Suspensions (Part 1)

Question 1

What is the definition of a suspension, and what are the different types of suspension?

Answer 1

• A suspension is a dispersion of finely divided solid particles in a liquid medium.
• The liquid medium may be aqueous or non-aqueous.

Types of suspension:

• Coarse suspension: particle size > 1 micron
• Colloidal suspension: particle size < 1 micron

Question 2

What are the applications of suspensions?

Answer 2

1. For oral use
   » Means to administer insoluble drugs in liquid form.
   » Means of supplying distasteful drugs in a form that is pleasant to taste.
   » Effective means of administering insoluble compounds for their absorptive or antacid properties in the treatment of gastric disorders.
2. For injection
   » Means of sustaining drug release as in depot therapy.
3. For external use
   » Means of applying insoluble drugs to the skin.

Question 3

What are the desirable properties of suspensions?

Answer 3

• chemically stable
• low rate of sedimentation of particles
• particles should be readily dispersed upon gentle shaking of container
• size of particle should remain fairly constant throughout storage
• should pour readily from container
• suspensions for injection should flow readily though a syringe needle
• suspensions for external use should spread easily, and yet not be so mobile that they run off the surface when applied.

Question 4

How may the particles in a suspension aggregate to form floccules?

Answer 4

Dispersed particles –> deflocculated suspension

Floccules –> flocculated suspension

Question 5

What is the formulation of a suspension?

Answer 5

• dispersed phase
• liquid vehicle
• wetting agent
• flocculating agent
• thickening agent
• pH buffer
• preservative
• colorant, flavour and fragrance

Question 6

What does the dispersed phase consist of, and what are the two types of dispersed phase?

Answer 6

• consists of insoluble solid particles.

Two types:

(i) . Lyophilic solids
- Solvent-loving, does not require the use of wetting agent for dispersion in the liquid vehicle.
- May also increase the viscosity of suspension.

(ii) . Lyophobic solids
- Not solvent-loving, require the use of wetting agent for dispersion in the liquid vehicle.
- Most drugs are hydrophobic.

Question 7

What are some characteristics of the liquid vehicle?

Answer 7

• also known as the dispersion medium or continuous phase.
• may be aqueous or non-aqueous.
• water is commonly used as a liquid vehicle.

Question 8

What is the function of a wetting agent?

Answer 8

To make the solid particles more penetrable by the dispersion medium.

Question 9

What are the modes of action of the wetting agent?

Answer 9

1. Reduces solid-liquid interfacial tension

2. Imparts hydrophilicity to particle surface

Question 10

What are some examples of wetting agents for the aqueous dispersion medium?

Answer 10

• surfactants (used at concentrations below CMC)
• hydrophilic polymers
  e. g. acacia, tragacanth, xanthan gum
• polyhydric alcohols (miscible with water)
  e. g. glycerin, propylene glycol, polyethylene glycols