lecture 6 emulsion technology - colloidal · pdf fileian morrison© 2008 lecture 6 -...

Ian Morrison© 2008

Emulsion Technology

Dispersions in liquids: suspensions, emulsions, and foamsACS National Meeting

April 9 – 10, 2008New Orleans

Lecture 6 - Emulsion technologyIan Morrison© 2008

Typical food emulsionsFood Emulsio

n typeDispersed phase Continuous phase Stabilization factors, etc.

Milk, cream O/W Butterfat triglycerides partiallycrystalline and liquid oils.

Droplet size: 1 – 10 μmVolume fraction: Milk: 3-4%

Cream: 10- 30%

Aqueous solution of milkproteins, salts, minerals,etc.

Lipoprotein membrane, phospolipids,and adsorbed casein.

Ice cream O/W(aerated

tofoam)

Butterfat (cream) or vegetable,partially crystallized fat.

Volume fraction of air phase: 50%

Water and ice crystals, milkproteins, carboxydrates(sucrose, corn syrup)

Approx. 85% of the watercontent is frozen at –20oC.

The foam structure is stabilized byagglomerated fat globules formingthe surface of air cells.

Added surfactants act as“destabilizers” controlling fatagglomeration. Semisolid frozenphase.

Butter W/O Buttermilk: milk proteins,phospholipids, salts.

Volume fraction: 16%

Butterfat triglycerides,partially crystallized andliquid oils; genuine milkfat globules are alsopresent.

Water droplets distributed in semi-solid, plastic continuous fat phase.

Imitationcream

(to be aerated)

O/W Vegetable oils and fats.Droplet size: 1 – 5 μm.Volume fraction: 10 – 30%

Aqueous solution of proteins(casein), sucrose, salts,hydrocolloids.

Before aeration: adsorbed proteinfilm.

After aeration: the foam structure isstabilized by aggregated fatglobules, forming a network aroundair cells; added lipophilicsurfactants promote the needed fatglobule aggregation.

Coffeewhiteners

O/W Vegetable oils and fats.Droplet size: 1 – 5 μm.Volume fraction: 10 – 15 %

Aqueous solution of proteins(sodium caseinate),carbohydrates(maltodextrin, corn syrup,etc.), salts, andhydrocolloids.

Blends of nonionic and anionicsurfactants together with adsorbedproteins.

Margarine andrelatedproducts(low caloriespread)

W/O Water phase may contain culturedmilk, salts, flavors.

Droplet size: 1 – 20 μmVolume fraction: 16 – 50 %

Edible fats and oils, partiallyhydrogenated, of animalor vegetable origin.

Colors, flavor, vitamins.

The dispersed water droplets are fixedin a semisolid matrix of fat crystals;surfactants added to reduce surfacetension/promote emulsificationduring processing.

Mayonnaise O/W Vegetable oil.Droplet size: 1 – 5 μm.Volume fractions: Minimum 65%

(U.S. food standard.)

Aqueous solution of eggyolk, salt flavors,seasonings, ingredients,etc.

pH: 4.0 – 4.5

Egg yolk proteins and phosphatides.

Salad dressing O/W Vegetable oil.Droplet size: 1 – 5 μm.Volume fractions: Minimum 30%

(U.S. food standard.)

Aqueous solutions of eggyolk, sugar, salt, starch,flavors, seasonings,hydrocolloids, andacidifying ingredients.

pH: 3.5 – 4.0

Egg yolk proteins and phosphatidescombined with hydrocolloids andsurfactants, where permitted bylocal food law.

Dickenson, E.; McClements, D.J.; Advances in Food Colloids; Chapman & Hall: New York; 1996.

4


Terminology -I

ExternalInternal

ContinuousDiscontinuous

MediumDispersed

SerumDroplet

Phase 2Phase 1

1


Terminology - IIMacroemulsions – At least one immiscible liquid dispersed in another as drops whose diameters generally exceed 100 nm. The stability is improved by the addition of surfactants and/or finely divided solids. Considered only kinetically stable.

Miniemulsions – An emulsion with droplets between 100 and 1000 nm, reportedly thermodynamically stable.

Microemulsions – A thermodynamically stable, transparent solution of micelles swollen with solubilizate. Microemulsionsusually require the presence of both a surfactant and a cosurfactant (e.g. short chain alcohol).

Becher, P. Emulsions, theory and practice, 3rd

ed.; Oxford University Press: New York; 2001.

2


Emulsion stability

+

0F AσΔ = Δ <

Drops coalesce spontaneously.

+

work of desorptionF AσΔ = Δ +

If the work of desorption is high, the coalescence is prevented.

6


Surface activity in emulsions

Emulsions are dispersions of droplets of one liquid in another.

Emulsifiers are soluble, to different degrees, in both phases.

5


Emulsion stability

Electrostatic stabilization – at lower volume fractions

Steric stabilization – at all volume fractions

Additional factors –

1. Steric stabilization is enhanced by solubility in both phases:

2. Mixed emulsifiers (cosurfactants) are common. They can come from either phase.

3. Temperature is important – solubility changes quickly.

+

+

+

12


Emulsion inversion

The emulsifier maintains the same orientation at the interface, hence about the same energy.

The internal and continuous switch.

New


Bancroft’s Rule

The same emulsifier in an O/W emulsion.

An oil-soluble emulsifier in a W/O emulsion.

The long tail on the surfactant is to represent the longer range interaction of a “hydrophobic”molecule through oil.

“The emulsifier stabilizes the emulsion type where the continuous phase is the medium in which it is most soluble.”

16

Newish


Bancroft’s rule - II

If the hydrophilic portion of the emulsifier is charged, its “effective” size is much larger.

-

-

-

-

-

-

-

-

-

-

-

New


Manufacture of butter*• Milk is a fairly dilute, not very stable O/W emulsion, about 4% fat.

• Creaming produces a concentrated, not very stable O/W emulsion,about 36% fat. And is skimmed off.

• Gentle agitation, particularly when cool, 13 – 18 C, inverts it to make a W/O emulsion about 85% fat.**

• Drain, add salt, and mix well.

• Voila – butter!

• What remains is buttermilk.

*Becher, Emulsions; Oxford; 2001, p. 291

**Which might have happened 5000 years ago in the saddle bags of horsemen in Asia.

3


Measuring emulsion concentrations

The speed of sound is proportional to volume concentration.

Even to very high concentrations.

A calibration curve of speed of sound vsconcentration is easy to construct.

The measurement can be made through steel walls!

It just can’t be measured through air!

www.appliedsonics.com

New


Creaming of emulsions

Volume fraction0.0 0.2 0.4 0.6

Hei

ght/m

m

0

10

20

30

40

50

18 hours 43 hours 127 hours 154 hours 223 hours

Volume fraction at various heights and times was determined by measuring the speed of sound.

11

The drops in emulsions are typically large, > 1 μm.

Therefore the drops will either rise, O/W emulsions or settle, W/O emulsions.


Ostwald ripening of emulsions

Change in size distribution with aging, 0.005 M sodium oleate and octane: 1a, measured on first day; 1b, measured on third day; 1c. measured on seventh day, 0.005M cesium oleate; 2a, measured on first day; 2b measured on third day; 2c. Measured on seventh day.

9

2prσ

Δ =

Smaller drops are at higher pressures:

Therefore smaller ones gradually get even smaller and the large ones gradually get even larger,

Newish


Coalescence of emulsions

An emulsion system with an initial particle size of 235 nm was destabilized by dilution in a solution of an ionic surfactant opposite in sign to that of the particle charge. The three figures show the resulting distributions at times up to 4 days as reported in the figures.

10


Emulsion inversion

A

B

As the concentration increases (A) the droplets get closer until they pinch off into smaller, opposite type of emulsion (B).

14


Emulsion processes

A – Inversion C – Sedimentation E - CoalescenceB – Creaming D – Flocculation F - Ripening

A

B

C D

E

F

8


Demulsification – breaking emulsions

First, determine type, O/W or W/O. Continuous phase will mix with water or oil.

• Chemical demulsification, i.e. change the HLB

• Add an emulsifier of opposite type.

• Add agent of opposite charge.

• Freeze-thaw cycles.

• Add electrolyte. Change the pH.

• Raise temperature.

• Apply electric field.

• Filter through fritted glass or fibers.

• Centrifugation.

13


Multiple emulsions

(a) W/O/W double emulsion O/W/O double emulsionConsider, for either diagram:

Each interface needs a different HLB value.The curvature of each interface is different.

(Rosen, p. 313) 15


Emulsifiers come in graded seriesNew

HLB = 16.7PEO(20)-Sorbitan monolaurate

Tween 20HLB = 8.6Sorbitan monolaurate(C12, saturated)

Span 20

HLB = 15.6PEO(20)-Sorbitan monopalmitate

Tween 40HLB = 6.7Sorbitan monopalmitate(C16, saturated)

Span 40

HLB = 14.9PEO(20)-Sorbitan monostearate

Tween 60HLB = 4.7Sorbitan monostearate(C18, saturated)

Span 60

HLB = 10.5PEO(20)-Sorbitan tristearate

Tween 65HLB = 2.1Sorbitan tristearate(3-C18, saturated)

Span 65

HLB = 15.0PEO(20)-Sorbitan mono-oleate

Tween 80HLB = 4.3Sorbitan mono-oleate(C18, double bond)

Span 80

HLB = 11.0PEO(20)-Sorbitan tri-oleate

Tween 85HLB = 1.8Sorbitan tri-oleate(3-C18, double bond)

Span 85

Sorbitan (wikipedia) PEO (wikipedia)


The HLB SchemaVariation of type and amount of

residual emulsion with HLB number of emulsifier.

1 0

O /W

W /O

Volume and

type of emulsion H L B

Optimum for

O/W

Optimum for

W/O

Emulsionbreaker

17


HLB Scale

Lipophilic End of Scale Hydrophilic end of scale

Stearane Steric Acid SodiumStearate

SodiumLaurate

Sucrose Sodium Sulfate

Soluble in oil;insoluble in

water

Soluble in oil;insoluble in

water

Soluble in oil;and in hot

water

Slightly oil-soluble;

soluble inwater

Insoluble inoil;

soluble inwater

Insoluble in oil;soluble in water

Nonspreadingon watersubstrate

Spreads onwater substrate

Spreads onwater substrate

Reducessurface

tension ofaqueous

solutions

Does notaffect the

surfacetension in

aqueoussolution

Increases surfacetension in aqueous

solution

Does not affectinterfacial

tension at oil–water interface

Reducesinterfacial


Reducesinterfacial


Reducesinterfacial

tension at oil–water

interface

Does notaffect

interfacialtension at oil–

waterinterface

Increases interfacialtension at oil–water

interface

Does notstabilize

emulsions

Stabilizes waterin oil emulsions

Stabilizeseither type of

emulsion

Stabilizesoil in wateremulsions

Does notstabilize

emulsions

Decreases thestability ofemulsions

1___________ HLB Scale

20___________

18


Applications of the HLB scale

HLB Range Application

3.5–6 W/O emulsifier

7–9 Wetting agent

8–18 O/W emulsifier

13–15 Detergent

15–18 Solubilizer

19


Group Numbers for Calculating HLB Values

7 ( ) ( )HLB H L= + −∑ ∑

G roup N um berH ydrophilic G roups

- +3O SO N a− 38.7- +COO K− 21.1- +COO Na− 19.1

N (tertiary amine) 9.4Ester (sorbitan ring) 6.8Ester (free) 2.4

C O O H− 2.1O H (free)− 1.9O− − 1.3OH (sorbitan ring)− 0.5

2 2( C H C H O ) n− − 0.33n

Lipophilic G roups

C H− −2C H− − 0.475

3C H −C H= −

3 2( C HC H C H O ) n− − 0.15n

20


HLB and C.M.C.

4 0

2 0

0- 1 - 2 - 3 - 4 - 5

s o d iu m a lk y l s u l f a

A e r o s o l s e r ie s

A t la s T w e e n s

A t la s S p a n s

α −m o n o g ly c e

HLB

Log C.M.C.21


Phase inversion temperature

Water Emulsion Oil

30oC 40oC 50oC 60oC 70oC 75oC 80oC 90oC 100oC

www.bias-net.com/chimica/pdf/set_baglioni.pdf

22


HLB and the Phase Inversion Temperature

Phase Inversion Temperature (oC)

0 30 60 90 120

HLB

num

ber (

at 2

5oC

)

0

4

8

12

16

Cyclohexane/Water

Water/Cyclohexane

23


Particles as emulsion stabilizers

θ θ

h

Liquid 1(oil)

Liquid 2(water)

r

Almost all particles are only partially wetted by either phase.

When particles are “adsorbed” at the surface, they are hard to remove – the emulsion stability is high, sometimes thousands of kT.

Crude oil is a W/O emulsion and is old!!

24


Stability as a function of contact angle

θ0 30 60 90 120 150 180

Δ Fde

sorp

tion /

kT

0

3000

6000

9000

12000

ΔF2 ΔF1

25


The thermodynamics is rich

P. A. Kralchevsky,*,† I. B. Ivanov,† K. P. Ananthapadmanabhan,‡ and A. Lips‡ Langmuir 2005, 21, 50-63

Figure 7. Sketch of a particle of radius a, which is bridging between the surfaces of a film from phase 2 formed between two drops of phase 1. h is the film thickness. õ is the contact angle.

Figure 8. Definitions of phases, angles, and emulsions: By definition, the particles are initially dispersed in phase 2. The contact angle, õ, is always measured across phase 2. The emulsion 1-in-2 is a Bancroft-type emulsion, in which the particles are dispersed in the continuous phase. In contrast, the emulsion 2-in-1 is of anti-Bancroft type.

26


Figure 3. Microscopic image of a paraffin-in-water emulsion stabilized by P2 particles. Inset: same image taken at T ) 25 °C under crossed polarizers, confirming the presence of crystalsin the droplets.

Figure 1. Microscopic image of a paraffin-in-water emulsion stabilized by CTAB alone. T ) 25 °C.

Wax dispersed with fumed silica

J. Giermanska-Kahn,† V. Laine,† S. Arditty,† V. Schmitt,† and F. Leal-Calderon Langmuir 2005, 21, 4316-4323

Hydrophilic silica stabilizing a wax/water emulsion

27


Bubbles stabilized with fumed silica

Figure 1. Fraction (F) of bubbles remaining as a function of time (t) formed in dispersions of 1wt%of 33% SiOR particles at different NaClconcentrations: 3 mol dm-3 ([), 2 mol dm-3 (0), 1 mol dm-3 (2), and 0.5 mol dm-3 (4).

Hydrophobic silica stabilizing a foam in water with added salt.

Thomas Kostakis, Rammile Ettelaie, and Brent S. Murray Langmuir 2006, 22, 1273-1280

28


Physical properties of emulsions

• Identification of “internal” and “external” phases; W/O or O/W

• Droplet size and size distributions – generally greater than a micron

• Concentration of dispersed phase – often quite high. The viscosity, conductivity, etc, of emulsions are much different than the continuous phase.

• Rheology – complex combinations of viscous (flowing) elastic (when moved a little) and viscoelastic (when moved a lot) properties.

• Electrical properties – useful to characterize structure.

• Multiple phase emulsions – drops in drops in drops, …

29


Variation in properties with concentration

The variation of properties of emulsions with changes in composition. If inversion occurs, there is a discontinuity in properties, as they change from one curve to the other. Above 74% there is either a phase inversion or the droplets are deformed to polyhedra.

0 10 20 30 40 50 60 70 80 90 100

Emul

sion

Pro

perty

Volume Fraction Oil

W/O

PhaseinversionSpherical droplets

Polyhedraldroplets

Oil in water emulsion

30


Conductivity of emulsions

The specific conductivity of aqueous potassium iodide and phenolemulsions as a function of composition (Manegold, p. 30).

P h en o l (% V o lu m e)0 2 0 4 0 6 0 8 0 1 0 0

Con

duct

ivity

(Ω-1

m-1

)

0 .0 0

0 .0 5

0 .1 0

0 .1 5

0 .2 0

0 .2 5

O /W

W /O

Phenol in water Inversionzone

Water inPhenol

31


Interfacial viscometer

Bicone suspendedat oil/water

interface.

Light reflectsoff mirror into

detector.

Torsional wiresupporting bicone.

Laser

Position Detector

Mirror

Stepping motor

32


Rheology of O/W interfaces

( )2 4 where 4

Bk Tr D Da

τ τπη

Δ = =

( )2 4 where 4

Bk Tr D Da

τ τπη

Δ = =

Wu and Dai, Langmuir, 23, 4324 – 4331, 2007.

By single-particle tracking

2 23

Bk TraGπ

Δ =′

For viscous liquids:

For elastic liquids:

The particles have to sit properly at the O/W interface.

33


Making emulsions

• Method of phase inversion

• PIT method

• Condensation methods - solubilize an internal phase in micelles

• Electric emulsification

• Intermittent milling

34


Intermittent milling

+

Well stabilized drops

Mill to smaller size,hence larger area.

Marginallystable drops.

Dilute intostable dispersion.

Continuedmilling.

Unstabledrops coalesce.

Smaller,stable drops.

35

lecture 6 emulsion technology - colloidal · pdf fileian morrison© 2008 lecture 6 -...

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