Foam, emulsion
Márta Berka
University of Debrecen
Dept of Colloid and Environmental
Chemistry
http://dragon.unideb.hu/~kolloid/
1
http://www.tutornext.com/classification-colloidal-solutions/7245
Type of sols
categorized by inner / outer phases
•
aerosols
L/G liquid in air:
fog, mists, spray
S/G solid aerosol,
solid in gas: smoke,
colloidal powder
Complex, smog
•
lyosols
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)
xerosols,
xerogels
G/S solid foam: polystyrene
foam, bread, cake, whipped
cream
L/S solid emulsion: opals,
pearls
S/S solid suspensions:
pigmented plastics
2
Aeroszol (L/G, S/G)
Atmospheric aerosols
enhanced aerosol concentrations cause the droplets in a cloud to be smaller and
more numerous within a cloud of fixed water amount.
Carbon Black
Aggregates
Primarily used as
reinforcing filler
• Tire
• Black pigment.
Elastomer composites,
• Plastics, Pipe, •
Printing Inks, Coatings
Carbon black in its nascent form is fluffy powder.
The particle size is roughly the size of virus.
Fumed (a continuous flame hydrolysis technique )
Silica (silicon dioxide) Aggregates
Thermal conductivity: 12 to 16 mW/m·K
Light transmission: 20 to 80% at 2 cm
Particle 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
3
and hydrophobic fumed silicas
Dispersion of gas, G/L; foams
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.
http://www.tcd.ie/Physics/Foams/index.php
4
Foam
Stabilizers are needed
Foam structure of a spherical
foam at 400X magnification
Foams can be made by
Mixing or agitation; examples are draught
beer, whipped cream and sea foam.
Evolution of dissolved gas; for example
canned beer, soft drinks, shaving foam and
hair mousse.
Bubbling gas through liquid.
/www.ctmw.com/articles/Rita/2.htm
Spherical bubbles
<70-75%
Polyhedral cells
Foam structure of a hexagonal
foam at 400X magnification
5
The Kelvin Problem - Filling Space with Bubbles , Kelvin's solution, the tetrakaidekahedron
Foam forming
2γ
ΔP =
r
p
pr
C
p
The pressure is at C > A > B places
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
The medium is the continuous phase!
porous filter minimum pressure,
p=2γ/r. At first the largest bubbles
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The Laplace pressure is low (see next slide), because of the negative
come out.
curvature, hence water will flow
Formation of bubbles
Polyhedral cells
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.
2γ
ΔP =
r
r<0
Cross section of a Plateau border and foam lamellae.
(The arrows show the direction of streaming.)
Interference
Soap Bubbles as Art
black
Different
colors
http://www.tcd.ie/Physics/Foams/duran.php
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.
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Stabilization of a foam film
Electrostatic stabilization of a foam
film
Each interface is electrically charged.
As the film thins, the repulsion increases.
Steric stab. Liquid crystals stabilize
foams
Good emulsifying are also good
foaming agent.
The factors which influence emulsion stability, against droplet coalescence
and foam stability against bubble collapse are similar
Antifoams
L
With an antifoam on one surface,
electrostatic stabilization is lost.
The Laplace pressure is low, because of the negative
curvature, hence water will flow, until they become
unstable.
(a) Antifoam drop. (b) Entering the
surface. (c ) Leading to rupture of the
film.
•Antifoams - added to existing foams, in
the form of small droplets, which spread
on the lamellae, thinning and breaking it.
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Foam Stability, Inhibition and Breaking
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.
•Foam inhibitors - added before foam forms, displace foaming
agents, 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.
Simethicone is an
oral anti-foaming agent
used to reduce bloating
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Applications
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.
Marshmellow - foam formed from egg white, gelatin, and sugar.
Ice cream - refrigerated and aerated at the same time. Ice crystals and fat crystals form the
matrix.
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 carbon
dioxide. Mostly stabilized by proteins. Protein-polysaccharide complexes are especially
stabilizing.
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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 lighter
than 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, terminology
The emulsion is a dispersed system in which the
phases are immiscible or partially miscible.
Droplet size: 0.1-10 μm
in miniemulsion
Polyhedral cells
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
The medium is the continuous phase!
The globules of the dispersed liquid are generally between 0.1 micron and 10 micron, and so are 16
larger than the particles found in sols.
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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Terminology
Macroemulsions – At least one immiscible liquid dispersed in
another as drops whose diameters generally exceed 10 μm.
The stability is improved by the addition of surfactants and/or
finely divided solids. Considered only kinetically stable.
Becher, P. Emulsions, theory
and practice, 3rd
ed.; Oxford University Press:
New York; 2001.
Miniemulsions – An emulsion with droplets between 0.1
and 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).
• 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 emulsions
Emulsions 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 thermodynamically
unstable
ΔG = γΔ A < 0
ΔA < 0
but
emulsifiers
ΔG = γΔ A + work of desorption > 0
If the work of desorption of emulsifier is high, the coalescence is prevented ,
and the emulsions are thermodynamically stable.
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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, Sonolator
In 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. Wetting Agents: Anionic, Cationic, Nonionic
5.
Finely divided solids: Bentonite, Magnesium Hydroxide, Aluminum Hydroxide o/w
emulsion; carbon black w/o
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Emulsion stability
Factors 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
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anything from seconds to years, depending the emulsion formulation and manufacturing condition.
Emulsion Inversion
As the concentration increases (A)
the droplets get closer, and the
agitation pinches them off into
smaller, opposite type of emulsion
(B).
water
making milk into butter
w
• 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 ...
oil
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Typical food emulsions
Food
Milk, cream
Emulsion 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,
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 –20 oC.
W/O
Buttermilk: milk
proteins, phospholipids,
salts.
Volume fraction: 16%
O/W
Vegetable oils and fats.
Droplet size: 1 – 5 μm.
Volume fraction: 10 –
30%
Butterfat triglycerides,
partially crystallized
and liquid oils; genuine
milk fat globules are
also present.
Aqueous solution of
proteins
(casein), sucrose, salts,
hydrocolloids.
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
Imitation cream
(to be aerated)
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 emulsions
Food
Emulsion type
Dispersed phase
Continuous phase
Stabilization factors, etc
Coffee
whiteners
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, and
hydrocolloids.
Blends of nonionic and
anionic surfactants together
with adsorbed
proteins.
Margarine and related
Products (low calorie
spread)
W/O
Water phase may contain
cultured
milk, salts, flavors.
Droplet size: 1 – 20 μm
Volume fraction: 16 – 50 %
Edible fats and oils, partially
hydrogenated, of animal or
vegetable origin. Colors,
flavor, vitamins.
The dispersed water droplets
are fixed
in a semisolid matrix of fat
crystals;
surfactants added to reduce
surface
tension/promote
emulsification
during 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 egg
yolk, sugar, salt, starch,
flavors, seasonings,
hydrocolloids, and
acidifying ingredients.
pH: 3.5 – 4.0
Egg yolk proteins and
phosphatides
combined with hydrocolloids
and surfactants, where
permitted by local food law.
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1
HLB Scale
Griffin: HLB = 20 * Mh / M
Davies' method:
20
Mh, M the hydrophilic part and the whole
molecule (Mh+Ml)
HLB=7+ hydrophilic groups – lipophilic groups
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%)
http://www.snowdriftfarm.com/what_is_hlb.html
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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
HLB=7+ hydrophilic groups – lipophilic groups
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/solution
13- clear solution
SDS is an exemption with HLB 40
Ratio of solubility in octanol and water, logKOW
27
Variation of type and amount of residual emulsion with HLB
number of emulsifier.
(antagonistic action)
Nature of the emulsifying agent determine the type of emulsion
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 in drops, …
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Multiple emulsions
W/O/W
double emulsion
O/W/O
double 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
an emulsification technique that encapsulates two different inner drops inside an oil drop using glass
capillary devices with a dual bore injection tube
Drug delivery
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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, ethoxylated
surfactants 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 cludiness
results.
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
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