Essential Chemistry and Biochemistry Associated with Low Temperature and Freezing - Freeze-drying of Pharmaceuticals and Biopharmaceuticals - page 54

Figure 4 Simulation of typical concentration changes during the cooling of a NaCl
solution showing undercooling of water, ice nucleation and growth, transient
supersaturation and final NaCl crystallisation, with the re-establishment of
equilibrium conditions
the eutectic temperature of -211C. With further cooling, the solution
becomes supersaturated in NaCl until, after 45 min, when the NaCl
concentration has reached 6 M, nucleation of NaCl becomes effective,
salt spontaneously and rapidly precipitates, and eutectic conditions are
re-established. Any labile biochemical product, present in this particular
solution, would therefore have been exposed to a highly supersaturated
salt solution for 5 min.
The actual crystallisation behaviour of any given solute during freez-
ing depends on its nucleation rate, which, in turn, is affected by the
following factors:
Solution volume and concentration
Cooling rate
Crystal structural complexity
Competing solutes (excipients or contaminants).
Under the conditions shown in Figure 4, a supersaturation degree
reaching approximately 30% is maintained over 5 min before eutectic
equilibrium is established. This indicates that the crystal structure of