Settling In Suspensions

• Settling in Suspensions affects stability, dosage uniformity, and quality of pharmaceutical formulations.
• Settling In Suspensions is a natural consequence in suspensions due to gravity acting on particles denser than the liquid.

Stokes’ Law

• The rate of settling V of spherical particles in a fluid is described by:
• $V = \frac{2 r^{2} (\rho_{p} – \rho_{f}) g}{9 \eta} $
• Where:
  • r = radius of the particle
  • ρ_p​ = density of the particle
  • ρ_f​ = density of the fluid
  • g = gravitational acceleration
  • η = viscosity of the fluid

Key Factors Influencing Settling

1. Particle Size: Larger particles settle faster.
2. Density Difference: Greater density difference increases settling rate.
3. Viscosity: Higher viscosity slows down settling.
4. Temperature: Higher temperature lowers viscosity, increasing settling.

Sedimentation vs. Flocculation

• Sedimentation: Settling of individual particles
• Flocculation: Loose clusters form and settle faster (but are easily redispersed)

Problems with Settling

• Caking: Formation of a hard sediment that is difficult to redisperse.
• Non-uniform dose: Especially problematic in oral suspensions.

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