emulsions
A system of two immiscible liquids
in which one is dispersed as droplets. (e.g., water phase + oil phase
o/w
oil in water
dispensed: oil
continuous: water
w/o
water in oil
dispersed: water
continuous: oil
oral applications of emulsion
- O/W to mask the taste of an oil (e.g., mineral oil emulsion as a laxative)
– O/W to enhance absorption of an oil (Vitamin A and D, cod liver oil)
external applications of emulsion
– O/W: Water-washable; Vanishing cream
– W/O: For cleansing skin; Cold cream
instability of emulsions: creaming
-strokes law
-reversible as long as the interfacial film is effective in maintaining the integrity of individual droplets
intravenous lipid emulsion (O/W) applications of emulsion
– For parenteral nutrition
– 100 mL of 20% lipid emulsion provides 200 kcal, while 100 mL of 5% dextrose only provides 20 kcal.
– Smallest capillaries are 5 μm. It is critical that the droplet size is less than 1 μm to avoid embolisms.
emulsifying agents
-Emulsifying agents are added to stabilize the emulsion.
-Types of emulsifying agents
◦ Surface active agents (= surfactants)
◦ Hydrophilic colloids (polymers)
◦ Finely divided solid particles
surface active agents
- Molecules that contain both a hydrophilic region and a hydrophobic region
- Orient at the liquid-liquid or liquid-air interface and lower interfacial or surface tension
finely divided solid particles
- Particles less than a micron can adsorb at the
interface and form a film of fine particles. - Examples:
– Bentonite, magnesium aluminum silicate, aluminum hydroxide: Hydrophilic, for O/W emulsion.
– Charcoal: Hydrophobic, for W/O emulsion
instability of emulsions: coalescene
◦ Droplet size increases because the interfacial film is unable to maintain the integrity of individual droplets.
◦ Irreversible and will ultimately lead to a layer of oil and a layer of water (broken emulsion)
◦ Cannot fix by shaking the bottle.
◦ Must be reformulated.
hydrophilic colloids
- Hydrophilic polymers
- Used in O/W emulsions.
- Form a multimolecular film at the interface and increase the viscosity of water.
- Do not lower the interfacial tension.
- Examples: Acacia, tragacanth, gelatin
instability of emulsions: phase inversion
◦ Due to Phase volume ratio exceeding 74%
* Rule of thumb: Keep the phase volume ratio <50%
◦ O/W emulsion stabilized with sodium stearate can be inverted to W/O type using “hard” water (containing calcium)
* Conversion of Na stearate to Ca stearate
