Stability (Macro/nano)

Question 1

Emulsion stability

Answer 1

• Droplets retain their initial character and
  remain uniformly distributed throughout the continuous phase

Question 2

Cause of emultion instability

Answer 2

• Phase inversion
• Creaming
• Flocculation
• Coalescence
• Ostwald ripening

Question 3

How do you stablise an oil in water emultion

Answer 3

• 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

Question 4

Phase inversion

Answer 4

Once droplets are in contact, interfacial
surfactant film re-aligns forming water-in-
oil droplets and phase inversion occurs from oil in water

Question 5

Creaming

Answer 5

• Density is lighter than the continuous phase forming a layer on top cream on milk
• To avoid this, increase the oil density or viscosity of the emulsion

Question 6

Flocculation

Answer 6

• 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 aggregation occours

Question 7

Coalescence

Answer 7

• 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
• Arrested coalescence of adjoining inner cores

Question 8

Ostwald ripening

Answer 8

• Collision between two droplets may
  cause one bigger droplet and one smaller droplet
• Repeated collisions, the small droplets become very small becomes solubilised in the continuous medium
• Eventually diffuse and re-deposit on larger droplets making them even larger in size

Question 9

Asspumtion of DLVO

Answer 9

• Van der Waals forces of attraction (VA)
• Electrostatic repulsive forces (VR)

Question 10

Energy of attraction

Answer 10

• Vary with distance (H) between pains of atoms and molecules inverse of 6th power

Question 11

Electrical repulsion

Answer 11

• 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

Question 12

Increasing charges on double layer causes…

Answer 12

• Optimise the concentration of
  surfactant (don’t forget about
  the associated counterions)
• Optimise the pH

Question 13

Zeta potential of particle depends on stability

Answer 13

• 0-5mV is rapid coagualtion
• 10-30mV incipient instability
• 30-40mV Moderate stability
• 40-60mV Good stability
• 61< mV Excellent stability

Question 14

What does DVLO show?

Answer 14

• Van der Waals attraction explains why some of the colloidal particles aggregate e.g. suspension to floc
• Electrical repulsion explains why some colloidal particles stay seperate

Question 15

Secondary minimum

Answer 15

• Large distance seperated particles experience minimal attraction
• Forces of attraction are weak, flocculation occours and this can be redispersed when shaking

Question 16

Primary maximum

Answer 16

• As the particles get closer together start to experience repulsion this peaks at primary maximum
• Vmax (height of primary max) determines the stability of the system
• High value means coagulation is slow long term stability
• Energy barrier that leads to irreversible particle aggregation

Question 17

Primary maximum heigh is determined by

Answer 17

• Diffrent surfacant
  Elecrolyte concentration:
• Neutralisation or reduction of charge on droplets
• Decrease in Vmax
• Destabilisation of the emulsion

Question 18

Primary minimum

Answer 18

• 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

Question 19

Non-ionic surfacant

Answer 19

• No electrostatic charge is present to stabilise the droplet
• Entropic (steric) effects
• Osmotic (solvation) forces

Question 20

Entropic (steric) effects

Answer 20

• 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 decrease of entropy
  This situation is thermodynamically unfavourable and forces the droplets apart again

Question 21

Osmotic (solvation) forces

Answer 21

• 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 apart

Question 22

Steric stabilisation

Answer 22

• Vt = Va + Vs
• Surfactants are used, electrostatic forces are more efficient at stabilising emulsion droplets than steric/solvation forces alone

Question 23

What are the important forces for charged colloids

Answer 23

• Vander waals and electrostatic forces

Question 24

What are the important forces for uncharged colloids

Answer 24

• Vander waals and steric and solvation forces