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8/2/2019 06 Emulsion
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Dodecane droplets in a
continuous phase of water/glycerol mixture.
Sodas: Oil in Water emulsion
Milk: Oil in Water
emulsion
Balm: Water in oil emulsion
Mayonnaise: Oil in
Water emulsion
Emulsions
Emulsion
suitable for
intravenous
injection.
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Outline
• Introduction
• Types of emulsions
• Emulsifying agents
• Tests for emulsion types
• Emulsion Stability
• Phase Inversion, Creaming
• Emulsion Breaking
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Introduction
Emulsion – Suspension of liquid droplets (dispersed phase) of
certain size within a second immiscible liquid (continuous
phase).
Classification of emulsions
- Based on dispersed phaseOil in Water (O/W): Oil droplets dispersed in water
Water in Oil (W/O): Water droplets dispersed in oil
- Based on size of liquid droplets
0.2 – 50 mm Macroemulsions (Kinetically Stable)
0.01 – 0.2 mm Microemulsions (Thermodynamically Stable)
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Metal cutting oils Margarine Ice cream
Pesticide Asphalt Skin cream
Emulsions encountered in everyday life!
Stability of emulsions may be engineered to vary from
seconds to years depending on application
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Stable suspensions of liquids constituting the dispersed
phase, in an immiscible liquid constituting the continuousphase is brought about using emulsifying agents such as
surfactants
Surfactants must exhibit the following characteristics to beeffective as emulsifiers
- Good surface activity- Should be able to form a condensed interfacial film
- Diffusion rates to interface comparable to emulsion forming
time
Emulsifying Agents
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Surfactants
Anionic – Sodium stearate, Potassium laurateSodium dodecyl sulfate, Sodium sulfosuccinate
Nonionic – Polyglycol, Fatty acid esters, Lecithin
Cationic – Quaternary ammonium salts,
Amine hydrochlorides
Solids
Finely divided solids with amphiphilic properties such assoot, silica and clay, may also act as emulsifying agents
(Pickering Emulsions: Attribute of high stability)
Common Emulsifying Agents
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• Conceptual framework that relates molecular parameters
(head group area, chain length and hydrophobic tail
volume) and intensive variables (temperature, ionic
strength etc.) to surfactant microstructures
• Critical Packing Parameter /
Packing Parameter
v: Volume of hydrocarbon corel: hydrocarbon chain length
a0: Effective head group area
Surfactant Packing Parameter
CPP or P v
l a0
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v: Volume of hydrocarbon chain= 0.027(nc + nMethyl)
l: hydrocarbon chain length= 0.15 + 0.127nc
Where nc = number of carbon atoms without the methyl
group
nMethyl = number of methyl groups
ao: Effective head group area: difficult to calculate.
Surfactant Packing Parameter
CPP or P v
l a0
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Packing Parameter is inversely related to HLB
Mid Point of
Packing ParameterP = 1
analogous to
HLB 10
At P = 1/ HLB = 10,
surfactant has equa
affinity for oil and
water
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Bancroft’s Rule:Relation to HLB & CPP of Surfactant
Surfactant
WaterOil
Surfactant
WaterOil
Surfactant more soluble in
water (CPP < 1, HLB > 10)
O/W emulsion
Surfactant more soluble in oil
(CPP > 1, HLB < 10)W/O emulsion
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Bancroft’s Rule:Relation to HLB & CPP of Surfactant
Surfactant
WaterOil
Surfactant
WaterOil
Surfactant more soluble in
water (CPP < 1, HLB > 10)
O/W emulsion
Surfactant more soluble in
oil (CPP > 1, HLB < 10)
W/O emulsion
Packing Parameter = 1
Microemulsion
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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. Refractive index measurement
5. Filter paper test
Tests for Emulsion Type
(W/O or O/W emulsions)
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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(Analogous to stable foam bubbles)
Mixed surfactant system preferred over single surfactant.
(Lauryl alcohol + Sodium lauryl sulfate: hydrophobic interactions)NaCl added to increase stability (electrostatic screening)
Emulsions are Kinetically Stable!
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(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 chargedpositively)
No correlation between droplet charge and emulsion stability in W/O
emulsions
Steric barrier – dehydration and change in hydrocarbon chainconformation.
Emulsions are Kinetically Stable!
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(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.)
Emulsions are Kinetically Stable!
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(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) TemperatureTemperature , usually emulsion stability
Temp affects – Interfacial tension, D, solubility of surfactant,
Brownian motion, viscosity of liquid, phases of interfacial film.
Emulsions are Kinetically Stable!
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Phase Inversion in 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.
Based on the Bancroft’s rule, it is possible to change anemulsion from O/W type to W/O type by inducing changes
in surfactant HLB / CPP.
In other words...
Phase Inversion May be Induced.
C id t f 2 i i ibl d 1 i ibl i f li id
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Consider systems of 2 immiscible and 1 miscible pairs of liquids
Acetic Acid
WaterBenzene
Surfactant
WaterOil
Acetic acid & water are miscible in
all proportions
Benzene & water - partly miscible,
acetic acid & water - partly miscibleAcetic acid added to a mixture of
benzene & water, preferentially
partitions into water (slope of tie line)
Surfactant and water are miscible
in all proportions
Oil and water - partly miscible,
surfactant and oil - partly miscible
Tie line
Surfactant added to a mixture of oil
& water, preferentially partitions
into water (slope of tie line)
I T At ifi t t f t t b Oil
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Increase T: At a specific temperature, surfactant becomes Oil
Soluble across all proportions, Acetic Acid does not!
Acetic Acid
WaterBenzene
Acetic Acid
WaterBenzene
Surfactant
WaterOil
Surfactant
WaterOil
Increase inT, P
Increase in T,
Electrolyte
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Why does Phase Inversion Take Place for system with Surfactants?
Surfactant
WaterOil
Surfactant
WaterOil
O/W emulsion W/O emulsion
Temperature for Non Ionics, Salting out electrolytes for ionics
B ft’ R l M if t d i R f S f t t S l bilit
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Bancroft’s Rule: Manifested in Response of Surfactant Solubility
O/W emulsion W/O emulsion
Temperature for Non Ionics, Salting out electrolytes for ionics
Temperature and electrolytes disrupt the water moleculesaround non-ionic and ionic surfactants respectively, altering
surfactant solubility in the process
– Also reflected by change in curvature of the interface
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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/Oemulsion and vice versa.
3. Phase volume ratio
Oil/Water ratio W/O emulsion and vice versa
Inversion of Emulsions (Phase inversion)
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Droplets larger than 1 mm 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
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. This is achieved by producing
a polyphase emulsion
kT gH a 3
3
4
Creaming of Emulsions
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Methods of Destabilizing Emulsions
1. Physical methods
(i) Centrifuging
(ii) Filtration – media pores preferentially wetted by
the continuous phase
(iii) Gently shaking or stirring
(iv) Low intensity ultrasonic vibrations
2. Heating
Heating to ~ 700C will rapidly break most emulsions.
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3. Electrical methods
• Most widely used on large scale
• 20 kV results in coalescence of entrained water
droplets (W/O) e.g. in oil field emulsions and jet
fuels. (mechanism – deformation of water drops intolong streamers)
• For O/W, electrophoretic migration of charged
groups to one of the electrodes. Ex. Removing tracesof lubricating oil emulsified in condensed water.
Methods of Destabilizing Emulsions
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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 thetype of emulsion.
Basic requirements:
1. Good surface activity
2. Ability to form a condensed interfacial film
3. Appropriate diffusion rate (to interface)
G l G id li
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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.
General Guidelines:
G l G id li
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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.
General Guidelines
G l G id li
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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 10-400
C lower than the storagetemperature is desired.
General Guidelines
G l G id li
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3. Cohesive energy ratio (CER) methodInvolves matching HLB’s of oil and emulsifying agents;
also molecular volumes, shapes and chemical nature.
Limitation – necessary information is available only fora limited no. of compounds.
General Guidelines