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Figure 8 Temperature dependence of ice nucleation rate in undercooled water, according
to Equation (5), from DSC data obtained in the isothermal (3) and scanning
(
) modes. Adapted from Michelmore & Franks
23
Table 1 Dimensions and concentrations of active nuclei in undercooled water
17
Nuclei (g
1
)
Temperature (K)
Number of molecules per nucleus
2.3
10
315
263
15,900
3.8
10
14
243
566
7.1
10
9
223
122
shows the partial equilibrium phase diagram of water. The high-pressure
polymorphic forms of ice are omitted. They are of no importance to
freeze-drying, because they are stable only at pressures in excess of 200
MPa; some of them cannot coexist at all with liquid water. Included in
Figure 9 is the pressure dependence of the nucleation temperature. The
pronounced effect of pressure on the nucleation rate, as expressed by the
nucleation temperature, may help to resolve the question: what are the
structural features of an active nucleus? Given the low density of liquid
water and ice-Ih, the effect of pressure would be to distort any specific
molecular configurations that are required for the promotion of ice
growth. Figure 9 demonstrates a so far unexplained parallelism between
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