Lecture 12 Flashcards
emulsions (2/15)
emulsion
a system of two immiscible liquids in which one is dispersed as droplets
example - water phase + oil phase
two phases of emulsions
dispersed phase
continuous phase (external) – determines organoleptic properties
applications of emulsions
oral, external, intravenous lipid (O/W)
O/W oral
used to mask the taste of an oil (mineral oil emulsion as a laxative)
used to enhance absorption of an oil (vitamin a/d, cod liver oil)
o/w external
water-washable
vanishing cream
w/o external
for cleansing skin
cold cream
intravenous lipid emulsion
o/w only
for parenteral nutrition
100mL of 20% lipid emulsion provides 200kcal, while 100mL of 5% dextrose only provides 20kcal
smallest capillaries are 5 mcm (critical that the droplet size is less 1 mcm to avoid embolisms)
emulsifying agents
emulsifiers
added to stabilize the emulsion
types of emulsifying agents
surface active agents (surfactants, monomolecular film)
hydrophilic colloids (polymers, multimolecular film)
finely divided solid particles (solid particle film)
surfactants
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
act as clamp to bind oil and water together
types of surfactants
anionic - alkyl sulfate, alkylbenzene sulfonate
cationic - alkyltrimethylammonium bromide, alkylpyridinium chloride
zwitterionic - alkyl betaine, alkyldimethylamine oxide
nonionic - alcohol ethoxylate, poly-poly-poly block copolymer
hydrophile-lipophile balance (HLB)
a measure of the relative contributions of the hydrophilic and lipophilic regions of a surfactant
calculated according to an empirical formula
ranges from 0 to 20 for non-ionic surfactants
low HLBs
indicate greater lipid solubility
HLB of a mixture equation
aX + (1-a)Y
X – HLB of surfactant 1
Y – HLB of surfactant 2
a – fraction of surfactant 1 in the surfactant mixture
rule of bancroft (1913)
a relative solubility of the surfactant determines the type of emulsion (= the phase in which the surfactant is more soluble becomes the continuous phase)
surfactant with high HLB
soluble in water
forms an o/w emulsion
micelles
small aggregates that form as surfactant molecules self-associate, occurs as the concentration of a surfactant increases above a critical concentration (CMC)
have hydrophobic groups of the surfactants oriented toward the center/core
core of the micelle
represents a lipid-like region that is capable of dissolving water-insoluble drugs
hydrophilic colloids
hydrophilic polymers
used in O/W emulsion
form a multimolecular film at the interface and increase the viscosity of water
does not lower the interfacial tension
examples - acacia, tragacanth, gelatiun
finely divided solid particles
particles less than a micron can adsorb at the interface and form a film of fine particle
hydrophilic finely divided solid particles
bentonite
magnesium aluminum silicate
aluminum hydroxide
used for O/W emulsion
hydrophobic finely divided solid particles
charcoal
used for W/O emulsions
predicting type of emulsion
based on surface active agents, hydrophilic colloids, or finely divided solid
surfactant predicts emulsion type
HLB less than 10, W/O
HLB greater than 10, O/W
hydrophilic colloids predicts emulsion type
only O/W
finely divided solids predict emulsion type
hydrophilic if contact angle is less than 90, O/W
hydrophobic if contact angle is greater than 90, W/O
phase volume ratio
volume of oil phase / total volume of the emulsion
0-26 –> O/W only
26-74 –> either O/W or W/O
74-100 –> W/O only
creaming
reversible as long as the interfacial film is effective in maintaining integrity of individual droplets
Stokes Law
Stokes Law
velocity of sedimentation
(squared diameter of droplets)(density of internal phase - density of external phase)980
/
18(viscosity of external phase)
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 be fixed by shaking
must be reformulated
phase inversion
due to phase volume ratio exceeding 74% (should keep the ratio below 50%)
O/W emulsion stabilized with sodium stearate can be inverted to W/O type using hard water – containing calcium (high HLB to low HLB)
instability of emulsion factors
creaming
coalescence
phase inversion
