Crystallisation Flashcards
What is crystallisation and what are the two major steps?
Formation of crystals from another phase via nucleation and growth.
What is the driving force behind crystallisation?
Thermodynamics, minimisation of G total. Consider as a chemical equilibrium.
What is a phase diagram?
Graphical representation of physical states of a system under different conditions of temperature, pressure and composition.
What is the phase rule?
F = C +2 - P
f = degrees of freedom
c = components - composition of system
p = phases
2 - two intensive physical conditions
What can we do with the phase rule?
Allows us to construct phase diagrams as can characterise and predict chemical state and phases of a system under many physical conditions. NB. applies to thermodynamic equilibria, nothing to do with kinetics.
What is supersaturated and why is it important?
Supersaturation - how far we are from equilibrium in a solid-solution dissolution equilibrium
Important as equilibrium not reached instanteously, supersaturated requires generation
How can supersaturation be mathematically expressed?
∆μ eq-SS = RTlnS
where S ~ cA ^SS/ cA ^eq
S = supersaturation parameter
∆μ arises from equilibrium distance..∆G type relations.
What are the stable, metastable and unstable zones of the solubility phase diagram and what physical processes are happening in each?
Saturated curve between stable and metastable regions. There exists a higher curve that indicates the boundary between metastable and unstable regions.
In unstable region, we see immediate crystal formation,and in metastable region, nucleation is occurring. Nothing is happening in the stable region.
How can supersaturation be generated?
Evaporation, cooling and anti-solvent addition
How does generation of supersaturation by evaporation work?
Evaporate solvent to past saturation curve and nucleate, solution concentration after crystal formation on curve at same temperature as start.
How does generation of supersaturation by cooling work?
Cool solution past saturation curve to nucleate, solution concentration after crystal formation on curve at same temperature as cooled to.
How does generation of supersaturation by anti-solvent addition work?
Solubility curve changes with addition of anti-solvent. By adding anti-solvent, decrease solubility of A in mixture of solvent, leading to crystallisation.
When do you use each method of supersaturation generation?
Flat solubility curve = evaporative
Steep = cooling
High solubility = anti-solvent crystallisation
How can polymorphs be identified by solubility curves?
Two types: monotropic or enantiotropic.
Monotropic - one polymorph more stable across whole range, solubility lower.
Enantiotropic - inflection points in curve suggest change in forms. super saturation of one form can lead to formation of other form.
