Suspensions Flashcards
What is a medicinal suspension?
Dispersion of fine, insoluble solid (disperse phase) particles within a liquid (continuous phase)
What is a coarse suspension?
Solid particles of a drug with a mean size above 1μm and less than 5μm
What is a colloidal dispersion?
Solid particles of a drug with a mean size below 1 μm
What is the continuous phase of a suspension?
Usually aqeous but can be organic or oil-based
What are the main properties of a suspension?
Disperse phase particles should be small, uniformly sized and not settle too rapidly
Settled particles should not deflocculate
Product should have an agreeable odour, colour, taste and texture
Continuous phase should not support microbial growth
What does deflocculated mean?
Particles form a solid and cannot be redispersed/resuspended
Advantages of suspensions
Used when swallowing solid dosage forms is not possible
Can mask taste
High surface area is desirable for fast drug dissolution in the GI tract
Some drugs cannot be prepared as solution due to drug hyrolysis
Insoluble derivatives can be formulated as oral suspensions
Disadvantages of suspensions
Sedimentation
Presence of water can result in hydrolytic degradation
Ostwald Ripening
What is Ostwald ripening?
Smaller particles can recrystallise on the surface of larger particles
What is aggregation?
A collection of individual particles into a group
What is coagulation?
Occurs when particles approach each other with sufficient energy to overcome repulsive forces
Stoke’s Law
v = 2α^2 g(σ-ρ)/9^η
η= kinematic viscosity of liquid
α = radius of sphere
ρ = density of liquid
σ = density of sphere
Sedimentation of flocculated suspension
Rate of sedimentation is fast
Appearance can alter after a few minutes of shaking
Sediment exhibits higher volume due to loose, open structure of floccules
Sediment is easily redispersed by mild shaking
Sedimentation of deflocculated suspension
Rate of sedimentation is slow
No apparent time for change for some time after changing
Many hours to form a sediment
Weeks or months to sediment
Forms a cake and cannot be resuspended
Degree of Sedimentation
F = Vs/Vt
Vs = Ultimate apparent volume of sediment
Vt = Total volume of suspension
F = sedimentation value
Degree of Flocculation
β = F/F∞
β = degree of flocculation
F = ultimate sedimentation volume of flocculated suspension
F∞ = ultimate sedimentation volume of deflocculated suspension
Why are indiffusible solids not suitable for suspending?
They cannot be suspended in a continuous phase for a sufficient length of time to measure a metered dose - without a suspending agent
Why must particles be small for formulating suspensions?
To allow for a slow rate of sedimentation
Particles larger than 5μm give a gritty texture
What is a disadvantage to smaller particles in suspension?
Small particles dissolve and recrystallise on larger particles causing a positive shift in particle size distribution
What are hydrophilic colloids used for?
Viscosity Enhancers
Suspending agents
How do hydrophilic colloids work?
Coat the surface of the solid hydrophobic particles with. multi-molecular layer
This promotes wetting by imparting a hydrophilic character on the surface of the particles
Why is a suspending agent needed with water?
Water is a low viscosity Newtonian fluid so is not good at preventing sedimentation
Why are suspending agents needed?
To slow the rate of sedimentation
How do suspending agents work?
Increase the viscosity of the continuous phase by the addition of a water soluble polymer
