Suspensions Flashcards
What are suspensions?
Suspensions are made of solid particles with low solubility dispersed in another phase.
They are colloid suspensions
When are suspensions used?
Unpalatable soluble drugs given as insoluble form to hide bad taste.
Lotions used to deposit fine powder on skin surface
Slow release formulation to give a prolonged effect
What are the desirable qualities of a suspension?
Particles should settle (sediment) slowly
Sediments should disperse easily on shaking
No caking/clumping
Should be pourable from bottle and spread easily
What governs the stability of a suspension?
Gravity - solid particles will eventually sediment
Stokes law - rate of sedimentation is controlled by this: larger particles will sediment faster than smaller ones
How does caking occur?
Particles at the bottom are gradually compressed by the weight of those above: this provides enough energy to overcome the primary maximum (repulsive forces) and form an irreversible interaction at the primary minimum = caking
What is flocculation?
A cluster of particles held together in a loose open structure. Flocs are spontaneously formed and broken down.
What is the influence of DLVO on flocculation?
It means that a deep secondary minimum is required for their formation (decrease of repulsion potential)
What are the characteristics of a flocculated system?
Sediment contains voids
Easy to redisperse
No problems from caking
Depends on electrolyte and type
Are normal suspensions flocculated or deflocculated?
Deflocculated.
Particles do not form flocs, they sediment independently.
Potential of repulsion is high therefore particles are often separated.
Pressure on particles in sediment may overcome primary energy barrier to become irreversibly bound together = caking
What are the characteristics of a deflocculated suspension?
Compact with few voids
Sedimentation is slow
Difficult to redisperse
Susceptible to caking
What do pharmaceutical suspensions need to have?
Controlled flocculation.
How is controlled flocculation achieved?
Using electrolytes and polymers (concentration and type is important)
How is controlled flocculation monitored?
Zeta potential
Sediments’ height
What is the effect of increasing concentration of ions on zeta potential?
As salt conc increases, zeta potential decreases.
More of counterion is adsorbed on the surface of the solid particle
What is the effect of the type of ion on flocculation?
Flocculation depends on the charge number of the ion and is independent of the type of ion
