2. Emulsions
food
cosmetics
pharmaceutics
biological systems
bituminous carpet (asphalt)
etc.
3. Emulsion
suitable for
intravenous
injection.
Emulsions
Balm: Water in oil emulsion 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.
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. Introduction
Emulsion – Suspension 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
- Based on size of liquid droplets
0.2 – 50 mm Macroemulsions (Kinetically Stable)
0.01 – 0.2 mm Microemulsions (Thermodynamically Stable)
6. Emulsifying agents
Stable suspensions of liquids constituting the dispersed phase, in an
immiscible liquid constituting the continuous phase is brought about
using emulsifying agents such as surfactants
Surfactants must exhibit the following characteristics to be
effective as emulsifiers
- good surface activity
- should be able to form a condensed interfacial film
- diffusion rates to interface comparable to emulsion forming time
7. Common Emulsifying Agents
Surfactants
Anionic – Sodium stearate, Potassium laurate
Sodium dodecyl sulfate, Sodium sulfosuccinate
Nonionic – Polyglycol, Fatty acid esters, Lecithin
Cationic – Quaternary ammonium salts, Amine hydrochlorides
Solids
Finely divided solids with amphiphilic properties such as
soot, silica and clay, may also act as emulsifying agents
(Pickering emulsions: attribute of high stability)
8. Making emulsions
surfactant
oil droplet in
water oil droplet in
(unstable) water
(stabilized)
po
lym
solid
er
particles
oil droplet in
water
(stabilized)
9. ∆G = γ H ∆A >> 0
emulgeation requires large energy input
∆G = γ H ∆A << 0
drop coalescence proceeds continuously
∆G = γ H ∆A + desorption energy
high desorption energy
prevents/hinders coalescence
11. Surfactant Packing Parameter
• 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
CPP or P =
l ⋅ a0
v: volume of hydrocarbon core
l: hydrocarbon chain length
a0: effective head group area
12. Surfactant Packing Parameter
v
CPP or P =
l ⋅ a0
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.
14. Packing Parameter is inversely related to HLB
mid point of packing
parameter
P=1
analogous to
HLB 10
at P = 1/ HLB = 10,
surfactant has equal
affinity for oil and water
15. 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).
16. optimum for
O/W emulsions
HLB
oil
optimum for water
W/O emulsions
water
oil
17. The type of emulsion (O / W or W / O) is affected by:
• the ratio of the oil to water (non-polar to polar) phase;
• the chemical properties and the concentration of the emulsification agent;
• the temperature; the presence of additives;
• for solid particles as the stabilizing agents (Pickering emulsions)
the wetting conditions (contact angles of the oil and water phases on the solid)
Bancroft’s rule (1912): the dispersion medium of an O+W emulsion is the phase
in which the solubility of the emulsifying agent is higher.
HLB APPLICATIONS
1-3 antifoaming agents; inverse micelles
3-8 W/O emulsifiers
7-9 wetting agents
10-16 O/W emulsifiers
13-16 detergents
15-18 solubilizers
Application of surfactants on the basis of their HLB
19. HLB values for typical nonionic surfactants structures
tenzid kereskedelmi név HLB
20. Bancroft’s Rule: Relation to HLB & CPP of Surfactant
Surfactant
Surfactant
Oil Water
Oil Water
Surfactant more soluble in Surfactant more soluble in oil
water (CPP < 1, HLB > 10) (CPP > 1, HLB < 10)
O/W emulsion W/O emulsion
21. Bancroft’s Rule: Relation to HLB & CPP of Surfactant
Surfactant
Surfactant
Packing Parameter = 1
Oil Water
Oil Water
Microemulsion
Surfactant more soluble in Surfactant more soluble in
water (CPP < 1, HLB > 10) oil (CPP > 1, HLB < 10)
O/W emulsion W/O emulsion
22. 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. refractive index measurement
5. filter paper test
24. Emulsions are kinetically stable!
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)
25. Emulsions are kinetically stable!
(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.
26. Emulsions are kinetically stable!
(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, η ↑ as the no. of droplets↑
(many emulsion are more stable in concentrated form than
when diluted.)
27. Emulsions are kinetically stable!
(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.
28. 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 an
emulsion from O/W type to W/O type by inducing changes
in surfactant HLB / CPP.
In other words...
Phase Inversion May be Induced.
29. Phase inversion induced by the change in the HLB / CPP
Na-soap Ba-soap
+ BaCl2
water oil
O/W W/O
O/W
W/O
30. Why does phase inversion take place for system with surfactants?
Surfactant Surfactant
Oil Water Oil Water
O/W emulsion W/O emulsion
temperature for non ionics, salting out electrolytes for ionics
31. Bancroft’s rule: manifested in response of surfactant solubility
temperature for non ionics, salting out electrolytes for ionics
O/W emulsion W/O emulsion
Temperature and electrolytes disrupt the water molecules
around non-ionic and ionic surfactants respectively, altering
surfactant solubility in the process
– Also reflected by change in curvature of the interface
32. 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
33. 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.
34. Creaming of emulsions
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
4 3
πa ∆ρgH 〈〈 kT
3
a - droplet radius, ∆ρ - density difference,
g - gravitational constant, H - height of the vessel,
Creaming can be prevented by homogenization. Also by reducing
∆ρ, creaming may be prevented. This is achieved by producing
a polyphase emulsion
35. 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.
36. Methods of destabilizing emulsions
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 into
long streamers)
• For O/W, electrophoretic migration of charged
groups to one of the electrodes. Ex. Removing traces
of lubricating oil emulsified in condensed water.
37. 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)
38. 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.
39. 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.
40. 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.
41. 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.
50. Emulsions – microemulsions - internal structure
O/W W/O
Bicontinuous structure (µE)
- bicontinuous µEs do exist;
- bicontinuos emulsions do not exist!
51. The interfacial tension (IFT) for microemulsions is ca.
1000-times less than the IFT of O/W or W/O emulsions !!!
IFT [mN/m]
O/W µE W/O
100 % water 100 % oil
54. Winsor-microemulsions
phase inversion may be generated by the variations of temperature/salinity
nonionic surfactants: temperature increases
ionic surfactants: electrolyte (NaCl) concentration increases
55. Winsor-microemulsions
pure oil pure water
O/W bicontinuous W/O
Winsor-I Winsor-III Winsor-II
