Preformulation - Theoretical Emulsification

Question 1

What is meant by an emulsion?

Answer 1

• Metastable dispersion of liquid droplets in an immiscible liquid

Question 2

Describe a single emulsion.

Answer 2

• 2-component system
• Liquid droplets in another liquid

Question 3

Describe a double emulsion.

Answer 3

• 3-component system
• Primary emulsion droplets in another liquid

Question 4

Describe pharmaceutical emulsions.

Answer 4

• Droplet diameter 100 nm-25 micrometre
• Colloids: multi-component dispersed phase systems where the disperse phase is within the size range of 1-1000nm.
• Oil/Water emulsions most common
• Topical, parenteral, oral

Question 5

Describe the three phases of emulsification.

Answer 5

1. Separate phases: low energy, thermodynamically favourable, stable
   SHAKE
2. Unstable dispersion: high interfacial energy, thermodynamically unfavourable, unstable
   STAND
3. Phase separation: low energy, thermodynamically favourable, stable

Question 6

How to make droplets last?

Answer 6

• Emulsifier: Form interfacial film around droplets, stabilise
  droplets by lowering interfacial energy.
• Viscosity enhancer: Inhibit droplet diffusion, prevent
  coalescence.

Question 7

Describe the mechanisms of emulsification.

Answer 7

• Most emulsifiers are amphiphilic (e.g. surfactants).
• Mediate molecular interactions at phase boundry.
• Lower droplet interfacial energy (γ).

Question 8

Describe emulsion stability.

Answer 8

• Emulsions are metastable.
• Droplets undergo brownian motion and may interact with
  each other.
• Attraction promotes droplet consolidation → instability.
• Repulsion promotes droplet separation → stability.

Question 9

What phenomena can happen as a result of emulsion instability?

Answer 9

• Flocculation: reversible
• Coalescence: Irreversible
• Creaming: reversible
• Sedimentation: reversible
• Cracking: Irreversible

Question 10

Describe the DVLO theory.

Answer 10

– Electrical repulsion: electrical double layer.
– Van der Waals attraction: induced dipole-induced dipole
interactions.

Question 11

What does the DVLO theory assume?

Answer 11

• No other interaction forces between the particles/droplets

Question 12

Describe VA, VR and VT of the DVLO theory.

Answer 12

• The attractive (VA) and repulsive energies (VR) change at different rates with particle distance.
• The sum of these energies (VT) explains particle dispersion or flocculation.

Question 13

Describe the three stages of the DVLO theory graph.

Answer 13

1. Primary minimum: Net attraction, irreversible coagulation.
2. Primary maximum: Net repulsion, stable dispersion.
3. Secondary minimum: Net attraction, reversible flocculation.

Question 14

Define sedimentation.

Answer 14

Particles (droplets)
sink in continuous phase under opposing forces.

Question 15

What promotes sedimentation?

Answer 15

− Larger droplet size.
− Greater density difference
between droplet and continuous phase.
− Lower fluid viscosity.

Question 16

What determines emulsion type?

Answer 16

1. Phase volume ratio: disperse phase typically <70%.
2. Bancroft rule: “The phase in which the emulsifier is more
   soluble is the continuous phase.”
   – Hydrophilic emulsifier → O/W emulsion.
   – Lipophilic emulsifier → W/O emulsion.

Question 17

Describe the Bancroft rule.

Answer 17

Rationale: The bulker portion of the emulsifier, regardless of whether that portion is hydrophilic or hydrophobic, tends to face outwards.

Question 18

What is HLB?

Answer 18

• Numerical scale indicating overall hydrophilicity/lipophilicity
  of an emulsifier.

Question 19

Describe how HLB is an ordinal scale.

Answer 19

Double the HLB value not necessarily twice as
hydrophilic.