Monday, November 1, 2010

Sedimentation:

Sedimentation: Factors governing the physical stability of a suspension are due to unequal gravitational pull on the suspended particles and the medium, and are depicted by Stocke’s equation:

v = d2 (ρ – ρ0) g/18η
Where,
V = sedimentation velocity
d = particle diameter
ρ = particle density
ρ0 = medium density
g = gravitational constant
η = viscosity of continuous external phase

Form the above equation it is clear that the reduction of (ρ – ρ0) value causes decreases the rate of sedimentation significantly. Physical stability of a suspension can be improved by increasing the viscosity of the external phase. In case of non-Newtonian suspending medium, too high viscosity is undesirable because it is difficult to re-disperse the settled material. By using the polyols (e.g. – glycerin or sorbitol) we may increase viscosity but it is rarely used to stabilize pharmaceutical suspensions.

No comments:

Post a Comment