Part 1 - Crystal growth Flashcards
What happens below the critical concentration for nucleation?
Only growth of particles will relax the supersaturated system towards the equilibrium.
How is the growth mechanism controlled?
Either by diffusion (mass transport) or the reaction (surface integration)
Draw a schematic showing the degree of supersaturation in the bulk of the solution, in the stagnant film surrounding the crystal and the crystal surface.
What are the driving force for diffusion and the reaction?
See slides lecture 4.
Driving force for diffusion is the difference in supersaturation from the bulk solution and the stagnant film. The driving force for the reaction is the difference in supersaturation from the stagnant film and the area just above the crystal growth facet, where it is the saturation concentration.
Which law can be used to describe the diffusion controlled growth?
Fick’s law.
dr/dt = DV (C - C*)/r
where D is the diffusion constant, V is the molecular volume, C is the bulk concentration, C* is the solubility concentration and r is the radius of the particle.
On a surface, where will atoms, ions or molecules attach primarily?
On active sites: kinks and edges
What can happen for high values of S on surfaces?
2D nucleation of growth sites can form, which generates the kinks necessary for further growth. The nucleation rate will be higher than the lateral growth rate, and the surface will be rough.
What happens if the lateral growth rate is faster than the nucleation rate for 2D-nuclei?
The surface will be smooth.
What are the three mechanisms we can distinguish between for reaction controlled growth?
- Growth by 2D-nucleation
- Spiral growth
- Rough growth
What is “Growth by 2D-nucleation”?
It is when, at high supersaturations, 2D-nuclei forms on a surface, and allows for further growth.
What is “Spiral growth”?
Growth that comes from step formation due to screw dislocations.
What is “Rough growth”?
That for high supersaturations, the growth units will attach anywhere on the crystal surface so that the crystal surface becomes rough.
How can we for organic compounds achieve the transition between smooth growth and rough growth?
By increasing the supersaturation or increasing the temperature.
What is the general expression for reaction controlled growth?
G = k_g * (S-1)^g
Where k_g is a constant proportional to the solute concentration, S is the supersaturation and g is a parameter that is dependent on the type of growth.
For reaction controlled growth, give examples of different values of the parameter g.
Spiral growth (low S) = 2 Spiral growth (high S) = 1 2D-nucleation > 2 Rough growth = 1
What can be said for the link between growth rate and surface diffusion?
The growth rate will be proportional to the surface if there is enough time for species to diffuse on the surface.
