Dispersions

A dispersion is a system containing one or more constituents distributed throughout a homogeneous medium. Dispersions can be classified into three categories based on particle size:

 True Solutions—less than 0.001 micron.
Molecular dispersions, particles are invisible even with the electron microscope and pass through filter paper and semi- permeable membranes.
 Colloids—0.001 to 0.5 microns. An intermediate state between true solutions and suspensions. Particles cannot be seen with an ordinary microscope but can be seen with the electron microscope.
In addition, while the particles of a colloidal dispersion will still pass through filter paper they will not pass through semi-permeable membranes. They are of very high surface area. Particles diffuse more slowly than those of a true solution.

 Coarse Dispersions (Suspensions) — greater than 0.5 microns. Particles (dispersed phase) are often visible with the naked eye (unaided) and will not pass through filter paper or semi- permeable membranes. Particles seldom diffuse. Coarse dispersions are extensively used in pharmaceutical products.
(Much overlapping in such a classification)
There can be many types of colloidal dispersions. Each phase may be a solid, liquid or gas. The most important colloidal pharmaceutical preparations include:
1. Foams
2. Aerosols
3. Emulsions
4. Suspensions
5. Ointments
6. Gels