Contributed by:
The highlights are:
1. Emulsions
2. Emulsifying Agents
3. Tests for emulsions
4. Creaming of emulsions
5. Inversion of emulsions
6. Selection of emulsifiers
1.
Emulsion
suitable for
intravenous
injection.
Balm: Water in oil emulsion
Emulsions Sodas: Oil in Water emulsion
Milk: Oil in Water
emulsion
Dodecane droplets in a Mayonnaise: Oil in
continuous phase of Water emulsion
water/glycerol mixture.
2.
Emulsion – Dispersion of liquid droplets (dispersed phase) of
certain size within a second immiscible liquid (continuous
phase).
Classification of emulsions
- Based on dispersed phase
Oil in Water (O/W): Oil droplets dispersed in water
Water in Oil (W/O): Water droplets dispersed in oil
Water in Oil in water (W/O/W): Water in Oil emulsion
dispersed in water – multiple emulsion
- Based on size of liquid droplets
0.2 – 50 m Macroemulsions
0.01 – 0.2 m Microemulsions
3.
Advantages
• Administration of Distasteful oil, mask the
unpleasant taste
• Better and faster absorption
• Less irritation to the skin
• Sustained release medication
• Nutritional supplement
• Diagnostic purposes
4.
Emulsions encountered in everyday life!
Pesticide Asphalt Skin cream
Metal cutting oils Margarine Ice cream
Stability of emulsions may be engineered to vary from
seconds to years depending on application
5.
Emulsifying Agents
Stable dispersions of liquids constituting the dispersed phase,
in an immiscible liquid constituting the continuous phase is
brought about using emulsifying agents such as
Carbohydrates: acacia, tragacanth, agar, chondrus and pectin
Proteins: gelatin, egg yolk and casein
High mol wt alcohols: stearyl alcohol, cetyl alcohol,
glycery monostearate, cholesterol – w/o stabilisers
Surfactants: SPAN, TWEEN, organic soaps
( triethanolamine oleate),
Non ionic- pH 3-10, cationic – 3-7, anionic- greater than 8
6.
Common Emulsifying Agents
Anionic – Sodium stearate, Potassium laurate
Sodium dodecyl sulfate, Sodium sulfosuccinate
Nonionic – Polyglycol, Fatty acid esters, Lecithin
Cationic – Quaternary ammonium salts,
Finely divided Solids
Finely divided solids with amphiphilic properties such as
silica and clay, may also act as emulsifying agents
Others: bentonite, magnesium hydroxide, Al(OH)3
7.
Tests for Emulsion Type
(W/O or O/W emulsions)
Based on the Bancroft’s rule, many emulsion properties are
governed by the properties of the continuous phase
1. Dye test
2. Dilution test
3. Electrical conductivity measurements
4. Filter paper test
8.
Thermodynamic instability
• ∆G=Ὺ .∆A
• Increase in the surface free energy =
interfacial tension X increased surface area
9.
Mechanism of emulsification
• Monomolecular theory
Surfactants
• Reduce interfacial tension
• Forms a protective film around globule
• Ionic surfactant exert repulsion between
globules
10.
Mechanism
Multimolecular theory
• Hydrocolloids form multimolecular
physical barrier around globules there by
prevent coalescence of oil globules
• Acacia, gelatin
Solid particle adsorption theory
11.
Physical Instability
Creaming: Concentration of globules at the top or bottom of
emulsion.
Reversible process but leads to breaking
Influenced by: Stoke’s equation
V = h = d2st (ρs–ρo) g
t 18ηo
-globule size
-Viscosity of dispersion medium
-Difference in the densities of dispersed and dispersion
medium
12.
Creaming of Emulsions
Droplets larger than 1 m may settle preferentially to the top or
the bottom under gravitational forces.
Creaming is an instability but not as serious as coalescence or
breaking of emulsion
Probability of creaming can be reduced if
4 3
a gH kT
3
a - droplet radius, Δρ - density difference,
g - gravitational constant, H - height of the vessel,
Creaming can be prevented by homogenization. Also by reducing
Δρ, creaming may be prevented.
13.
Creaming can be
reduced/prevented by
• Reducing the globule size by
homogenization
• Increasing the viscosity of dispersion
medium
• Reducing the difference in densities
14.
Coalescence
Separation of two phases due to fusion of globules. Also
called cracking of emulsion.
Irreversible process.
Sheath of EA around globules is lost.
Creaming leads to breaking- globules comes nearer
Breaking of emulsion is observed due to:
Insufficient amount of EA
Incompatibility between EA
Alteration in the properties of EA
15.
Inversion of Emulsions (Phase inversion)
O/W W/O
1. The order of addition of the phases
W O + emulsifier W/O
O W + emulsifier O/W
2. Nature of emulsifier
Making the emulsifier more oil soluble tends to produce a W/O
emulsion and vice versa.
3. Phase volume ratio
Oil/Water ratio W/O emulsion and vice versa
16.
Inversion of Emulsions (Phase inversion)
4. Temperature of the system
Temperature of O/W (polyoxyethylenated nonionic
surfactant) makes the emulsifier more hydrophobic and the
emulsion may invert to W/O.
5. Addition of electrolytes and other additives.
Strong electrolytes to O/W (stabilized by ionic surfactants)
may invert to W/O
Example. Inversion of O/W emulsion (stabilized by
sodium cetyl sulfate and cholesterol) to a W/O type upon
addition of polyvalent Ca.
17.
W/O vs. O/W emulsions
Bancroft's rule
Emulsion type depends more on the nature of the emulsifying agent
than on the relative proportions of oil or water present or the
methodology of preparing emulsion.
The phase in which an emulsifier is more soluble constitutes the
continuous phase
In O/W emulsions – emulsifying agents are more soluble in water
than in oil (High HLB surfactants).
In W/O emulsions – emulsifying agents are more soluble in oil than
in water (Low HLB surfactants).
18.
Emulsions
Rate of coalescence – measure of emulsion stability.
It depends on:
(a) Physical nature of the interfacial surfactant film
For Mechanical stability, surfactant films are characterized
by strong lateral intermolecular forces and high elasticity
Mixed surfactant system preferred over single surfactant.
(Lauryl alcohol + Sodium lauryl sulfate: hydrophobic interactions)
combination of SPAN and TWEEN
19.
Emulsions
(b) Electrical or steric barrier
Significant only in O/W emulsions.
In case of non-ionic emulsifying agents, charge may arise due to
(i) adsorption of ions from the aqueous phase or
(ii) contact charging (phase with higher dielectric constant is charged
positively)
No correlation between droplet charge and emulsion stability in W/O
emulsions
Steric barrier – dehydration and change in hydrocarbon chain
conformation.
20.
Emulsions
(c) Viscosity of the continuous phase
Higher viscosity reduces the diffusion coefficient
Stoke-Einstein’s Equation
This results in reduced frequency of collision and therefore
lower coalescence. Viscosity may be increased by adding
natural or synthetic thickening agents.
Further, as the no. of droplets
(many emulsion are more stable in concentrated form than when
diluted.)
21.
Emulsions
(d) Size distribution of droplets
Emulsion with a fairly uniform size distribution is more stable than
with the same average droplet size but having a wider size
distribution
(e) Phase volume ratio
As volume of dispersed phase stability of emulsion
(eventually phase inversion can occur)
(f) Temperature
Temperature , usually emulsion stability
Temp affects – Interfacial tension, D, solubility of surfactant,
Brownian motion, viscosity of liquid, phases of interfacial film.
22.
Preparation of emulsion
Dry gum method
Wet gum method
Bottle method
23.
Selection of Emulsifiers
Correlation between chemical structure of surfactants and
their emulsifying power is complicated because
(i) Both phases oil and water are of variable compositions.
(ii) Surfactant conc. determines emulsifier power as well as the
type of emulsion.
Basic requirements:
1. Good surface activity
2. Ability to form a condensed interfacial film
3. Appropriate diffusion rate (to interface)
24.
General Guidelines:
1. Type of emulsion determined by the phase in which emulsifier
is placed.
2. Emulsifying agents that are preferentially oil soluble form W/O
emulsions and vice versa.
3. More polar the oil phase, the more hydrophilic the emulsifier
should be. More non-polar the oil phase more lipophilic the
emulsifier should be.
25.
General Guidelines
1. HLB method – HLB indicative of emulsification behavior.
HLB 3-6 for W/O
8-18 for O/W
HLB no. of a surfactant depend on which phase of the final emulsion
it will become.
Limitation – does not take into account the effect of temperature.
26.
General Guidelines
2. PIT method – At phase inversion temperature, the hydrophilic
and lipophilic tendencies are balanced.
Phase inversion temperature of an emulsion is determined
using equal amounts of oil and aqueous phase + 3-5%
surfactant.
For O/W emulsion, emulsifier should yield PIT of 20-600C
higher than the storage temperature.
For W/O emulsion, PIT of 10-400C lower than the storage
temperature is desired.
27.
General Guidelines
3. Cohesive energy ratio (CER) method
Involves matching HLB’s of oil and emulsifying agents;
also molecular volumes, shapes and chemical nature.
Limitation – necessary information is available only for
a limited no. of compounds.
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