Disperse systems

Disperse systems
Disperse systems include many types of liquid preparations containing undissolved or immiscible drug distributed throughout a vehicle.
In these preparations the substance distributed is referred to as the dispersed phase, and the vehicle is termed the dispersing phase or dispersion medium, together they produce a disperse system.
Dispersed phase may be:
1. Solid materials that are insoluble in the dispersing phase like suspension.
2. Liquid that is neither soluble nor miscible with the liquid of dispersing phase like an emulsion.
3. Small air bubbles throughout a solution like an aerosol, dispersion also consist of droplets of a liquid (solution or suspension) in air.
The particles of the dispersed phase vary widely in size from large particles down to particles of colloidal dimension between 1 nm and 0.5 µm, so we have:
• Coarse dispersion : dispersion containing coarse particles 10 to 50 µm, they include suspensions and emulsions.
• Fine dispersion : dispersion containing smaller particles 0.5 to 10 µm.
• Colloidal dispersion : dispersion containing particles in colloidal range 1nm to 0.5 µm they include Magmas and gels.
Particles in coarse dispersion have greater tendency to separate from the dispersion medium than do particles of a fine dispersion, this because of their greater density than the dispersion medium so complete and uniform redistribution of the dispersed phase is essential to the accurate administration of uniform doses, this should be accomplished by moderate agitation of the container.




Suspensions
Preparations containing finely divided drug particles distributed uniformly throughout a vehicle (usually aqueous) in which the drug exhibits a minimum degree of solubility, it is a coarse dispersion.
They are available either in in ready to use form that is already distributed through a liquid vehicle with or without stabilizers and other additives or as dry powder intended for suspension in liquid vehicles.
Depending on nature of drugs we have:
? For oral suspension: dry powder mixtures for reconstitution at the time of dispensing e.g many antibiotics.
? Oral suspension: prepared suspensions not requiring reconstitution at the time of dispensing

Reasons for suspensions:
1. Chemically unstable drugs in solution can be stable when suspended.
2. Drugs that are insoluble in the delivery vehicle can be suspended effectively.
3. The disadvantage of disagreeable taste of certain drugs in solution can be overcome when the drug is administered as undissolved particles of an oral suspension.
4. To achieve controlled/ sustained drug release.

Features desired in a pharmaceutical suspension:
1. Suspension should settle slowly and should be readily redispersed upon gentle shaking of the container.
2. The particle size of the dispersed phase should remain constant throughout long periods of undisturbed standing.
3. The suspension should pour readily and evenly from its container.




Classification of suspensions:
? Based on general classes
1. Oral suspension e.g antacids, anthelmintic
2. Externally applied suspension e.g Calamine lotion.
3. Parenteral suspension e.g procaine penicillin G, Insulin zinc suspension.

? Based on proportion of sol