Suspensions and Colloids

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gabe salvador

Cards (156)

What is a suspension?
Coarse dispersion where insoluble particles dispersed in liquid medium
Particles normally bigger than 1 micrometre
What is the difference between suspensions and colloids?
Suspensions large so feel gravity's effect
Colloids aren't large
Give some examples of suspensions
Calprofen
Pepto-bismol
Ciprodex ear drops
Humulin (insulin)
How do you classify suspensions based on general classes?
Oral
External
Parenteral
How do you classify suspensions based on the proportion of solid particles?
Dilute (2-10% w/v)
Concentrated (50% w/v)
How do you classify suspensions based on the electrokinetic nature of solid particles?
Flocculated/deflocculated suspension
How do you classify suspensions based on the size of their solid particles?
Colloidal suspension (< 1 micron)
Coarse suspension ( > 1 micron)
What are the advantages of suspensions?
Faster absorption
Can be used if patients struggle with swallowing
What are the disadvantages of suspensions?
Physically unstable
Difficult to formulate
Uniform and accurate dose can't be achiever unless suspension's packed in unit dosage form
What makes the ideal suspension?
Suspended particles shouldn't settle rapidly
Should be easily re-suspended - controlled flocculation
Easy to pour but not watery or gritty
Should have pleasing:
Odour
Colour
Palatability
Good syringeability
Physically, chemically and microbiologically stable
How are suspensions physically unstable?
Suspensions small like colloids - high SA:V ratio
2 phases don't want to be in contact - want to return to having high SA:V ratio
Suspensions could need lots of energy input to form and could be unstable
What instabilities could be present in a suspension?
Sedimentation
Coagulation
Flocculation
Ostwald ripening
Adhesion of particles to containers
How are suspensions prepared?
Prepared by milling
Poorly-soluble drug and liquid vehicle exposed to high shear
Adding surfactant to suspension reduces surface tension - stabilised
What is caking? 
Suspensions feel force of gravity - settle over time
Could form cake - bad
Stop caking or reduce sedimentation to improve stability
How does sedimentations occur?
Occurs due to suspension experiencing force of gravity
How is sedimentation velocity expressed?
Velocity expressed by Stoke's Law
How do you reduce sedimentation velocity?
Reduce particle radius (not always possible)
Increase viscosity of dispersion medium (may reduce ability to resuspend)
What is sedimentation volume?
How suspensions are assessed
F = Vf/V
F = sedimentation volume
Vf = final volume of sediment
Initial volume of suspension before settling
What is coagulation? 
Caused by interaction at primary medium
Strong energy of interaction
Doesn't resuspend with shaking
What is flocculation? 
Caused by interaction at secondary medium
Weak energy of interaction
Resuspends w shaking
Is deflocculation in a suspension good?
Flocculated and deflocculated suspensions both sediment
Deflocculated = coagulation to form cake
Want flocculation
Describe a deflocculated suspension
Particles behave independently
Sedimentation slow (days - weeks)
Initial sediment small w cloudy supernatant
Sediment volume increases and supernatant starts clearing after longer time
Caked = dense and compacted sediment
Describe a flocculated suspension 
Particles arranged in loose aggregates
Behave as large porous particles
Sediments quickly (mins)
Clear supernatant
Maximum sedimentation within hours - days
Sediment easily re-dispersed by shaking
Large sedimentation volume
What is Ostwald ripening? 
Suspensions have range of particle sizes
Temp increases = equilibrium solubility increases - suspended particles start dissolving
Dissolution = surface phenomenon and smaller particles = higher SA:V ratio so dissolve first
Temp decreases = very saturated solution - excess dissolved drug starts to precipitate on particle surfaces
Larger droplets = larger SA - more rapid growth - increased particle size
What are some excipients that could be present in a suspension formulation?
Vehicle - normally aqueous buffer/syrup
Salts (flocculating agents)
Hydrophilic polymers (flocculating agents)
Surfactants
Preservatives
Sweetening agents
Viscosity modifiers
Chemical stabilisers
Describe hydrophilic polymers 
Polymer chains adsorb onto particles surface
Steric repulsion stabilising disperse systems
Entropic effects - polymer entanglement not entropically favoured
Osmotic effects - high solute concentration between particles = osmotic pressure
Can be used as flocculant - chain interpenetration leads to 'bridging'
Dependent on polymer concentration
What do surfactants do for suspensions?
Affect attractive and repulsive forces when adsorbed
Affects wettability of insoluble particles - increases homogeneity
How do you measure the effectiveness of a flocculating agent?
Effectiveness expressed as degree of flocculation
What do suspending agents do?
Stabilise suspension against sedimentation
What are some examples of suspending agents?
Hydroxyethylcellulose
Hydroxypropylmethylcellulose
Gum tragacanth
Why is solution rheology important for suspending agents?
Shear-thinning behaviour and thixotropy could help w ability to resuspend and consume
What is shear thinning in terms of suspending agents?
Solution viscosity decreased w increased shear (how hard you shake it)
What is thixotropy in terms of suspending agents?
Solution viscosity decreases w duration of shear (how long you shake it)
What are the excipients of calprofen?
Xanthan gum
Propylparaben
Saccharin sodium
Purified water
Ibuprofen
Polysorbate 80
What is the role of xanthan gum in calprofen? 
Viscosity modifier
What is the role of propylparaben in calprofen? 
Preservative
What is the role of saccharin sodium in calprofen? 
Sweetener
What is the role of purified water in calprofen?
Vehicle
What is the role of ibuprofen in calprofen? 
API
What is the role of polysorbate 80 in calprofen?
Surfactant