Bonding 3

Crystalline & Amorphous Solids

• 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.

Crystalline Solids

• 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

• 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.

Pharmaceutical Uses of Amorphous Solids

• 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.

Crystallization

• 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.

Pharmaceutical Importance

• Morphology

• Particle size

• Polymorphism

• Solvation

• Hydration

  • Affect how easily a drug can be injected, compressed for tablets. Ibuprofen (examples)