E2 L4 - Emulsion Flashcards
A system of two immiscible liquids in which one is dispensed as droplets (e.g. water phase + oil phase)
Emulsion
Dispersed phase
“water in oil” (water is dispersed, oil is continuous)
Continuous phase (=external phase)
Determines organoleptic properties (taste, smell, feel) of the emulsion
Oral: Oil in water
to mask the tase of an oil
to enhance absorption of an oil
External: Oil in water
water washable vanishing cream
External water in oil
for cleansing skin – cold cream
IV Lipid emulsion
Oil in water
Oil has to be very small so it does not block the blood vessel
Oil can provide more calories than solutions
Emulsifying agents
Added to stabilize the emulsion
Three types of emulsifiers
Surfactants
Polymers
Fine divided particles
Three strategies of emulsifiers
Use surface activation (surfactants)
Use polymer (provide steric interest so they do not merge)
one purpose: Improve stability
Surfactant
Empty vials
Hydrophilic and hydrophobic (amphipathic)
Align (hydrophilic - water; hydrophobic - oil)
Separate
Surface activation
Hydrophilic colloid - polymer
Align with surface of the barrier
create barrier
Fine particles
Smaller than polymer
Particles CANNOT MERGE because there is barrier created on the surface
Which emulsifier is more different than the other two?
Surfactant
Surfactant - reduces interfacial tension
other two - do not reduce interfacial tension, just stop the things from merging
Surface active agents - surfactants
Molecules that contain both a hydrophilic and hydrophobic region
Orient at the liquid-liquid or liquid-air interface and lower interfacial tension
Log C
Concentration of surfactant
Surfactants reduce surface tension to a certain extent why?
Over a certain concentration, it stops reducing tension
Forms micelles
Too much - they start to form their own assemblies, so they are not reducing surface tension anymore (since they are forming small vesicles themselves
When this happen: CMC - critical micelle count
Know different types of surfactant chains, if they are anionic or cationic, and if they are a zwitterion or not
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Hydrophile-lipophile balance
A measure of the relative contributions of the hydrophilic and lipophilic regions of a surfactant
Calculated according to an empirical formula
Ranges 0-20 for non-ionic surfactants
Low HLBs indicate greater lipid solubility
In practice, a mix of emulsifying agents is used to get a desired HLB
HLB equation
HLB = aX + (1-a)Y
X: HLB of surfactant 1
Y: HLB of surfactant 2
A: Fraction of surfactant 1 in the surfactant mix
Bigger than 10 - hydrophilic
Smaller than 10 - hydrophobic
HLB = X * a + y(1-a)
Rule of Bancroft (1913)
A relative solubility of the surfactant determines they type of emulsion (i.e. the phase in which a surfactant is more soluble becomes the continuous phase)
E.g. a surfactant with a high HLB (>10, soluble in water) forms an o/w emulsion
Micelles
As the concentration of a surfactant increases above a critical concentration, (the CMC) the surfactant molecules self-associate into small aggregates called MICELLES
The micelles formed in water have the hydrophobic groups of the surfactant oriented TOWARD the core of the micelle. The center of the micelle represents a lipid-like region that is capable of dissolving water-insoluble drugs
Hydrophobic colloids
Hydrophilic polymers
Used for O/W emulsions – hydrophilic
Form a multimolecular film at the interface and increase the viscosity of water
Do NOT lower the interfacial tension
Ex: Acacia, tragacanth, gelatin
Natural polymers
Finely divided solid particles
Particles less than a micron can adsorb at the interface and form a film of fine particles
Ex:
Bentonite, magnesium, aluminum silicate, aluminum hydroxide: Hydrophilic O/W emulsion
Charcoal: Hydrophobic, for W/O emulsion
Predicting type of emulsion
The phase in which the emulsifier is most soluble will be the external (=continuous) phase
Surface active agents
HLB < 10: W/O
HLB > 10: O/W
Hydrophilic colloids: Only O/W
Finely divided solids
Hydrophilic if contact angle < 90degrees O/W
Hydrophobic if contact angle >90 W/O
**Obtuse – hydrophobic
Acute – hydrophilic**
Phase volume ratio
Phase volume ratio:
Volume of oil phase/Total volume of the emulsion
0-26%: O/W only
26-74%: either
74-100% W/O only
T/F: emulsifier is a surfactant
FALSE - Correct to say surfactant is an emulsifier, NOT that emulsifier is surfactant (it is a subtype of emulsifier, but not the only one)
Instability of emulsion - creaming
V = d2(pi-pe)980/18n (Stokes Law)
V = velocity of sedimentation
D: diameter of droplets
Pi: density of internal phase
Pe: density of external phase
980 cm/sec2: gravity constant
Reversible as long as the interfacial film is effective in maintaining the integrity of individual droplets
Droplets stay as droplets – shake before use
Natural process
Instability of Emulsion - Coalescence
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 fic by shaking bottle
Must be reformulated
Molecules bump together and fuse – irreversible
Instability of Emulsion - Phase inversion
Big problem in IN
Hard water
Due to phase volume ratio exceeding 74%
Rule of thumb: keep the phase volume ratio <50%
O/W emulsion stabilized w/sodium stearate can be inverted to W/O type using “hard” water (containing calcium)
Conversion of Na stearate to Ca stearate
