14 Questions
What is the effect of decreasing the particle size on sedimentation?
Decreases sedimentation rate
What is the most efficient equipment for size reduction?
Fluid-energy mill
What is the density range of most organic drugs?
1.1-1.5 g/mL
What is the effect of increasing the viscosity of the vehicle?
Decreases sedimentation rate
What is the purpose of suspending agents like bentonite dispersions?
To entraps the drug particles and retard their sedimentation
What is the limitation of increasing the viscosity of the vehicle?
It may result in non-pourable liquid suspension
What is the term used to describe vehicles that retard sedimentation?
Structured vehicles
What is the effect of high viscosity on re-dispersibility of sediments?
Decreases re-dispersibility
Which of the following is a natural suspending agent?
Acacia
What is the characteristic of Sodium carboxymethyl cellulose?
Anionic
Which of the following suspending agents is insoluble in water?
Ethylcellulose
What is the effect of adding a strong base to a Crabopol dispersion in water?
It causes the polymer to ionize and dissolve, forming a clear gel
Which of the following suspending agents is used mainly in non-aqueous suspensions?
Ethylcellulose
What is the characteristic of Crabopol in terms of pH?
It is insoluble at pH
Study Notes
Sedimentation
- 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.
Viscosity and Density
- 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
- 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).
Cellulose derivatives, also known as cellulose polymers, are water-soluble substances synthesized from cellulose through various reactions.
- 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.
Synthetic Suspending Agents
- Crabopol (Carbomer) is a polyacrylic acid that forms a dispersion in water with low viscosity, but becomes a clear gel when strong base is added.
Learn about the factors affecting sedimentation velocity in suspensions, including particle size and its impact on settling rates.
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