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Emulsion Technology

Lecture 6

ACS© 2005

Terminology -I

Phase 1 Droplet Dispersed Discontinuous Phase 2 Serum Medium Continuous

Internal

External

ACS© 2005

Lecture 6 - Emulsion Technology

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Terminology - II

Macroemulsions ­ 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. Microemulsions usually 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.

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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. · 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

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Typical food emulsions

Food Milk, cream Emulsio n type O/W Dispersed phase Butterfat triglycerides partially crystalline and liquid oils. Droplet size: 1 ­ 10 µm Volume fraction: Milk: 3-4% Cream: 10- 30% Butterfat (cream) or vegetable, partially crystallized fat. Volume fraction of air phase: 50% Continuous phase Aqueous solution of milk proteins, salts, minerals, etc.

Stabilization factors, etc. Lipoprotein membrane, phospolipids, and adsorbed casein.

Ice cream

O/W (aerated to foam)

Water and ice crystals, milk proteins, carboxydrates (sucrose, corn syrup) Approx. 85% of the water content is frozen at ­ 20oC.

The foam structure is stabilized by agglomerated fat globules forming the surface of air cells. Added surfactants act as "destabilizers" controlling fat agglomeration. Semisolid frozen phase. Water droplets distributed in semisolid, plastic continuous fat phase.

Butter

W/O

Buttermilk: milk proteins, phospholipids, salts. Volume fraction: 16% Vegetable oils and fats. Droplet size: 1 ­ 5 µm. Volume fraction: 10 ­ 30%

Imitation cream (to be aerated)

O/W

Butterfat triglycerides, partially crystallized and liquid oils; genuine milk fat globules are also present. Aqueous solution of proteins (casein), sucrose, salts, hydrocolloids.

Coffee whiteners

O/W

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

Margarine and related products (low calorie spread) Mayonnaise

W/O

Water phase may contain cultured milk, salts, flavors. Droplet size: 1 ­ 20 µm Volume fraction: 16 ­ 50 % Vegetable oil. Droplet size: 1 ­ 5 µm. Volume fractions: Minimum 65% (U.S. food standard.) Vegetable oil. Droplet size: 1 ­ 5 µm. Volume fractions: Minimum 30% (U.S. food standard.)

Aqueous solution of proteins (sodium caseinate), carbohydrates (maltodextrin, corn syrup, etc.), salts, and hydrocolloids. Edible fats and oils, partially hydrogenated, of animal or vegetable origin. Colors, flavor, vitamins. Aqueous solution of egg yolk, salt flavors, seasonings, ingredients, etc. pH: 4.0 ­ 4.5 Aqueous solutions of egg yolk, sugar, salt, starch, flavors, seasonings, hydrocolloids, and acidifying ingredients. pH: 3.5 ­ 4.0

Before aeration: adsorbed protein film. After aeration: the foam structure is stabilized by aggregated fat globules, forming a network around air cells; added lipophilic surfactants promote the needed fat globule aggregation. Blends of nonionic and anionic surfactants together with adsorbed proteins.

O/W

The dispersed water droplets are fixed in a semisolid matrix of fat crystals; surfactants added to reduce surface tension/promote emulsification during processing. Egg yolk proteins and phosphatides.

Salad dressing

O/W

Egg yolk proteins and phosphatides combined with hydrocolloids and surfactants, where permitted by local food law.

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Surface activity in emulsions

Emulsions are dispersions of droplets of one liquid in another. Emulsifiers are soluble, to different degrees, in both phases.

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Emulsion stability

+

F = A < 0

Drops coalesce spontaneously.

F = A + work of desorption

+

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

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Stability of emulsions*

Types: · Creaming ­ less dense phase rises · Inversion ­ internal phase becomes external phase · Ostwald ripening ­ small droplets get smaller · Flocculation ­ droplets stick together · Coalesence ­ droplets combine into larger ones

*Dickenson in "Food Structure"; Butterworths; 1988; p. 43.

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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.

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Breaking 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.

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Creaming of Emulsions

50

40

Height/mm

30

20

10

18 hours 43 hours 127 hours 154 hours 223 hours

0 0.0 0.2 0.4 0.6

Volume fraction

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

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Stability of emulsions - II

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.

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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. Ion exchange Raise temperature. Apply electric field. Filter through fritted glass or fibers. Centrifugation.

Lecture 6 - Emulsion Technology

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Emulsion Inversion

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

A

B

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Bancroft's Rule

"The emulsifier stabilizes the emulsion type where the continuous phase is the medium in which it is most soluble." A hydrophilic solute in an O/W emulsion. The long tail on the surfactant is to represent the longer range interaction of a "hydrophilic" molecule through water.

A hydrophilic solute in a W/O emulsion.

Lecture 6 - Emulsion Technology

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The HLB Schema

Variation of type and amount of residual emulsion with HLB number of emulsifier.

Optimum

O /W

Emulsion

breaker

for O/W

Volume and type of emulsion

10

HLB

W /O

Optimum

for W/O

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HLB Scale

Lipophilic End of Scale

Stearane Steric Acid Sodium Stearate Sodium Laurate

Hydrophilic end of scale Sucrose Sodium Sulfate

Soluble in oil; insoluble in water Nonspreading on water substrate

Soluble in oil; insoluble in water Spreads on water substrate

Soluble in oil; and in hot water Spreads on water substrate

Slightly oilsoluble; soluble in water Reduces surface tension of aqueous solutions Reduces interfacial tension at oil­ water interface

Insoluble in oil; soluble in water Does not affect the surface tension in aqueous solution Does not affect interfacial tension at oil­ water interface Does not stabilize emulsions

Insoluble in oil; soluble in water Increases surface tension in aqueous solution

Does not affect interfacial tension at oil­ water interface

Reduces interfacial tension at oil­ water interface

Reduces interfacial tension at oil­ water interface

Increases interfacial tension at oil­water interface

Does not stabilize emulsions

Stabilizes water in oil emulsions 1 ___________

Stabilizes either type of emulsion HLB Scale

Stabilizes oil in water emulsions

20 ___________

Decreases the stability of emulsions

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Applications of the HLB Scale

HLB Range 3.5­6 7­9 8­18 13­15 15­18

Application W/O emulsifier Wetting agent O/W emulsifier Detergent Solubilizer

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Group Numbers for Calculating HLB Values

G N ber roup um H ydrophilic G roups

-O 3 a+ SON

38.7 21.1 19.1 9.4 6.8 2.4 2.1 1.9 1.3 0.5 0.33n

HLB = 7 + ( H ) - ( L)

-C OK+ O-C ON + O- a N(tertiaryam ine) Ester (sorbitanring) Ester (free) -CO H O -O (free) H -O- -O (sorbitanring) H (-C 2 HO )n HC 2 -

L ipophilic G roups

-CH- -C 2 - H C 3- H =CH- (-C C 3 HO )n H HC 2 -

0.475

0.15n

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HLB and C.M.C.

40 s o d i u m a lk y l s u lf a A e r o s o l s e r ie s

HLB

20 A t la s S p a n s

A t la s T w e e n s

- o n o g ly c e m 0 -1 -2 -3 -4 -5

Log C.M.C.

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Phase inversion temperature

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

Water

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Emulsion

Lecture 6 - Emulsion Technology

Oil

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

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HLB and the Phase Inversion Temperature

16

HLB number (at 25oC)

12

Cyclohexane/Water

8

4

Water/Cyclohexane

0 0 30 60 90 120

Phase Inversion Temperature (oC)

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Multiple emulsions

(a) W/O/W double emulsion

O/W/O double emulsion

Consider, for either diagram: Each interface needs a different HLB value. The curvature of each interface is different.

ACS© 2005 Lecture 6 - Emulsion Technology

(Rosen, p. 313)

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Particles as emulsion stabilizers

Liquid 1 (oil)

r h

Liquid 2 (water)

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

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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, ...

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The Variation in Emulsion Properties with Concentration

Oil in water emulsion W/O Polyhedral droplets

Emulsion Property

Spherical droplets

Phase inversion

0

10

20

30

40

50

60

70

80

90

100

Volume Fraction Oil

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.

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Conductivity of Emulsions

0 .2 5

Conductivity ( m )

-1 -1

0 .2 0 0 .1 5 0 .1 0 0 .0 5 0 .0 0 0 20 40 60 80 100 W /O O /W

P h e n o l (% V o lu m e )

Phenol in water

Inversion zone

Water in Phenol

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

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Viscosities of Two Types of Emulsion

Deflection of inner cylinder

300

200

Benzene in water

100 ?

Water in benzene

0 0 10 20 30 ? 40 50 60 70 80 90 100

Percent benzene

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Interfacial viscosimeter

Torsional wire supporting bicone.

ser La

Light reflects off mirror into detector. Mirror

Position Detector

Bicone suspended at oil/water interface.

Stepping motor

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Bibliography for emulsions

Becher, P., Ed. Encyclopedia of emulsion technology, Vol. 1 Basic Theory, 1983; Vol. 2 Applications, 1985; Vol. 3 Basic theory, measurement, applications, 1988; Vol. 4, 1996; Marcel Dekker: New York. Becher, P.; Yudenfreund, M.N., Eds. Emulsions, Latices, and Dispersions; Marcel Dekker: New York; 1978. Becher, P. Emulsions: Theory and practice; Reinhold Publishing: New York; 1957; 3rd ed.; Oxford University Press: New York; 2001. Dickenson, E. An introduction to food colloids; Oxford University Press: New York; 1992. Dickenson, E.; McClements, D.J.; Advances in Food Colloids; Chapman & Hall: New York; 1996. Flick, E. W. Industrial surfactants; Noyes Publications: Park Ridge, NJ; 2nd ed. 1993. Lissant, K.J., Ed. Emulsions and emulsion technology; Marcel Dekker: New York; Parts 1 and 2, 1974; Part 3, 1984. McCutcheon's: Emulsifiers & Detergents, American Edition, MC Publishing: Glen Rock, NJ; (An annual publication.) Rosen, M.J. Surfactants and interfacial phenomena; John Wiley & Sons: New York; 1st ed, 1978; 2nd ed., 1989. Sherman, P, Ed. Rheology of emulsions; Macmillan Company: New York; 1963. Sherman, P., Ed. Emulsion science; Academic Press: New York; 1968. Shinoda, K.; Friberg, S. Emulsions and solubilization; John Wiley & Sons: New York; 1986. Sjöblom, J., Ed Emulsions and emulsion stability; Marcel Dekker: New York; 1996.

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