Bonding 3

Study Notes

Crystalline solids have ordered arrays of ions, atoms, or molecules held together by electrostatic or non-electrostatic forces.
Crystalline solids have well-defined faces and edges.
Amorphous solids lack order in their arrangement of constituent particles.
Amorphous solids are often powders.
Some amorphous solids are super-cooled liquids with high viscosities (e.g., different types of glass).
Amorphous solids are less thermodynamically stable than crystalline forms.
Amorphous solids can float like a gas or flow like a liquid.

Molecules in crystalline materials are arranged in an ordered pattern, with this repeating pattern contained within the unit cell (smallest repeating unit).
Crystalline solids have long-range ordering; the order of atoms extends throughout the entire crystal.
Crystalline solids have narrow, and reproducible melting ranges.
Crystalline solids have well-defined faces and edges.
Crystalline solids' properties often depend on the orientation of the particles (anisotropic).
Crystalline solids diffract X-rays, producing a distinctive diffraction pattern.
Crystalline solids fracture or cleave under pressure.

Amorphous solids have broad, variable melting ranges (approximately 15°).
Amorphous solids have no well-defined faces or edges.
The properties of amorphous solids do not depend on particle orientation (isotropic).
Amorphous solids do not diffract X-rays.
Amorphous solids yield and flow under pressure.

Amorphous solids are used as packing materials (e.g., glass bottles).
Amorphous solids are used as excipients (additives) in dosage forms, e.g., fused silica (thickening agent or good flow properties).
Amorphous forms of active ingredients (e.g., Novobiocin), can be more therapeutically active and more soluble/dissolve quicker than their crystalline counterparts.
Fine amorphous powders can be dispersed in liquids for use in injections and aerosol formulations.

Crystallisation is the production of a single-component crystalline phase from a multi-component fluid phase.
The process involves three steps:
- Supersaturation of the solution (creating a solution with more dissolved solute than a saturated solution at maximum solubility). This can be achieved by cooling a saturated solution, evaporating a saturated solution or by adding a precipitant (e.g. H₂O to an alcoholic solution).
- Formation of crystal nuclei (process of crystallization will start once crystals of the same/similar structure as the desirable drug are introduced, but will spontaneously occur beyond the metastable region).
- Crystal growth (drug concentration depletion from solution; occurs around growing crystals — resulting in a depletion of drug molecules in the solution; concentration gradient from the bulk solution to the crystal face; larger supersaturation = higher growth rate.