Suspensions Flashcards
What are the particle sizes of suspensions?
> 1micrometers
What is the definition of a suspension?
Formulation of small insoluble solid particles distributed throughout a liquid continuous phase
What are the advantages compared to a tablet?
Faster absorption
What are the disadvantages compared to solution?
Slowe absorption
Most pharmaceutical suspensions are solid drug particles in aqueous vehicles, but what are the other possible aqueous phases and why are they sometimes used?
Oily or Organic liquids are sometimes used to slow the rate of absorption in some injections.
What is the concept of wetting?
THe ability of a solid to stay in contact with the liquid.
WHy is wetting related to suspensions?
Weting is the first step in the formulation of a suspension to stop the particles from remaining on the surface, attaching to container, or form large clumps within the liquid.
How easy is it to wet diffisuable and indiffusable solids?
Diffusable solids are easily wetted and straightforward to suspend
Indiffusable solids are hydrophobic and not easily wetted. They require a wetting agent.
WHat are wetting agents?
They decrease the interfactial tension of indiffusable solids to promote wetting and help dispersion
e.g. alcohol
What type of particles are difficult to wet?
Finly divided substances are difficult to wet due to the precense of an adsorbed layer of air around the particels.
Describe the concept of interfacial tension:
Molecules rather be in contact with molecules like themselves rather than with different molecules.
Interfacial tension involves cohesive and adhesive forces. The bigger the difference between these forces, the bigger the interfacial tension and therefore the higher the degree of wetting is required.
What are cohesive and adhesive forces?
Cohesive forces are those that form between ‘like’ molecules
Adhesive factors are those that form between different molecules
What are the different types of Wetting agents?
SurfaceActive Agents (detergents)
Hydrophilic colloids
Solvents
How do SurfaceActive Agents (detergents) work?
Hydrocarbon chains are absorbed by hydrophobic particle surfaces while polar groups enter the liquid and become hydrated. They can cause excessive foaming or deflocculate the system.
How do Hydrophilic colloids work?
They coat the hydrophobic particles with a layer, giving the particle a hydrophilic character.
They can be used in combination with SAAs,
but can also lead to deflocculation.
How do solvents work?
Water miscible solvents decrease the interfacial tension by penetrating the powder clumps and displacing the air.
How do we asses suspension stability?
- Rate of sedimentation
- Final volume or height of sedimentation (how densly sedimentated)
- Ease of redispurion
- Particle size distribution.
What rate of shear should be used during the measurement of stability and why?
A low rate of shear should be used to mimic the change in strucutre upon different storage.
What would a high rate of shear do during the measurement of stability?
A high rate may damage the suspension and give an inacurate representation of what would happen on the shelf.
What is the advantage and disadvantage of using ACCELERATED STABILITY TESTING?
Shortens time of experiments
Not always possible to accurately predict the normal behaviour from the results.
What are the two types of Accelerated stability testing:
- Centrifugation
2. Temperature cycling
Describe Centrifugation:
This works by increasing the rate of sedimentation.
A disadvantage is that if the system is flocculated, then the system will be destroyed when centrifuged forming a tightly packed sediment which would not happen in real conditions.
Describe Temperature cycling:
This is useing two extreme temperatures (very cold and very hot) as a worse case scenario of storage conditions. A disadvantafe is that it increases the rate of crystal growth which wouldn’t happen in normal conditions
What is the equation of sedimentation volume?
Sedimentation vol (F) = Vu (ultimate vol) / Vo (original volume)
In the sedment/time graphs, what does a constant line tell us?
That sedimentation is complete
What if F (Sedimentation volume) is = to 1?
If F = 1, then the volume of sediment is equal to the original volume (think of the sediment equation) and the product is said to be at ‘Flocculation equilibrium’. This is a desirable state in pharmaceuticals.
What is the normal values of F?
F is normally less than 1, becasue the volume after sedimentaton is normally lower than the volume of the original suspension.
What happens if the F is >1?
This is not desirable because the floccs in the system become loose and fluffy.
What is the equation for the degree of flocculation?
Degree of flocculation = Flocculated sedementation / Deflocculated sedementation.
It is more useful than F but takes longer.
