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Sols (liosols S/L), xerosols (*/S solid medium), gels

Bányai István

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

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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 stabilization

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 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

the agitation 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 emulsions (reading)

Food 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 emulsions (reading) 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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HLB Scale 1 2

0

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

Test examples:

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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/solution 13- clear solution Ratio of solubility in octanol and water,

logKOW

HLB=7+ hydrophilic groups – lipophilic groups

SDS is an exeption 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/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

Drug delivery

an emulsification technique that encapsulates two different inner drops inside an oil drop

using glass capillary 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) • 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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Type of colloids on the basis of structure (appearance)

Porodin

colloids

Incoherent (fluid-like) Coherent (solid-like) gel

Colloidal Dispersions sols

Macromol. solutions

Association Colloids

Colloidal solutions

(porous) Reticular Spongoid

corpuscular fibrillar lamellar diszpersion macromolecular association

liofób liofil liofil

(IUPAC proposal)

categorized by inner / outer phases

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Type of sols (incoherent)

• 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 L/S solid emulsion: opals, pearls S/S solid suspensions: pigmented plastics

Definitions

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• Sol stability: kinetic (DLVO theory, steric stabilization)

• Sol: incoherent or coherent

• Xerosol: solidified sol, no aggregation no skeleton structure

• Gel: coherent, skeleton structure

• Cream: concentrated emulsion (L/L),

• Grease: fat

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Preparation of sols

Importance of monodispersity:

It is a basic duty to make well controlled size ans size distribution.

Top to bottom technique: With dispergation it is almost impossible.

Precipitation (bottom to top)

AgI sol (AgNO3+ KI), Au (gold) sol (HAuCl4 boiling+ Na-citrate rubin colored sol) sulphur sol (Na2S2O3 and HCl ) iron-hydroxid sol , FeCl3 hydrolysis

Rate of nucleation and growing nuclei are important

Processes after preparation (changes in size)

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Ageing of colloids ; Ostwald-ripening Moving slowly to equilibrium

LaMer-diagram (1950): precipitation

20 Example: ceria nanoparticles

2007.03.20 14.előadás

Gels

• Definition – Coherent colloid system, in which one of the

components forms a skeleton (network) and the medium is around it.

– Transition between liquids (vapor pressure, cinductivity) and solids (shape)

• Types – porodin gels: consist of particles and – Reticular gels: fibers, coarse fibers, bunch of fibers

by means of primary or secondary bonds. – spongoid gels: lamellas, films,

Definition by IUPAC

• Gel:Nonfluid colloidal network or polymer network that is expanded throughout its whole volume by a fluid.[3]

• Note 1: A gel has a finite, usually rather small, yield stress.

• Note 2: A gel can contain:

• (i) a covalent polymer network, e.g., a network formed by crosslinking polymer chains or by nonlinear polymerization;

• (ii) a polymer network formed through the physical aggregation of polymer chains, caused by hydrogen bonds, crystallization, helix formation, complexation, etc., that results in regions of local order acting as the network junction points. The resulting swollen network may be termed a “thermoreversible gel” if the regions of local order are thermally reversible;

• (iii) a polymer network formed through glassy junction points, e.g., one based on block copolymers. If the junction points are thermally reversible glassy domains, the resulting swollen network may also be termed a thermoreversible gel;

• (iv) lamellar structures including mesophases, e.g., soap gels, phospholipids, and clays;

• (v) particulate disordered structures, e.g., a flocculent precipitate usually consisting of particles with large geometrical anisotropy, such as in V2O5 gels and globular or fibrillar protein gels. (above) rather than of the structural characteristics that describe a gel.

• Hydrogel: Gel in which the swelling agent is water.

• Note 1: The network component of a hydrogel is usually a polymer network.

• Note 2: A hydrogel in which the network component is a colloidal network may be referred to as an aquagel.

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Typical gels

a/ reversible polymer gel b/ reversible porodin gel c/ irreversible polymer gel d/ irreversible solid-gas xerogel

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a) Ionic b) hydrophobic c) H-bridge d) van der Waals b) Hairy-micelles f-6 coordination bond

2007.03.20 14.előadás

Porodin gel (silica)

• Silica gel (SiO2x nH2O)

2007.03.20 14.előadás

Porodin gel (clay)

• Example: (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·(H2O)n

(montmorillonit)

silt: 1. viscosity: taking up solids 2. Cooling and lubrication 3. Pressure to keep away

liquids(density) 4. Cover the wholes of the wall 5. Keeps the stability of the wall

composition: clay+ barit (weight)+xantan carboxi-methyl cellulose (viscosity)

4-5 times water to weight

2007.03.20 14.előadás

Drilling

2007.03.20 14.előadás

Drilling 2

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Sol-gel technology

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Aerogel „frozen smoke”

Az aerogels are the lightest solid material. Good insulators. Silica based aerogel was the first, but today: Al, Cr, Zn or carbon are used

http://www.youtube.com/watch?v=mAJWyRIDDVQ

http://www.youtube.com/watch?v=HoCAxS4vqwQ

Structure of aerogel

http://stardust.jpl.nasa.gov/photo/aerogel.html

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Aerogel

http://www.resonancepub.com/aerogel.htm

http://en.wikipedia.org/wiki/Aerogel

State-of-the-Art Manufacturing Technology

Exhcange the liquid to gas!

Si or Al biocompatible

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In addition, there is no surface tension in a supercritical fluid, as there is no liquid/gas phase boundary. By changing the pressure and temperature of the fluid, the properties can be “tuned” to be more liquid- or more gas-like.

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Sythesis of silica AEROGELS

First hydrogel is made.

Two basic processes:

Hydrolisys: Si(OC2H5)4(al) + H2O Si(OC2H5)3(OH)(al) + C2H5OH

⌠A Si(OC2H5)4(al) tetra-aetoxy-silane in alcohol.⌡

Condensation: ≡Si–OH(al) + HO–Si≡(al) ≡Si–O–Si≡(al) + H2O

≡Si–OR(al) + HO–Si≡(al) ≡Si–O–Si≡(al) + ROH

In this phase Si–O–Si bonds form and nanoparticles

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The size is determined by pH. In acid the hydrolisis is faster The condensation is slow: small particles form. In alcalic medium: opposite: bigger particles, loose structure

TEM pictures

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Lyogels (solvent in the skeleton)

Polimer gels (“intelligent” gels), reversible transformations (T, pH, salt content, etc.)

Disposable diapers

drug delivery

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Hydrogels

http://www.gcsescience.com/o69.htm

(poly (sodium propeonate)) poly acrylic acids. monomer:

randomly coiled molecules, in water swelling happens

Examples of hydrogels: foods, fruit jellys,

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By addition of salt water flows out.

Disposable diapers

Solidification of waste liquid

• Easier to handle

• storage

• Destruction is easier

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Intelligent gels

PDMS: poly(dimethyl-siloxane)

elastomers

Magnetic nano particles

Poli-aspartic acid gel: artifical muscle

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In acidic medium no ions incorporated in it.

Temperature (NIPA)

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N-isopropy.acrylamide gel: 34 oC

PEM (proton exchange mebrán)

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Xerogel coating

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• Interference (anti-reflection coatings for the areas of UV, VIS and NIR.

• Applications: From Architectural

Application to UV protection

• 1992 Prinz Optics (Sol-Gel Dip

Coating Process).

xerogel coating: applications, modern opale

http://www.prinzoptics.de/en/home/index.php

http://www.variotrans-glas.de/htdocs_en/home/index.html

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http://www.molecularexpressions.com/primer/lightan

dcolor/interferenceintro.html