stability of micro/nano Flashcards
What are droplets like in a stable emulsion?
In a stable emulsion, droplets retain their initial character and
remain uniformly distributed throughout the continuous phase
What can cause emulsion instability?
- Phase inversion
- Creaming
- Flocculation
- Coalescence
- Ostwald ripening
What happens in phase inversion/ oil in water?
*stabilised by ionic surfactant/co-surfactant
* If charge on emulsion droplet is reduced
(with the addition of ions through buffer or drug), emulsion droplets will come
together
* Once droplets are in contact, interfacial surfactant film re-aligns forming water-in-oil droplets and phase inversion occurs
What is phase inversion?
Conversion from oil-in-water emulsion to water-in-oil emulsion
What is creaming?
Fat globules (coloured with dye) tend to accumulate as a cream
layer on top of the milk
* Due to the differences in densities between fat globules and the plasma
phase of milk
Why does creaming happen?
- Due to density difference between oil and water, the oil droplets
tend to concentrate at the top of the emulsion - To avoid this, increase the oil density or viscosity of the
emulsion
What is flocculation?
- Flocculation is when two or more emulsion droplets aggregate
without losing their individual identity - Larger droplets (> 2 μm) flocculate fastest and flocculation is
promoted by creaming
What is an example of flocculation?
adding salt (Na3PO4) causes flocculation
When does coalescence occur?
- Coalescence occurs when two or more droplets collide and
form one larger droplet and is irreversible - It is caused by various factors, including surfactant type and
concentration, pH, temperature etc
What happens in Ostwald ripening?
- With polydispersed droplets, collision between two droplets may
cause one bigger droplet and one smaller droplet - Upon repeated collisions, the small droplets become very small
and become solubilised in the continuous medium. They
eventually diffuse and re-deposit on larger droplets making
them even larger in size
What is van Der walls + how it works? (DVLO)
Energy of attraction varies with the distance (H) between pairs of atoms or molecules or neighbouring particles with the inverse of the 6th power. A= 1/H⁶
Vr (DVLO) how it arrises, what happens?
- Arises from the interaction of
the electrical double layers
surrounding pairs of particles - Repulsive forces decay
exponentially with distance - Repulsive forces decay more
rapidly than attractive forces
therefore the attractive forces
predominate over longer
distances
What will increasing the charge on the double layer do? (VR)
- Optimise the concentration of
surfactant (don’t forget about
the associated counterions) - Optimise the pH
What does 0 to +5 zeta potential mean in terms of stability?
Rapid coagulation or flocculation
What does +10 to +30 zeta potential mean in terms of stability?
Incipient instability
What does +30 to +40 zeta potential mean in terms of stability?
Moderate stability
What does +40 to +60 zeta potential mean in terms of stability?
Good stability
What does +61+ zeta potential mean in terms of stability?
Excellent stability
What does Va in DVLO prove in contrast to Vr?
- Explains why some colloidal
particles aggregate - Emulsions → coalesce
- Suspension → floc
Whereas the Vr =* Explains why some colloidal particles stay separate
What is the potential energy diagram of VT= Va + Vr?
picture
What does the secondary minimum mean?
- At large distances of
separation, particles
experience a minimal
attraction - Forces of attraction are weak,
flocculation not coalescence,
i.e. can redisperse upon
shaking
What happens at the primary maximum?
- As particles come closer
together, they start to
experience some repulsion
which will peak at the primary
maximum - The height of this repulsive
force (Vmax) determines the
stability of the system - The height varies with different
surfactants and electrolyte
concentration - A high value will ensure
coagulation is so slow that the
system displays long term stability - The energy barrier that leads to
irreversible particle aggregation
What if you add salt during primary maximum?
- Addition of electrolyte (e.g.
NaCl) induces: - Neutralisation or reduction of
charge on droplets - Decrease in Vmax
- Destabilisation of the emulsion
What happens at the primary minimum?
- At close approach, van der
Waals forces always
dominate over repulsive
electrostatic forces - A deep primary minimum is
present - At this short inter-particle
distance, particles/droplets
coagulate irreversibly
