Introduction to Crystallization of Fine Chemicals and Pharmaceuticals - Molecules: Nucleation, Aggregation and Crystallization - page 159

Fig. 6. Development of crystal shape. The shape is governed by the relative growth rate
of different faces and is dominated by those faces having the lowest growth rate. An arrow
describes the gradual movement direction of the surface when it is growing. Different
arrow colors refer to the growth rates of different faces and the arrow length is proportional
to the rate of growth.
largely related to the relative growth rates of the individual surfaces. Note
that the growth rate of a surface relates to the linear translation of the
phase boundary perpendicular to the plane of the surface. A surface that
accumulates new material relatively quickly may gradually decrease its
size and disappear into an edge or a point, whereas slow-growing surfaces
become large and will dominate the outer shape of the crystal.
Supersaturation, temperature, impurities, pH, choice of solvent, and
so on: all influence the growth rate of the individual surfaces. Hence, the
relation between the growth rates of the various surfaces can be affected,
and thus the shape of the final crystals may differ depending on condi-
tions. The sensitivity of the particle shape to various variables is such that
laboratory experiments can even be misleading with regard to what can be
expected on an industrial scale. Frequently, impurities in very low con-
centrations can affect the particle shape significantly. Conversely, this
means that additives can be used to control particle shape.
There is a growing interest in using additives to control the result
of crystallization processes. Additives can be used to affect the shape,
i.e. the relative growth rates of different surfaces, but also to influence
nucleation and overall growth rate. Additives are divided into tailor-made