Lecture 3: preformulation considerations 2 Flashcards
What is the characteristic of the arrangement of molecules in a crsytalline structure?
The molecules are packed in a defined order, and this order repeats over and over again throughout the solid structure.
What is a unit cell?
The unit cell is the smallest unit of volume that contains all of the structural and symmetry information of the lattice.
Why does temperature remain constant during a phase change?
Because the heat energy produced is used up in changing the state of matter and also for breaking numerous bonds or attractive forces.
True or false?
For a pure crystalline material, the melting point is always equal to the freezing point.
True
What are some ways to decrease the solubility of a drug in a solvent?
- Solvent evaporation
- Cooling
- Addition of an anti solvent which the drug has a low solubility in
What are the 2 steps for crystallisation?
- Nucleation: formation of a small mass onto which the crystal can grow
- Growth
Why is nucleation a slow process?
Because the initial crystal components must impinge on each other in the correct orientation and have placement for them to adhere
How do you provide nucleation sites for crytallisation?
- Scratching the container
- Dip foreign materials like strings into the container
What are crystal habits?
- Crystal habit refers to the external shape of the crystal
- It is a consequence of the rate at which different faces of the crystal grow.
True or false?
The same compound can crystallize to form different shapes in different solvents.
True
What are some crystallographic defects?
- Interstitial atom
- Vacancy
- Substitutional smaller/larger atom
- Frankel-pair
Why does fast solvent evaporation introduce more crystal defects?
Fast solvent evaporation forces the atoms to form the structure very quickly, and so there isn’t enough time for them to arrange themselves properly.
What are some processes that can cause defects to crystals?
- Rapid recrystallisation from solution
- Freeze drying (it produces amorphous materials)
- Incorporation of impurities
- Mechanical stress like milling and griding and compression
How does the melting point of a crystal change after more defects are introduced, and why?
- The melting point decreases
- Impurities change the intermolecular forces of the crystal structure and make it less stable, so less energy is required to break down the crystal lattice
What is polymorphism?
The ability of a substance to exist as two or more crystalline phases, with different arrangement of the molecules and/or different conformations of the molecules.
