Emulsion
description
Transcript of Emulsion
Colloid chemistry
Lecture 13: Emulsions
food
cosmetics
pharmaceutics
biological systems
bituminous carpet (asphalt)
etc.
Emulsions
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
EmulsionsEmulsions
Emulsion suitable for intravenous
injection.
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
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 waterWater in Oil (W/O): Water droplets dispersed in oil
- Based on size of liquid droplets0.2 – 50 mm Macroemulsions (Kinetically Stable)0.01 – 0.2 mm Microemulsions (Thermodynamically Stable)
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
Emulsifying agents
SurfactantsAnionic – Sodium stearate, Potassium laurate
Sodium dodecyl sulfate, Sodium sulfosuccinateNonionic – Polyglycol, Fatty acid esters, LecithinCationic – Quaternary ammonium salts, Amine hydrochlorides
SolidsFinely 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
oil droplet inwater
(stabilized)
oil droplet in water
(unstable)
Making emulsions
surfactant
polymersolidparticles
oil droplet inwater
(stabilized)
∆G = γ H ∆A << 0
drop coalescence proceeds continuously
∆G = γ H ∆A + desorption energy
high desorption energyprevents/hinders coalescence
∆G = γ H ∆A >> 0
emulgeation requires large energy input
O / W W / O
Making emulsions
• 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 lengtha0: effective head group area
Surfactant Packing Parameter
CPP or P =v
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 groupnMethyl = number of methyl groupsao: effective head group area: difficult to calculate.
Surfactant Packing Parameter
CPP or P =v
l ⋅ a0
Surfactant Packing Parameter
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
Bancroft's ruleBancroft's ruleEmulsion 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).
W/O vs. O/W emulsions
optimum for W/O emulsions
optimum forO/W emulsions
HLB
water
oil
oil
water
Application of surfactants on the basis of their HLB
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 phasein which the solubility of the emulsifying agent is higher.
solubilizers15-18detergents13-16
O/W emulsifiers10-16wetting agents7-9
W/O emulsifiers3-8antifoaming agents; inverse micelles1-3
APPLICATIONSHLB
θ θ
water
oil
oil
oil
waterwater
víz
Pickering emulsions
HLB values for typical nonionic surfactants structures
tenzid kereskedelmi név HLB
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
Bancroft’s Rule: Relation to HLB & CPP of SurfactantSurfactant
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
Based on the Bancroft’s rule, many emulsion properties are governed by the properties of the continuous phase
1. dye test 2. dilution test3. electrical conductivity measurements 4. refractive index measurement5. filter paper test
Tests for emulsion type (W/O or O/W emulsions ?)
Conductivity of emulsions
O / V
V / O
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!
(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.
Emulsions are kinetically stable!
(c) Viscosity of the continuous phaseHigher 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.)
Emulsions are kinetically stable!
(d) Size distribution of dropletsEmulsion 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 ratioAs 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!
Phase inversion in emulsionsBancroft's ruleEmulsion type depends more on the nature of the emulsifyingagent 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 changesin surfactant HLB / CPP.
In other words...Phase Inversion May be Induced.
Phase inversion induced by the change in the HLB / CPP
O / W
W / O
Na-soap Ba-soap
water oil + BaCl2
O/W W/O
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
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 molecules around non-ionic and ionic surfactants respectively, altering
surfactant solubility in the process
– Also reflected by change in curvature of the interface
O/W→ W/O
1. The order of addition of the phasesW →O + emulsifier → W/OO →W + emulsifier → O/W
2. Nature of emulsifierMaking the emulsifier more oil soluble tends to produce a W/O emulsion and vice versa.
3. Phase volume ratioOil/Water ratio↑ →W/O emulsion and vice versa
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.
Inversion of emulsions (phase inversion)
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, ∆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
kTgHa ⟨⟨∆ρπ 3
34
Creaming of emulsions
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. HeatingHeating to ~ 700C will rapidly break most 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.
Methods of destabilizing emulsions
Selection of emulsifiers
Correlation between chemical structure of surfactants andtheir 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 activity2. Ability to form a condensed interfacial film3. Appropriate diffusion rate (to interface)
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:
1. HLB method – HLB indicative of emulsification behavior.
HLB 3-6 for W/O8-18 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
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.
General guidelines
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 for a limited no. of compounds.
General guidelines
1 – phase separation(creaming/sedimentation)
2 – Ostwald ripening
3 – aggregation processes(flocculation;coagulation;coalescence)
4 – phase inversion
Breaking emulsions
primaryemulsion
coalescence breaking
flocculation creaming
Breaking emulsions
Stabilization of emulsions
• emulsifiers: mostly surfactants• hydration forces: O / W• steric forces: W / O• electrostratic forces: ionic surfactants• polymers: steric forces (entropy stabilization)• solid powders: hydrophobic forces (+ wetting)
Breaking emulsions
• sedimentation• centrifugation• filtration• thermal coagulation• electric treatment• ultrasonication• chemical additives (e.g. salting out)
oil + lipophilicsurfactant
aqueous phase
W / O emulsion
stirring
W / O emulsion
step 2
step 1
hidrophilicsurfactant
stirring
W / O / Wcomplex emulsion
Complex (multiphase) emulsions
primary emulsifier
oil phaseinner aqueous phase
szekunder emulgeálószer
outer aqueous phase
W / O / W emulsion
Complex(multiphase) emulsions
W / O / W O / W / O
10 µm 20 µm
secondary emulsifier
W / O / W O / W / O
Hypothetic phase diagram
surfactant
water oil
stablemetastableunstable
stability
macroemulsions
miniemulsions
microemulsions
normalmicelle
solubilizate
microemulsion O/W macroemulsion
Micelles, solubilizates, emulsions
thermodynamicallystable
thermodynamicallyunstable
Emulsions – microemulsions - internal structure
O/WBicontinuous structure (µE)
W/O
- bicontinuous µEs do exist;- bicontinuos emulsions do not exist!
The interfacial tension (IFT) for microemulsions is ca.1000-times less than the IFT of O/W or W/O emulsions !!!
O / W W / OµE
100 % water 100 % oil
IFT [mN/m]
microemulsion
emulsion
Appearance and properties
turbid; milkytransparent; translucent
optical properties
surfactants; polymers; solid particles (Θ.90)
surfactants;co-surfactants
stabilizing agents
($1 mJ/m2(.0interfacial tension
1-20 µm20-400 nmsize
thermodynamically unstable; kinetically
stable
thermodynamically stable
stability
O/W; W/O; + complex: O/W/O; W/O/W
O/W; W/O; bicontinuous structure
type
energy input requiredspontaneous, no energy input requied
formation
emulsionsmicroemulsionsproperty
Physico-chemical properties
nonionic surfactants: temperature increasesionic surfactants: electrolyte (NaCl) concentration increases
Winsor-microemulsions
phase inversion may be generated by the variations of temperature/salinity
Winsor-microemulsions
Winsor-I Winsor-IIWinsor-III
O/W W/O bicontinuous
pure oil pure water