Colloids 7 W4

Question 1

What is an emulsion

Answer 1

Two immiscible liquids, one finely subdivided in the other Two phases – continuous and disperse
Droplet size of the disperse phase usually between 1-100 μm Dispersed phase is typically ~ 15-30% of the total volume
Highly thermodynamically unstable

Question 2

2 types of emulsion

Answer 2

• oil in water (o/w) and
• water in oil (w/o)

Question 3

What is a cream

Answer 3

Semi-solid, highly ordered, not just concentrated emulsions

Question 4

Why use complex, thermodynamically unstable formulations?

Answer 4

• For lipid-soluble drugs, an aqueous-dispersible oil formulation (an emulsion) allows efficient oral or IV delivery.
- Ciclosporin (prevents organ transplant rejection) and Propofol (an intravenous anaesthetic) are essentially insoluble in water: (aqueous solubilities 0.04 and 0.14 mg/mL).
• For topical drugs, cream formulations allow high drug concentrations and can increase the permeability of the stratum corneum
- (see topical drug delivery lecture and workshop for more detail).

Question 5

There are different routes to breakdown of the dispersion to a two-phase ‘split’ or ‘cracked’ emulsion: creaming

Answer 5

Creaming is sedimentation where the dispersed phase is less dense than the continuous phase, so the ‘sedimenting’ droplets rise rather than fall.

Question 6

There are different routes to breakdown of the dispersion to a two-phase ‘split’ or ‘cracked’ emulsion: flocculation

Answer 6

Flocculation is the same process as in suspensions but now leads to coalescence and cracking – why?
Now, when droplets are in secondary minimum, they can bridge and merge due to the mobile interface between the droplets.

Question 7

Stabilising emulsions means

Answer 7

stabilising the interface

Question 8

Suspension vs emulsion : suspension

Answer 8

Solid particles ~ 0.1 – 10 μm diameter: Particles generally retain their size (although recall Ostwald Ripening).

Thermodynamically unstable: Stability managed by control of particle motion and interactions.

Particles move by Diffusion and Sedimentation: Viscosity modifiers used to control particle motion.

Particle interactions determine flocculation state: Flocculation is generally a quasi-stable state for suspensions

Question 9

Suspension vs emulsion : emulsion

Answer 9

Liquid droplets ~ 1 – 100 μm diameter:
Size distribution changes can occur rapidly as droplets coalesce.

(Even More) Thermodynamically unstable: Liquid:liquid interfaces very easily merge when droplets meet – more difficult to control.

Droplets move by Diffusion and Sedimentation: Viscosity modifiers used to control particle motion. ‘Sedimenting’ droplets may rise (creaming) rather than fall, depending on their densities.

Particle interactions determine flocculation state: Flocculation quickly leads to an unstable state for emulsions.