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Foam, emulsion

Márta BerkaUniversity of Debrecen

Dept of Colloid and Environmental Chemistry

http://dragon.unideb.hu/~kolloid/

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Type of sols

• aerosols • lyosols xerosols,xerogels

G/L gas phase in liquid (sparkling water, foam, whipped cream)

L/L emulsion, liquid in liquid, milk

S/L colloid suspension (gold sol, toothpaste, paint, ink)

categorized by inner / outer phases

L/G liquid in air: fog, mists, spray

S/G solid aerosol, solid in gas: smoke, colloidal powder

Complex, smog

G/S solid foam: polystyrene foam, bread, cake, whipped creamL/S solid emulsion: opals,pearlsS/S solid suspensions: pigmented plastics

http://www.tutornext.com/classification-colloidal-solutions/7245

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Aeroszol (L/G, S/G)

enhanced aerosol concentrations cause the droplets in a cloud to be smaller and more numerous within a cloud of fixed water amount.

Carbon Black AggregatesPrimarily used as reinforcing filler• Tire• Black pigment. Elastomer composites, • Plastics, Pipe, •Printing Inks, Coatings

Fumed (a continuous flame hydrolysis technique ) Silica (silicon dioxide) AggregatesThermal conductivity: 12 to 16 mW/m·KLight transmission: 20 to 80% at 2 cmParticle density: 140 kg/m³, Bulk Density: 40-100 kg/ m³ , Surface area: 700 m2/g ,Porosity: > 90%Particle size: 5μ - 5 mm . In liquids, the chains bond together via weak hydrogen bonds forming a three dimensional network, trapping liquid and effectively increasing the viscosity (thixotropy). Surface Chemistry hydrophobic, reinforcement, thickening & thixotropy, anti-scratch - hydrophilic and hydrophobic fumed silicas

Atmospheric aerosols

Carbon black in its nascent form is fluffy powder. The particle size is roughly the size of virus.

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Dispersion of gas, G/L; foams

http://www.tcd.ie/Physics/Foams/index.php

gas phase is the dispersed phase and liquid is the dispersion medium (soda water, sparkling water)

Lyosols: when the dispersed gas bubbles have colloidal size.

The foams are the concentrated dispersions of gas phase.

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FoamStabilizers are needed

Foam structure of a spherical foam at 400X magnification

Foam structure of a hexagonal foam at 400X magnification

/www.ctmw.com/articles/Rita/2.htm

Polyhedral cellsFoams can be made byMixing or agitation; examples are draughtbeer, whipped cream and sea foam. Evolution of dissolved gas; for examplecanned beer, soft drinks, shaving foam and hair mousse. Bubbling gas through liquid.

Spherical bubbles <70-75%

The Kelvin Problem - Filling Space with Bubbles , Kelvin's solution, the tetrakaidekahedron

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Foam forming

Liquid foams are made wherever gases and liquids are mixed. Ingredients such as soap or other surfactant help to form stable films, and therefore long-lived foams.

Bubbling gas through liquid, through a porous filter minimum pressure, p=2γ/r. At first the largest bubbles come out.

The medium is the continuous phase!

prp p2P

rγ

Δ =

The pressure is at C > A > B places

C

The Laplace pressure is low (see next slide), because of the negative curvature, hence water will flow

Formation of bubbles

The arrows show the direction of streaming , hence water will flow to these points, until they become unstable. If you add glycerol to a soap solution, the viscosity increases, and the drainage of the foam is slowed down: it takes a longer time before the foam collapses.

Polyhedral cells

2Prγ

Δ =

Cross section of a Plateau border and foam lamellae.(The arrows show the direction of streaming.)

r<0

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InterferenceSoap Bubbles as Art

http://www.tcd.ie/Physics/Foams/duran.php

black

Different colors

The iridescent colours of soap bubbles are caused by interfering of (internally and externally) reflected light waves and are determined by the thickness of the film. The same as the phenomenon causing the colours in an oil slick on a wet road.

Stabilization of a foam film

Each interface is electrically charged. As the film thins, the repulsion increases.

Steric stab. Liquid crystals stabilize foams

Electrostatic stabilization of a foam film

Good emulsifying are also good foaming agent.

The factors which influence emulsion stability, against droplet coalescenceand foam stability against bubble collapse are similar

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Antifoams

With an antifoam on one surface,electrostatic stabilization is lost. •Antifoams - added to existing foams, in

the form of small droplets, which spread on the lamellae, thinning and breaking it.

(a) Antifoam drop. (b) Entering the surface. (c ) Leading to rupture of the film.

L

The Laplace pressure is low, because of the negative curvature, hence water will flow, until they become unstable.

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Foam Stability, Inhibition and Breaking

•Foam inhibitors - added before foam forms, displace foamingagents, or solubilizing the foaming agents (in micelles)•Foam breaking - mechanical, shock waves, compression waves,ultrasonics, rotating discs, heating, an electrical spark.•Antifoams - added to existing foams, in the form of small droplets,which spread on the lamellae, thinning and breaking it.

The stability of a liquid foam is governed by three main processes:Drainage: A freshly formed foam is not in equilibrium under gravity, and liquid will drain through the Plateau border channels until an equilibrium state is reached. Coarsening: gas diffuses between bubbles - some grow while others shrink and disappear. The net result of this process is that the average bubble size grows in time. Film Rupture: if a foam film gets too thin and weak, it will rupture. Eventually the foam will collapse and vanish. Unstable foams are formed from aqueous solutions of short chain acids or alcohols. Metastable foams are typically formed from solution of soaps, synthetic detergents, proteins, saponins, etc.

Simethicone is anoral anti-foaming agent

used to reduce bloating

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Applications

Marshmellow - foam formed from egg white, gelatin, and sugar.Ice cream - refrigerated and aerated at the same time. Ice crystals and fat crystals form thematrix.Dynamic foams: cakes, sponges, bread, meringues, soufflés. Bubbles change at various stages of preparation.Foams on drying, especially in distillation columns. A foam blanket at the surface acts as an insulating layer - causing overheating.Metallic slags foam probably because of the high viscosity. Cooling stabilizes the foam.Paper making - Caused by lignin, resin, and fatty acids in wood, sulfate soaps from pitch. Also, sizing materials, dyes, fillers, oxidized starch, proteins, etc act as profoamers.Beer - foam should not affect taste, but it remains important. Too little, beer looks "flat".Sources of foam: entrained air in the pouring, in the pressurizing, and from dissolved carbondioxide. Mostly stabilized by proteins. Protein-polysaccharide complexes are especiallystabilizing.

Chemical processing. Food products, such as whipped cream and chocolate mousse. Toiletries, such as shaving foam and hair mousse. Household cleaning products, such as oven cleaner and limescale remover. Fire extinguishers.

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Applications

Firefighting Foams•Primarily for fire protection in petroleum storage.Airplane fires.•Foam is made in a self-aspirating branchpipe: high pressure pushes the water + foaming agent down a pipe, aspirating air, foaming because of the turbulence ( about 1mm bubbles) and is thrown from about 15 to 75 m.

•Types: (1) Protein foam liquid - solution of hydrolyzed protein (chicken feather)(2) liquid with various perfluorinated surfactants (high performance, non-biodegradable), (3) mixtures of perfluorinated surfactants with proteins

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Applications

Foams to Immobilize• To retard evaporation. Improve insulation.

• For fumigants (toxic to fungi), insecticides (to keep them in place)

• Applying thin layers, such as adhesives or etching formulations, dyes or bleaches

• Capture of aerosols.

• Aqueous foam is an excellent suspending medium for paper fibers. Pseudo plasticity

enables dispersion of long fibers. At low shear stress the fibers are "frozen" in position.

Enables the use of long fibers which otherwise orient on coating.

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Solid foams Solid foams are cellular materials, i.e. materials which are made up from a framework of solid material surrounding gas-filled voids (bubbles). Solid foams can be 100 times lighterthan the equivalent solid material.

Natural solid foams include wood, bone and sea sponges.The bee's honeycomb is a two-dimensional cellular structure:

Recent developments in metal foams, especially aluminium, have produced a new class of lightweight materials, which are excellent energy absorbers. This property is useful in reducing the impact of a car crash. Other applications of solid foams include: Cushioning materials in furniture. Structural materials such as sandwich board. Insulating materials, such as cavity wall insulation. Honeycomb concrete. Packaging materials, such as expanded polystyrene. Several food items, such as bread, cake and other snack foods. AlMgCu metal foam blown by an intrinsic gas source

very lightweight, but stronger than a block of steel

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Emulsion, terminologyThe emulsion is a dispersed system in which the phases are immiscible or partially miscible.

Droplet size: 0.1-10 μm in miniemulsion

Phase 1 Phase 2

Droplet Serum

Dispersed Medium

Internal External

Discontinuous Continuous

O/W (oil in water), W/O (water in oil ) emulsions and bicontinuous

Polyhedral cells The medium is the continuous phase!

The globules of the dispersed liquid are generally between 0.1 micron and 10 micron, and so are larger than the particles found in sols.

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Emulsion types

• Identification of emulsion type:

1. Generally, an O/W emulsion has a creamy texture and a W/O emulsion feels greasy

2. The emulsion mixes readily with a liquid which is miscible with the dispersion medium

3. The emulsion is readily coloured by dyes which are soluble in the dispersion medium

4. O/W generally have a much higher electrical conductivity than W/O emulsions

The liquid with the greater phase volume need not necessarily be the dispersion medium!Above 74% there is either a phase inversion or the droplets are deformed to polyhedra.

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TerminologyMacroemulsions – At least one immiscible liquid dispersed inanother as drops whose diameters generally exceed 10 μm.The stability is improved by the addition of surfactants and/orfinely divided solids. Considered only kinetically stable.

Miniemulsions – An emulsion with droplets between 0.1and 10 μm, reportedly thermodynamically stable.

Microemulsions – An emulsion with droplets below 100 nm. 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, 3rded.; Oxford University Press: New York; 2001.

• 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

The most important physical properties of an emulsion is its stability

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Surface activity in emulsionsEmulsions are dispersions of droplets of one liquid in another. Emulsifiers form an adsorbed film around the dispersed droplets.Emulsifiers are soluble, to different degrees, in both phases.

Drops flocculate and coalesce spontaneously. In general, emulsions are thermodynamicallyunstable

0G AΔ γΔ= <

0G A work of desorptionΔ γΔ= + >

If the work of desorption of emulsifier is high, the coalescence is prevented , and the emulsions are thermodynamically stable.

emulsifiers

but

0AΔ <

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Making emulsions• Method of phase inversion• High Speed Mixers• Condensation methods - solubilize an internal phase in micelles• Electric emulsification• Intermittent milling

Homogenizer, Mills, Microfluidizer, SonolatorIn which fluid streams at high velocities are forced against each other resulting in cavitations, turbulence, and shear.

Emulsification proceeds in two steps: -mechanical mixing -stabilization

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Emulsifiers

Emulsifiers: -surface active materials, -naturally occurring materials, -finely divided solids

(Pickering stabilization)

1. Carbohydrate Materials: Acacia gum (gumiarábikum), Tragacanth (tragantmézga),

Agar (agar-agar), Pectine. o/w emulsion.

2. Protein Substances: Gelatin, Egg yolk, Caesin o/w emulsion.

3. High Molecular Weight molecules: Stearyl Alcohol, Cetyl Alcohol, Glyceryl Mono

stearate o/w emulsion, derivatives of cellulose, Na carboxymethilcellulose, cholesterol

w/o emulsion. Polyethylen glycol

4.4. Wetting Agents: Anionic, Cationic, NonionicWetting Agents: Anionic, Cationic, Nonionic

5.5. Finely divided solids: Finely divided solids: BentoniteBentonite, Magnesium Hydroxide, Aluminum Hydroxide , Magnesium Hydroxide, Aluminum Hydroxide o/wo/w

emulsion; emulsion; carbon black w/o

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Emulsion stabilityFactors favor emulsion stability (see lecture about

colloid stability)

1. Low interfacial tension

2. Steric stabilization. Mechanically strong interfacial film (proteins, surfactants, mixed emulsifiers are common. Temperature is important)

3. Electrical double layer repulsions (at lower volume fractions)

4. Relative small volume of dispersed phase

5. Narrow size distribution

6. High viscosity (simple retards the rates of creaming, coalescence, etc.)

7. Reduce gravitational separation: reduce density difference, reduce droplet size, increase continuous phase viscosity

The term “emulsion stability” can be used with reference to three different phenomena creaming (or sedimentation) , flocculation and a breaking of the emulsion due to the droplet coalescence. Eventually the dispersed phase may become a continuous phase, separated from the dispersion medium by a single interface. the time taken for phase separation may be anything from seconds to years, depending the emulsion formulation and manufacturing condition.

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Emulsion Inversion

As the concentration increases (A)the droplets get closer, and theagitation pinches them off into smaller, opposite type of emulsion (B).

making milk into 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. Behold! – butter!• What remains is buttermilk.

Typical amulsions: food emulsion, pesticide, cosmetics, proofing, drilling oil ...

water

oil

w

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Typical food emulsionsFood Emulsion type Dispersed phase Continuous phase Stabilization factors, etc Milk, cream O/W Butterfat triglycerides

partially crystalline and liquid oils. Droplet size: 1 – 10 μm Volume fraction: Milk: 3-4% Cream: 10- 30%

Aqueous solution of milk proteins, salts, minerals,

Lipoprotein membrane, phospolipids, and adsorbed casein.

Ice cream

O/W (aerated to foam)

Butterfat (cream) or vegetable, partially crystallized fat. Volume fraction of air phase: 50%

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

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

Butter W/O Buttermilk: milk proteins, phospholipids, salts. Volume fraction: 16%

Butterfat triglycerides, partially crystallized and liquid oils; genuine milk fat globules are also present.

Water droplets distributed in semisolid, plastic continuous fat phase.

Imitation cream (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 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.

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Typical food emulsionsFood Emulsion type Dispersed phase Continuous phase Stabilization factors, etc

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 anionic surfactants together with adsorbedproteins.

Margarine and relatedProducts (low caloriespread)

W/O Water phase may contain culturedmilk, salts, flavors.Droplet size: 1 – 20 μmVolume fraction: 16 – 50 %

Edible fats and oils, partially hydrogenated, of animal or 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 egg yolk, salt flavors,seasonings, ingredients,etc.pH: 4.0 – 4.5

Egg yolk proteins and phosphatides. Lecitin (O/W), cholesterine (W/O)

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 and surfactants, where permitted by local food law.

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HLB Scale1 20

http://www.snowdriftfarm.com/what_is_hlb.html

Griffin: HLB = 20 * Mh / M Mh, M the hydrophilic part and the whole molecule (Mh+Ml)

50 % Span 60 (HLB = 4.7) és 50 % Tween 60 (HLB = 14.9)?4.7 x 0.5 + 14.9 x 0.5 = 9.8

? which combination of emulsifiers is appropriate from Span 80 (HLB = 4.3) and Tween 80 (HLB = 15.0) for “required” HLB 12.0?(4.3*(1-x) + 15*x = 12; 28% & 72%)

Davies' method: HLB=7+ hydrophilic groups – lipophilic groups

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HLB (hydrophilic -lipophilic balance) values

The amphiphilic nature of many emulsifying agents (particularly non/ionic surfactant) can be expressed in terms of an empirical scale of so-called HLB

Applications Dispersibility in water

3-6 W/O emulsions Nil

7-9 wetting agents 3-6 poor

8-15 O/W emulsions 6-8 unstable milky dispersions

13-15 detergent 8-10 stable milky dispersions

15-18 solubiliser 10-13 Translucent dispersion/solution13- clear solution

Ratio of solubility in octanol and water, logKOW

HLB=7+ hydrophilic groups – lipophilic groups

SDS is an exemption with HLB 40

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

Nature of the emulsifying agent determine the type of emulsion

(antagonistic action)

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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 in drops, …

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Multiple emulsions

W/O/W double emulsion

O/W/Odouble emulsion

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

Particles as emulsion stabilizers

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 very old!!(Pickering stabilization)

bentonite clays tend to give O/W whereas carbon black tends to give W/O emulsions

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Multiple phase emulsions – drops in drops in drops

Drug delivery

an emulsification technique that encapsulates two different inner drops inside an oil drop using glasscapillary devices with a dual bore injection tube

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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 (antagonistic action).Add agent of opposite charge.

• Freeze-melt cycles.• Add electrolyte. Change the pH. Ion exchange• Raise temperature. HLB depends on the temperature. (Solubility, PIT)• Apply electric field.• Filter through fritted glass or fibers.• Centrifugation.

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Phase inversion temperature

1. As temperature is increased, ethoxylatedsurfactants become less water-soluble, because the hydrogen bonding between the oxygen of ethylene oxide and the hydrogen of water is inhibited. The molecules have more movement and cludinessresults.

2. inversion O/W- W/O and oil is separated out.

The oil-in-water emulsions measure just 100 – 300 nanometers, are of very low viscosity and can thus be applied by spraying.

SEM can provide a visual phase inversion: http://www.chemistrymag.org/cji/2001/03c058pe.htm