Part 1 - Nucleation Flashcards
What is nucleation?
The formation of an initial new phase in an existing phase (crystal nucleus in a solution)
What kind of process is nucleation?
Stochastic. Will happen randomly above a certain threshold value of saturation.
What is the nucleation rate?
The number of particles of the new phase per unit time.
How do we classify different types of nucleation?
We distinguish between primary and secondary nucleation. Primary nucleation is the focus of this course, and it consists of homogenous and heterogenous nucleation.
Secondary nucleation are things such as contact, shear, fracture, attrition and needle nucleation.
What are shape factors?
Factors that relate the particle’s volume and surface area to the size of the particle.
What distinguishes primary and secondary nucleation mechanisms?
Primary nucleation is the formation of a nuclei from nothing, while secondary nucleation is when nucleation is affected by the presence of other crystals
What is the difference between homogenous and heterogenous primary nucleation?
Homogenous nucleation happens unaffected by solids, for example a droplet of water in water vapor.
Hetereogenous nucleation happens when a foreign solid influences the rate of nucleation, for example formation of crystals on a reactor wall.
When considering the thermodynamics of nucleation, which energetic competition takes place?
Bulk energy gain from formation of new phase vs. increase in surface energy of new phase.
What is the critical Gibbs free energy for a nucleation process?
When the system energy gain of bulk formation is equal to the system energy loss of interface formation.
Derive the nucleation Gibbs free energy.
See lecture notes / exercise.
The rate of nucleation formation can be written as an Arrhenius velocity of the form J = A exp(-∆G*/kT). What is the pre-exponential factor A?
This depends on the model used to describe the cluster to super-nuclei formation.
How is the critical nucleation size dependent on temperature?
Higher temperature, higher radius. The reason being that supersaturation increases with lower temperature, and the Gibbs free energy associated with the volume is a function of the supercooling, ∆T.
How does a solid surface influence the Gibbs free energy barrier of nucleus formation?
It lowers the energy.
How does the critical Gibbs free energy change of heterogenous nucleation relate to the homogenous critical Gibbs free energy?
It relates with a factor phi. Phi is a function of the wetting angle from Young’s equation.
What is true for primary homogenous nucleation in the industry?
They rarely occur. The reason being that the solutions usually contain foreign particles that act as a substrate for nucleation -> heterogenous nucleation. But for very high supersaturation, the nucleated number can exceed the number of foreign particles, and homogenous nucleation can occur.
