief overview. 

Figure 8. Left panel phase diagram of ice T> T (P)) and transition lines corresponding to the ice Ih-to-HDA, LDA-to-HDA, and HDA-to-LDA transformations T

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