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Sols, foams, emulsions

Novák Levente & István Bányai

University of Debrecen Dept of Colloid and Environmental

Chemistry

http://kolloid.unideb.hu/

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Type of colloids on the basis of structure

Porodin

colloids

Incoherent (fluid) Coherent, gel

Colloidal

Dispersions

sols

Macromol.

solutions Association

Colloids

Colloidal solutions

(porous)

Reticular Spongoid

corpuscular fibrillar lamellar diszpersion macromolecular association

liofób liofil liofil

(IUPAC ajánlás)

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Type of sols

• aerosols • lyosols xerosols,

xerogels

L/G liquid in air:

fog, mists, spray

S/G solid aerosol,

solid in gas: smoke,

colloidal powder

Complex, smog

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)

G/S solid foam: polystyrene

foam, bread, cake, whipped

cream

L/S solid emulsion: opals,

pearls

S/S solid suspensions:

pigmented plastics

categorized by inner / outer phases

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

Aggregates

Primarily used as

reinforcing filler

• Tire

• Black pigment.

Elastomer composites,

• Plastics, Pipe, •

Printing Inks, Coatings

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

and hydrophobic fumed silicas

Atmospheric aerosols

Carbon black in its nascent form is fluffy powder.

The particle size is roughly the size of virus.

Preparation of aerosols

• Dispersion

– Decreasing presure: dispersin liquids

– Dispersing solids: cutting

– Redispersing settled solid or liquid

• Condensation

– Vapor condensation

– Dust aggregation

• Chemical reaction

– Gas phase reactions

– Reactions in liquid or solid with large heat development

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Stability of aerosols

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Stability of aerosols

The stability of aerosols are low: always in the fast coagulation limit

concentration

What determines the stability

low concentration – high kinetic stability

Dynamics of system

Brown motion causes

perikinetic coagulation

Fluid motion and sedimentation

causes orthokinetic coagulation

The combined effect of

diffusion and sedimentation

Optimum size of particle

stays the longest time

in the dispersion medium

rate of diffusion rate of sedimentation

Size of particle

combination

G/L sols

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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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Foam

Stabilizers 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

Spherical bubbles

<70-75%

Polyhedral cells 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.

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=2g/r. At first the largest bubbles

come out.

The medium is the continuous phase!

pr p p 2

Pr

g

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

2P

r

g

Cross section of a Plateau border and foam lamellae.

(The arrows show the direction of streaming.)

r<0

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Interference Soap 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 foams

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Static stabilization of foams

Electrostatic stabilization by ionic surfactants

steric stabilization by non-ionic surfactants

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 coalescence

and foam stability against bubble collapse are similar

Dynamic stabilizaion

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Dynamic stabilization of foams

(liquid flow)

Marangoni-effect: moderates the thinning of the distance by drainige. Stabilizes the foam.

Marangoni - effect

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When a surfactant-stabilized film undergoes sudden expansion, then immediately the expanded portion of the film must have a lower degree of surfactant adsorption than unexpanded portions because the surface area has increased (see Figure 3.24). This causes an increased local surface tension which provides increased resistance to further expansions. If unchecked, further thinning would eventually lead to film rupture. However, the local rise in surface tension produces an immediate contraction of the surface. Since the surface is coupled by viscous forces to the underlying liquid layers, the contraction of the surface induces liquid flow from the low-tension region to the high-tension region. The transport of bulk liquid due to surface tension gradients is termed the Marangoni effect; it re-thickens the thin films and provides a resisting force to film thinning

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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

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Foam Stability, Inhibition and Breaking

•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.

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 an

oral anti-foaming agent

used to reduce bloating

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Applications

Marshmallow - 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. . 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.

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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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 mm

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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Terminology Macroemulsions – At least one immiscible liquid dispersed in

another as drops whose diameters generally exceed 10 mm.

The stability is improved by the addition of surfactants and/or

finely divided solids. Considered only kinetically stable.

Miniemulsions – An emulsion with droplets between 0.1

and 10 mm, 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, 3rd

ed.; 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 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

0G A g

0G A work of desorption g

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

and the emulsions are thermodynamically stable.

emulsifiers

but

0A

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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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Emulsification

non-stable oil drop in water

Solid particle

stabilized oil drop in water

stabilized oil drop in water