Settling of Suspensions - Effects of Brownian Movement

FREE SETTLING OF PARTICLES

• Stokes' Law is only valid in dilute pharmaceutical suspensions consisting of less than 0.5 to 2 g of solids per 100 mL of liquid.

• In dilute suspensions, the particles do not interfere with one another during sedimentation, and free settling occurs, making Stoke's law valid

HINDERED SETTLING OF PARTICLES

in 5 %, 10 % or higher concentration suspensions, the particles interfere with one another, and_____ occurs and Stoke’s law no longer applies.

Settling of Suspensions

• The estimation of the physical stability of suspension may be obtained by diluting the suspension to a concentration of 0.5 to 2.0 % of the dispersed phase. 

• Though this is not always recommended because the addition of a diluent will affect the degree of flocculation (deflocculation) of the system, affecting particle size distribution. 

• There is a need for computation of velocity of settling to assure uniformity of distribution of the particles, a quality of acceptable suspension.

• Suspended materials should not settle rapidly and particles that settle should not form a hard cake at the bottom. The particles in Suspension should be kept uniformly distributed throughout the dispersion.

Stoke’s Law

• measures the velocity of sedimentation.

• The rate of settling of the dispersed phase in the dispersion medium is a function of:

  • Particle size

  • The viscosity of the dispersion medium

  • The difference in density between the dispersed phase

  • the dispersion medium

V

settling velocity of the solid

g

acceleration of gravity

p1

mass density of solid

p

mass density of the fluid

d

diameter of the solid (assuming spherical)

v

kinematic viscosity of liquid

1. Decrease mass density of the solid / liquid (directly proportional) 

2. Increase viscosity (inversely proportional)

To decrease velocity of settling:

Effects of Brownian Movement

is eliminated when the sample is dispersed in 50% glycerin solution, having a viscosity of 5 centipoises.