Essential Physics of Low Temperature and Freezing - Freeze-drying of Pharmaceuticals and Biopharmaceuticals - page 33

choice of silver iodide to promote nucleation of water and ice in
supersaturated water vapour. The present consensus, however, identifies
three factors that determine the nucleating efficiency of a substrate: (1) it
should have a small lattice mismatch with ice, (2) it should have a low
surface charge and (3) it should not be wetted by water. The heteroge-
neous nucleation temperature, T het , is not as easily defined as T hom .It
has been suggested that the two processes can be related by

DG het ¼ DG hom f(m, R)
where R is the radius of the (spherical) catalytic particle and m is a wetting
parameter that describes the relative ease of wetting of the particle by ice and
undercooled water, respectively, with m varying as 1 o m o þ 1. For
maximum catalytic efficiency, R 410 nm and m 1.

Great interest was generated by the discovery, some 25 years ago, that
enhanced heterogeneous nucleation is one of the mechanisms employed
by some freeze-tolerant organisms, in particular insects and plants, to
minimise the undercooling of their body fluids when the environmental
temperature drops to below T m . This enables the freezing of the plasma
to proceed under quasi-equilibrium conditions where lethal cytoplasmic
freezing can be avoided. An early, well-documented example is the
alpine plant Lobelia telekii. It grows on the slopes of Mount Kenya, at
temperatures that fluctuate daily between 10 and þ 101C. Its eores-
cence contains a potent ice nucleation catalyst that substantially sup-
presses undercooling. When the air temperature falls to ca. 0.51C,
freezing occurs and the latent heat of crystallisation released prevents
further falls in the temperature of the plant during the night. 25 The
catalyst, probably of polysaccharide origin, has also been shown to be
active in vitro (see Figure 11), where it is seen to inhibit the undercooling
of microdroplets of saline solution (H. Levine, unpublished data).
The environmental implications of biological ice nucleation are still a
subject of intensive research. For instance, ice-nucleating ability has
been observed in spider web fibroin, 26 but it is uncertain whether this
process has any ecological consequences. Unfortunately, no agreement
has yet been reached on nomenclature. Care is required to distinguish
between terms such as ''nucleation temperature'', ''kinetic freezing
point'', ''supercooling point'', and their multifarious interpretations. 27
Heteronucleation does not necessarily require the presence of foreign
solid matter in the bulk or at the surface of the undercooled liquid. It can
also be induced by a sudden compression, an electric field or irradiation.
The mechanism whereby extraneous factors can influence the nucleating
potential within an undercooled liquid is completely unknown. What is
important, however, is that at any given temperature, J het c J hom .

B