Biology Reference
In-Depth Information
Fig. 3. Polymorphism: the same compound may appear as different solid materials hav-
ing different crystal structures (a classic example is carbon in the form of graphite and dia-
mond). The figure shows two main types of polymorphism in terms of solubility curves
for two different polymorphs of the same compound. The stable polymorph always has the
lowest solubility. In a monotropic system, one structure is the stable structure over the
entire temperature range and hence always has the lower solubility. In enantiotropic sys-
tems, there is a change in relative stability at a certain temperature below the melting point,
and hence there is a change in relative solubility.
When crystallization is undertaken in polymorphic systems, it is by no
means uncommon for a metastable form to appear initially. This has even
been called the Ostwald Rule of Stages . Accordingly, the form that crystal-
lizes first is not only a question of thermodynamics but also depends on the
rate of nucleation and growth of the respective polymorphs. The rate of
nucleation and growth can be slower for the stable form than for the
metastable one. A crystallized metastable phase can transform more or less
quickly into the thermodynamically stable form. The phase transformation
will only occur with certainty if the stable phase is present, either as a result
of nucleation or by seeding. When dealing with problems of polymor-
phism, it is important to consider both thermodynamic and kinetic aspects.
Particle size
Size is not always the most important product property, but there are few
processes where size does not matter. Particle size can be important for
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