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
What does DVLO only apply to?
ionic surfactants
What if a non-ionic surfactant were to stabilise an emulsion?
- When a non-ionic surfactant stabilises an emulsion, no
electrostatic charge is present to stabilise the droplet (the
charge on the droplet is neutral)
How do hydrophilic polymer chains stabilise emulsions?
one of the two ways:
* Entropic (steric) effects
* Osmotic (solvation) forces
What is the entropic steric effect and what happens in it?
- 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
What is the osmotic salvation force, what happens in it?
- 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
How does steric stabilisation apply? + new equation?
- Modified equation
VT = VA + VS - A = attractive
- S = steric & solvation forces
- Generally when mixtures of
surfactants are used,
electrostatic forces are more
efficient at stabilising
emulsion droplets than
steric/solvation forces alone
What are the 4 main types of forces?
- Van der Waals (attractive)
- Electrostatic (repulsive)
- Steric forces (repulsive)
- Solvation forces (repulsive)
What forces apply for charged colloids?
- For charged colloids (emulsions stabilised by ionic surfactants),
van der Waals and electrostatic forces are the most important
What forces apply for uncharged colloids?
- For uncharged colloids (emulsions stabilised by non-ionic
surfactants) van der Waals and steric and solvation forces are
the most important
What equation is used when both ionic and nonionic stabilise an emulsion?
- DLVO equation becomes
VT = VA + VS + VR
BEST STABILITY
