Crystallization and matrix formation - Crystallization dynamics - Nucleation

5 important questions on Crystallization and matrix formation - Crystallization dynamics - Nucleation

What are 3 different ways to get a nucleus?

  1. Seeding --> manually adding small crystals
  2. Heterogeneous nucleation --> particles where molecules from the solution may be absorbed on 
  3. Homogeneous nucleation --> in very pure systems, large supersaturation causes dissolved molecules to associate themselves, concentration of those associates becomes so large it becomes a nucleus

What is the rate of homogeneous or heterogeneous nucleation strongly dependent on?

The amount of supersaturation. When the crystallisation temperature is crossed, the number of nuclei formed rises non-linearly with increasing supercooling or supersaturation.

What happens to the viscosity at larger supersaturation or supercooling?

It begins to increase. This makes it more difficult for the molecules to come together and form nuclei (heterogeneous or homogeneous)

How can nucleation be avoided if you want to keep a supersaturated solution?

Quickly cooling or quickly increasing the concentration of the solution. The nucleation rate goes to zero as very high supercooling/supersaturation.

What happens to the viscosity of a supersaturated solution, if nucleation does not take place, and how does this depend on the type of solution?

It becomes so viscous that it will form a glass.

In sugar solutions, the viscosity increases rapidly with higher concentration or lower temperature. The crystallisation and glass transition lines are close in the state diagram.
In systems where these lines are further apart, you might not see this slowing effect as clearly.

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