Essential Physics of Low Temperature and Freezing - Freeze-drying of Pharmaceuticals and Biopharmaceuticals - page 38

Figure 13 TTT curves for the crystallisation of water and glycerol. For details, see text
the two edges z . 28 Initially, a planar ice front is maintained. As the front
moves into the body of solution, a concentration gradient is established,
because diffusion of the salt away from the freezing interface cannot keep
up with the rate of advance of the ice front. The concentrated solute
produces a freezing point depression, over and above that due to the
equilibrium colligative effect. This effect is termed ''concentration polar-
isation''. 168 At some stage (between 72 and 84 s in Figure 14), the ice/
solution interface begins to show the first signs of instability. Freezing is
then no longer unidimensional, and dendrites begin to form, eventually
producing the familiar dendritic ice structure. Finally, the residual super-
saturated solution phase is forced into spaces between the dendrites,
where either eutectic crystallisation or vitrification will occur. It is of
importance to realise that, even under rigorously controlled cooling
conditions, i.e. thin films or microdroplets cooled at low rates, the process
becomes uncontrollable after the appearance of interfacial instabilities,
due to solute concentration polarisation ahead of the solid/liquid inter-
face. It follows that under conditions of bulk freezing of unseeded solu-
tions (i.e. undercooling and random nucleation), the freezing process
cannot be controlled.
z NaMnO 4 was chosen because (1) its eutectic temperature lies close to that of NaCl and (2) the
concentration can be monitored spectrophotometrically in the visible range.