W6 Stability (Micro/Nano) Flashcards
Emulsion instability can occur due to what? (5)
- Phase inversion
- Creaming
- Flocculation
- Coalescence
- Ostwald ripening
What is Phase inversion?
Conversion from oil-in-water emulsion to water-in-oil emulsion
Oil-in-water emulsion is 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 Creaming?
What is this due to?
How can it be avoided?
=Fat globules (coloured with dye) tend to accumulate as a cream layer on top of the milk
(oil droplets conc at the top of emulsion)
*This is due to the differences in densities between fat globules and the plasma phase of milk
Avoid- Increase the oil density or viscosity of the emulsion.
What is Flocculation?
What promotes it?
What is it caused by?
- 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
Addition of salt (Na3PO4) causes flocculation
What is Coalescence?
What is is caused by? (4)
- 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 is 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 does the DLVO graph assume? (interactions)
(It is a quantitative approach to the stability of lyophobic systems)
What are the features of a DVLO graph? (5)
What are the axis labels?
Assumes the only interactions involved are
* Van der Waals forces of attraction (VA)
* Electrostatic repulsive forces (VR)
Primary maximum
Primary minimum
Secondary minimum
Electrostatic Repulsion
Van der Waals
Labels: Distance- X axis
Attraction - Y axis Bottom
Repulsion- Y axis Top
What do Weak Van der Waals forces explain in the DVLO graph?
Flocculation or Coalescense
- Explains why some colloidal particles aggregate
- Emulsions = coalesce
- Suspension = floc
Zeta potential
Vr- Electrical repulsion arises from..?
(in more detail)
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
Increasing charge on the
double layer:
* Optimise the concentration of
surfactant (don’t forget about
the associated counterions)
* Optimise the pH
Explains why some colloidal particles stay separate
Different types of surfactant:
Cationic, Anionic, Zwitterionic
What are Entropic (steric) effects?
When two particles come into close contact, the polymer chains start to overlap
* This leads to a loss in the freedom of motion of the polymer chains, i.e. a loss of entropy
* This situation is thermodynamically unfavourable and forces the droplets apart again
Osmotic (solvation) forces
- When two particles come into close contact the polymer chains start to overlap, effectively leading to a concentrated polymer solution
- This induces an osmotic gradient in the solution: a concentrated polymer solution in the overlap region and a dilute solution in the bulk solution
- Water enters the concentrated region in an attempt to dilute it and in doing so forces the polymer chains (and droplets) apart
What causes the secondary minimum? (DLVO graph)
- 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 causes 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
- Addition of electrolyte (e.g. NaCl) induces:
- Neutralisation or reduction of
charge on droplets - Decrease in Vmax
- Destabilisation of the emulsion
