Suspensions Flashcards
What is a disperse system ?
- a two-phase heterogeneous system in which an insoluble or immiscible dispersed phase is distributed through a continuous phase.
What is a pharmaceutical suspension?
- a liquid disperse system consisting of particles distributed within a liquid vehicle.
Reasons for formulating pharmaceutical suspensions
- To deliver poorly water-soluble drugs which cannot be formulated as aqueous solutions
Ways to reduce sedimentation rate
Particle size reduction – increases diffusion
Increase medium density – e.g. adding dextrose.
Increase temperature
Increase medium density
Why is the difference between a drug solution and suspension ?
- solution contains a dissolved drug
- suspension contains undiscovered drug particles suspended in a liquid
What is a flocculated suspension?
- suspended particles form into floccules (clumps) rather than separate particles
- re dispersion is easy
What is a deflocculated suspension?
- Particles are individually dispersed
- form a cake at the bottom
Key properties of an oral suspension for uniform dosing:
- Particles settle slowly
- readily + uniformly re-dispersed upon shaking
- size remains consistent over time
3 methods of particle movement in suspensions?
- Brownian motion
- gravity
- external agitation (shaking by patients)
What is brownian diffusion?
- Irregular movement within the medium
- diffusion from high concentration to low
- small particles
What is sedimentation?
- It is the downward particle movement due to gravity
Which system is better to use to ensure reproducibility?
- A flocculated system
Which system ensures a slow sedimentation rate?
- Deflocculated system
What is relationship between particle size and sedimentation?
As particle size decreases sedimentation decreases
- submicron suspension (small) = Brownian motion keeps particles dispersed
- large particles = greater effect of gravity so settle quicker
Ways to reduce sedimentation rate
- Reduce particle size = increases diffusion
- reduce particle density
- increase medium density eg. Adding dextrose
- increase temperature
What is zeta potential?
- Zeta potential provides a measure of the magnitude of the electrostatic or charge repulsion/attraction between particles at the slipping plane, between the particle and its associated double layer and the the solvent.
3 Features of electrical double layer
- diffuse layer = ions diffuse more freely around the particle
- stern layer = attracts ions of opposite charge (+ve), ions are lightly bound to surface, drugs give negative charge in liquid
- slipping plane = boundary between stern + diffuse layer
What happens when zeta potential is reduced?
- attractive forces (van der waals) overcome the faces of repulsion
- particles come together to form floccules
What happens if suspension has a large negative or positive zeta potential ?
- particles tend to repel each other
effect of excipients like ionic salts on electrical double layer ?
- increase number of mobile charges
- low conc = affects diffuse layer, easier to neutralise the particle charge + thinning the layer
- high conc = affects fixed/stern layer, charge on partical surface decreases
effect of surfactants on electrical double layer ?
- CMC is the conc of surfactants above
which the micelles form - above CMC, micelles form with a hydrophobic core
- below CMC - surfactants will cover the particles surface
- conc below CMC used to reduce tension between particle + liquid
what are properties of an ideal suspension ?
- appropriate viscosity
- no hard caking
- sedimentation
- dispersion
what are suspending agents used for ?
- reduce particle sedimentation
what are chemical stabilisers used for ?
- improve chemical stability of the drug
what are flocculating agents used for ?
- decrease zeta potential
- cause aggregation of particles
what are wetting agents used for ?
- reduce tension between particle + liquid
- eg. surfactants below CMC
what are preservatives used for ?
- to prevent microbial growth
What is buffer used for in suspension ?
- To maintain the pH of the aqueous system.
• Ionic in nature, so will affect flocculation behaviour.
• A mixture of a weak acid/base and salts.
