14
Nucleation
When the temperature, the pressure, or the solute concentration of a
homogeneous system is changed, a new phase can be possibly formed.
Examples are the formation of a vapor phase when a liquid is heated to
above its boiling point and the formation of sucrose crystals when a sucrose
solution is cooled to below its saturation temperature. The formation of a
new phase is often very slow if the conditions are not far from equilibrium.
For the new phase to develop, nucleation has to occur, i.e., the formation of
small regions of the new phase that are large enough to grow spontaneously.
This is the subject of this chapter. The emphasis is on nucleation of a solid
phase in a liquid. Unless mentioned otherwise, the ambient pressure is
assumed to be constant at about 1 bar.
Most basic aspects are to be found in Chapters 2 and 10, especially
Sections 10.5.3 and 10.6.1.
14.1 PHASE TRANSITIONS
Assume the existence of a phasea which tends upon cooling to change
(wholly or partly) into a phase b. At one temperature Teq there is
equilibrium between the two phases; this implies that the free energyGof
the material in both phases will be equal. AtT<Teqthe value ofGwill be