Sedimentation in Suspensions

What is the effect of decreasing the particle size on sedimentation?

What is the most efficient equipment for size reduction?

What is the density range of most organic drugs?

What is the effect of increasing the viscosity of the vehicle?

What is the purpose of suspending agents like bentonite dispersions?

What is the limitation of increasing the viscosity of the vehicle?

What is the term used to describe vehicles that retard sedimentation?

What is the effect of high viscosity on re-dispersibility of sediments?

Which of the following is a natural suspending agent?

What is the characteristic of Sodium carboxymethyl cellulose?

Which of the following suspending agents is insoluble in water?

What is the effect of adding a strong base to a Crabopol dispersion in water?

Which of the following suspending agents is used mainly in non-aqueous suspensions?

What is the characteristic of Crabopol in terms of pH?

Velocity of sedimentation can be reduced by decreasing particle size, as large particles settle quickly and fine particles form hard cakes.
Particle size reduction is done using fluid-energy mill, which can achieve sizes as low as 10 micrometer.
Density of particles and vehicle should be minimized, as most organic drugs have a density between 1.1 and 1.5 g/mL, and water has a density of 1.0 g/mL.

Increasing viscosity of the vehicle reduces sedimentation, but it is limited by the potential for non-pourable liquids and difficulty in spreading and rubbing.
Viscosity and density are related, so changing one parameter affects the other.
Thickening agents and suspending agents can increase viscosity.

Suspending agents (e.g. bentonite dispersions or polymeric solutions) entrap drug particles, retarding sedimentation.
Such vehicles are referred to as structured vehicles.
Suspending agents can be classified as natural (e.g. acacia, tragacanth), semisynthetic (e.g. cellulose derivatives), and synthetic (e.g. Crabopol).

Water-soluble cellulose derivatives include methylcellulose, HPMC, and hydroxypropyl cellulose, which give high viscosity in water.
Water-insoluble cellulose derivatives include ethylcellulose, which is used in non-aqueous suspensions.