Suspensions Flashcards
Define suspension
A heterogenous system where fine particles are evenly DISPERSED throughout the liquid medium.
- particles are partially soluble and not fully dissolved
Outline 3 benefits of suspension dosage forms
- Dosing flexibility - beneficial to give to paediatric patients
- Ease of swallowing and formulation
- Flavour-adding flexibility to mask bitter tase
Outline 6 main characteristics of a good suspension
- Optimal viscosity
- Controlled flocculation (i.e. the level of closeness of the particles where they get re-dispersed immediately upon shaking
- Good wettability
- Good taste
- pH stability to maintain API stability
What does the stokes equation predict?
predicts the sedimentation rate of the particles over time (i.e. how fast the particles will settle at the bottom of the bottle)
Define caking
Particles at bottom are compressed over time
Define flocculated suspension
Double check slides and then write the answer.
List 3 factors that influence sedimentation rate
- Particle size
- Density difference
- Viscosity of medium
How does partial size affect sedimentation rate?
SMALLER particles are dispersed for a longer time and settles at a slower rate
- THUS, decreases sedimentation rate
LARGER particles settle faster
-THUS, increases SR
How does density difference (between particles and medium) affect SR?
- If PD (particle density) is higher than medium density (MD), the particles settle at a faster rate which increases SR
- If MD is higher than PD = floating of particles
OVERALL with a lower density difference particles will settle at a slower rate
How does viscosity of medium affect SR?
A thicker more viscous liquid slows the movement of particles which makes them settle at a slower rate THUS reduce in SR
In comparision, the more thin and flowing the liquid is, promotes faster particle movement leading to faster settling THUS SR.
When we reduce/decreasethe SR of particles what happens?
Particles are dispersed in the system for a longer period which STABILISES the suspension
List components that helps to stabilise a suspension
- Small particles
- Fluccolating agents
- electrolytes
- Preservatives
- Stabilisers
List the key excipients used in suspensions
- Wetting agents
- Flucoolating agents
- Viscosity enhancers
- Surfactants
- Buffering agents
Outline specific examples and the role of each class of excpieint
- Check ED discussion if we need to remember this
The particle diameter in the Stokes Law equation needs to be which unit
(m) - metres
how do you convert micrometers to m
divide by a million
What is the stern layer?
It is the layer that directly interacts with particles outer surface
- It is immobile (this layer moves with the drug particle)
- comprised of ion charges that is opposite to the particles charge
What is the slipping plane (shear plane)?
the outermost layer and located next to the stern plane
- it can freely move in the liquid
- I
Define Nernst potential
the effective charge on the surface of the particle
Define zeta potential
the charge of the particle that appears to other particles
What does a decrease in zeta potential mean?
Less negative or more positive charge:
1. Decrease in repulsive forces
2. Higher chances of aggregation/clumping
3. decrease in the STABILITY of the suspension
What does an increase in zeta potential mean?
More negative or less positive
1. Increase in repulsive forces
2. Lower chances of aggregation/clumping
3. Increase in the STABILITY of suspension
List the 3 factors that affect zeta potential
- Ionic strength
- Surfactants
- pH
How does ionic strength affect zeta potential?
With a higher ionic strength, there’s a decrease in repulsive forces therefore a decrease in zeta potential
How does surfactants affect zeta potential (refer to both cationic and anionic surfactants)
Cationic surfactants:
- increase in positive charge on the particle’s surface
- therefore a DECREASE in zeta potential = decrease in stability
Anionic surfactants
- increase in negative charge on particles surface
- INCREASE in zeta potential = increase in stability
How does pH affect zeta potential?
low pH:
- particles will gain a proton (H+) making them more positively charged
- decreased zeta potential
high pH
- particles will lose a proton (H+) making them more negatively charged = increased zeta potential = higher stability of suspension
