W11.1_Emulsions

Question 1

Define emulsions. State the pharmacological applications in emulsions. What are the types of emulsions? What is it composed of?

Answer 1

• Definition: inter-dispersion of two or more immiscible liquids
• Ex. commonly used topically, in TPN, oral administration of therapeutic agents
• Types: oil in water/regular (o/w), water in oil/inverted (w/o), multiple emulsions (w/o/w), microemulsions
• Continuous external phase + disperse internal phase + surfactants (reduce surface tension between liquids and act as interfacial phenomena)

Question 2

Describe the characteristics of emulsions. Define internal phase fraction.

Answer 2

• Inherently unstable (will eventually break down), thus physical stability is essential
• Formulation considerations: flow properties, aesthetically and texturally pleasing
• Size of droplets: coarse (white/milky) - fine (cloudy) - micro (transparent)
• Internal phase fraction/oil fraction/oil percentage: V(internal)/V(total) x 100%

Question 3

Explain the ways to determine the type of emulsions (4).

Answer 3

• Adding drops of emulsion to water -> droplets disperse in water for o/w (vice versa for w/o)
• Add oil soluble dye -> coloured droplets for o/w, clear droplets for w/o
• o/w has a higher electrical conductivity than w/o (higher water content)
• o/w spreads more rapidly on filter paper than w/o

Question 4

Explain how work is required to form emulsions and the role of emulsifying agents in emulsions.

Answer 4

• Formation of emulsion is not spontaneous (∵ high surface tension between two immiscible liquids) -> produce work to increase surface area between them
• Work: deformation of l/l interface by shear forces -> large droplets disrupted
• Emulsifying agents (ex. excipients, surfactants): applied to reduce the work to form emulsion

Question 5

Explain the different senario of instability in emulsions and how they would occur (3).

Answer 5

• Creaming (Stoke’s law): lack of uniformity in drug distribution -> variable dosage
• Upward creaming (dispersed phase is less dense: negative velocity of sedimentation)
• Downward creaming: (dispersed phase is more dense: positive velocity of sedimentation)
• Factors: particle size, viscosity of continuous phase, difference of density between phases
• Coalescence/cracking: forming a larger droplet after contact to reduce surface tension
• Reasons: insufficient amount of surfactants/polymers, incompatibilities between ingredients
• Breaking/phase separation: destruction of film surrounding particles (surfactants)
• Complete phase separation is irreversible

Question 6

Explain how stability in emulsions can be controlled by various methods (4).

Answer 6

• Critical value/fraction: assuming internal phase is in spheres
• o/w: around 74% (practically 50%) vs w/o: around 40%
• Lower volume of internal phase -> lower probability of droplets coalescence
• Non-ionic surfactants in o/w: charged surfactants -> charged stable droplets -> form hydrophilic layer on oil droplet -> converting hydrophobic droplet to hydrophilic
• w/o: surfactants stabilising through steric repulsive forces
• Macromolecules: adsorbing to surface and preventing coalescence by steric hindrance (ex. gums, proteins, synthetic polymers)
• Solid particles: preferentially wetted by either phase at interphase (ex. carbon particles/graphite for w/o, some salts for o/w)

Question 7

State the hydrophilic, water dispersable, and hydrophobic excipients. What is the purpose of using hydrophile lipophile balance? Explain how it is used by using an example.

Answer 7

• Refer to the photo
• Determining most effective emulsifier combination through volume ratio between hydrophilic and lipophilic part of surfactants (higher hydrophilicity = higher HLB value)
• Use HLB of each oil phase component -> find HLB of oil phase
• Find percentage of each surfactants that could reach the HLB