Competition. It often happens that some kinds of ‘‘foreign’’
molecules fit more or less in a vacant site in the crystal. These molecules
then compete with those of the crystallizing species for attachment sites.
Generally, an incorporated foreign molecule locally prevents further
crystallization, and it has to move out before a truly fitting molecule can
fill the site. The overall result is slowing down the crystallization.
We will illustrate the phenomenon for the crystallization oflactose.
Lactose is a reducing sugar, and in solution theaandbanomers are in
equilibrium with each other; the ratio b over a is about 1.6. The
crystallization ofa-lactose monohydrate, the most common crystalline
form, has been studied in detail. Figure 15.10 depicts a crystal as formed at
FIGURE 15.10 Common shape of an a-lactose monohydrate crystal. The
crystallographic axes (a, b, and c) and the Miller indices of the faces are given.