colloid chemistry

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