Water undercooled

Franks, F. Mathias, S.F. Trafford, K. The nucleation of ice in undercooled water and aqueous polymer solutions. Colloids Surf 1984,11, 275-285. 

FIGURE 15.13 Some properties of ice and (undercooled) water as a function of temperature. Given are mass density p, viscosity r, and specific heat at constant pressure cp, all at atmospheric pressure. 

The usefulness of undercooled water as a reaction medium has also been demonstrated in biochemical studies. Thus, the kinetics of biochemical reaction sequences can be sufficiently slowed down to become time resolved at subzero temperatures in the unfrozen state. Even more importantly, complex reaction pathways (mechanisms) are unaffected by undercooling, whereas the use of conventional cryoprotectants (glycerol, ethane diol, dimethylsulfoxide, etc.) alters the pathways, although not necessarily the nature of the end products. Finally, single cells or even cell clusters can be stored and kept intact for considerable periods in undercooled aqueous media. 

Figure 6 The volume free energy of homogeneous nucleation of undercooled water at —40°C as a function of the cluster radius at —40°C (see Equation (2)). According to Equation (3),r = 1.85 nm. The critical nucleus then contains ca. 200 molecules. The value for a was obtained from the nucleation data of Michelmore Franks ...

Apart from the dimensions of critical nuclei under varying conditions, it is also important to consider the kinetics of nucleation, especially relative to the kinetics of crystal growth. The nucleation rate (/) is commonly estimated by means of transition state theory. With the aid of the best available values for transport properties of undercooled water,it has been estimated that in the neighbourhood of —40°C, J increases rapidly with decreasing temperature, by about a factor 20 per degree nucleation is thus a well-defined event that is hardly affected by the rate of cooling. ... 

Reference was made earlier to the use of undercooled water in enhancing the long-term stability of enzymes in solution and the viability of isolated cells. The data in Table 1 and Figure 8 confirm that clean, undercooled water at —20°C, dispersed in the form of small droplets, is most unlikely to freeze, and can be safely used as storage medium for many years. 

Figure 8 Temperature dependence of ice nucleation rate in undercooled water, according to Equation (5), from DSC data obtained in the isothermal (o) and scanning ( ) modes. Adapted from Michelmore Franks ...

Table 1 Dimensions and concentrations of active nuclei in undercooled water ...

Figure 18 Photomicrographs of emulsified cells in undercooled water left, human erythrocytes and right, yeast (S. cerevisiae). The mean diameter of empty water droplets is 7 xm...

Figure 1 The effect of pH on the thermal stability of chymotrypsinogen according to Brandts.Circular symbols refer to a range of pH values triangles refer to 2.3 M urea. Tjj is the well-characterised thermal denaturation temperature. Tj is the experimentally determined denaturation temperature in undercooled water Franks ...

The mechanism of cold inactivation in undercooled water differs from that brought about by cryosolvents. 

R.W. Michelmore and F. Franks, Nucleation rates of ice in undercooled water and aqueous solutions of polyethylene glycol. Cryobiology, 1982, 19, 163-171. 

Next, let us examine the specific heat of undercooled water... 

These then are the properties of undercooled water which are so important in the subject of low temperature resistence of living organisms. Plants, insects and microorganisms seem to know all about these things and can turn them into effective mechanisms of survival. 

Freezing of deeply undercooled water in cold-hardened stems of 16 woody taxa was determined in DTA experiments on nonthawed, thawed and freeze-killed specimens [66], Freezing exotherms were observed down to onset temperatures of -53°C and appeared in size and position in dependence of the wood pre-... 

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