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

István Bányai and Levente Novák

Making emulsions

● Mode of action of making emulsions– Dispersion of one of the liquid phases in the other with cavitation, turbu-

lence, shear, etc. → energy is needed for emulsification● Emulsification proceeds in two steps

– Mixing– Stabilization

● Main methods– Shaking– Mechanical shear– Sonication– Condensation methods: solubilization of an internal phase into micelles– Electric emulsification– Phase inversion (“ouzo effect”)

Emulsifiers

● Surface active materials– Carbohydrates: acacia gum (gum arabic), tragacanth, agar, pectin → for o/w

emulsions.– Proteins: gelatin, egg yolk, casein → for o/w emulsions.– High molecular weight molecules: stearyl alcohol, cetyl alcohol, glyceryl

monostearate → for o/w emulsions, derivatives of cellulose, Na car-boxymethyl cellulose, cholesterol → for w/o emulsion

– Wetting agents (surfactants): anionic, cationic, zwitterionic, nonionic● Finely divided solids (Pickering stabilization)– Bentonite, clays– Silica (fumed)– Metal hydroxides (magnesium hydroxide, aluminum hydroxide) → for o/w

emulsions– Carbon black → for w/o emulsions

Emulsion stability

● The term “emulsion stability” can be used with reference to three different phenomena– creaming (or sedimentation)– flocculation– 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 formula-tion and manufacturing condition.

Emulsion stabilityFactors favoring emulsion stability

1. Low interfacial tension.

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

3. Steric stabilization.

4. Mechanically strong interfacial film (proteins, surfactants, mixed emulsi-fiers are common). Temperature is important.

5. Relative small volume of the dis-persed phase.

6. Narrow size distribution of the droplets (reduced Ostwald ripening).

7. High viscosity (simply retards the rates of creaming, coalescence, etc.).

8. Reduced gravitational separation: small density difference.

9. Reduced droplet size.

Emulsion inversion

● Emulsion inversion is the change of a given emul-sion type to an other type (e.g. o/w → w/o)

● Generally it proceeds by the action of– temperature– concentration– change of the composition of the phase(s) → e.g dilu-

tion by a solvent of different polarity

Increasing the concentration of droplets (A) make them get closer until they “pinch off” into smaller, opposite type of emulsion (B).

Making of butter

● Cow's milk is a fairly dilute, not very stable O/W emulsion, with about 4% fat.● Creaming produces a concentrated, not very stable O/W emulsion, about 36% fat.● Gentle agitation, particularly at 10–15 °C, inverts it to make a W/O emulsion about

85% fat.● Drainage, addition of salt, then thorough mixing produces

● Butter (solid phase)● Buttermilk (liquid phase)

Emulsion inversion

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 are more mobile and cloudiness results.

2. Inversion o/w → w/o, oil is sepa-rated out.

The oil-in-water emulsion droplets measure just 100–300 nm, in consequence they are of very low viscosity and can be applied by spraying.

Scanning electron microscope can provide a visual representation of the phase inversion: http://www.chemistrymag.org/cji/2001/03c058pe.htm

Hydrophilic-lipophilic balance (HLB)

● A practical (arbitrary) scale defining the relative balance between hydrophilic and lipophilic character of a surfactant

● Used mainly for non-ionic detergents● Two definitions in use

– Griffin's method: HLB=20×Mh/M (where Mh is the molar mass of the hydrophilic part of the molecule and M the molar mass of the whole molecule) → only valid for non-ionic surfactants

– Davies' method: HLB=7+fh×nh-fl×nl (nh: number of hydrophilic groups, nl: number of lipophilic groups, fh: weighing factor for hydrophilic groups, fl: weighing factor for lipophilic groups)

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

Applications by HLB Dispersibility in water by HLB

3-6 For W/O emulsions <3 None

7-9 wetting agents 3-6 Poor

8-15 For O/W emulsions 6-8 Unstable milky dispersions

13-15 Detergents 8-10 Stable milky dispersions

15-18 Solubilizers 10-13 Translucent dispersion/solution

>13 Clear solution

HLB = 7 + (number of hydrophilic groups) – (number of lipophilic groups)

Ionic detergents may have much higher HLB values:SDS has a HLB of 40

HLB values: applications

Variation of the type and amount of residual emulsion with the HLB value of the emulsifier

The nature of the emulsifying agent determines the type of emulsion

(antagonistic action)

Physical properties of emulsions

● Identification of “internal” and “external” phases (W/O or O/W)

● Droplet size and size distributions – generally greater than 1 µm

● Concentration of the dispersed phase – often quite high. The viscos-

ity, conductivity, etc, of emulsions are much different than for the con-

tinuous phase.

● Rheology – complex combinations of viscous, elastic and viscoelastic

properties.

● Electrical properties – useful to characterize the structure.

● Multiple phase emulsions – drops in drops in drops in drops, …

W/O/W double emulsion

O/W/O double emulsion

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

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 (several millions of years)!

(Pickering stabilization)

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

Emulsification by particles (Pickering emulsions)

http://www.rsc.org/delivery/_ArticleLinking/DisplayArticleForFree.cfm?doi=b501972a&JournalCode=SM

Multiple phase emulsions

● “Drops in drops”● More and more studied and used● Great potential in drug-delivery

Methods of breaking emulsions

● First, determine the 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-thaw cycles.● Add electrolyte. Change the pH. Ion exchange.● Raise temperature (HLB depends on the temperature)● Apply electric field.● Filter through fritted glass or fibers.● Centrifugation.

Type of colloidson the basis of structure (appearance)

Porodin

colloids

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

ColloidalDispersions sols

Macromolecular solutions

Association Colloids

Colloidal solutions

(porous)Reticular Spongoid

corpuscular fibrillar lamellardiszpersion macromolecular association lyophobic lyophilic lyophilic(IUPAC proposal)

categorized by inner / outer phases

Types of sols (incoherent)

• aerosols • lyosols xerosols, xerogels

L/G liquid in air: fog, mists, sprayS/G solid aerosol, solid in gas: smoke, colloidal powderComplex, 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

• Sol stability: property of a lyophobic sol to remain unaggregated → only kinetic (see DLVO theory, steric stabilization), lyophobic sols are thermodynamically unstable

• Sol: incoherent, dispersion colloidal system

• Xerosol: solidified sol, no aggregation, no skeleton structure → not a gel!

• Gel: coherent colloidal system, has a skeleton structure

• Cream: concentrated emulsion (L/L), o/w type

• Grease: high viscosity gel, with shear-thinning properties

Preparation of sols

Importance of monodispersity

It is important to make sols of well controlled particle size and size distribution for most uses.

• Top to bottom technique: it is almost impossible to achieve this by dispersion.

• Bottom to top technique: precipitation with chemical synthesis works often

AgI sol (AgNO3+ KI → KNO3 + AgI)

Gold sol (H[AuCl4] + Na3-citrate → ruby-colored Au sol)

Sulfur sol (Na2S2O3 + 2 HCl → 2 NaCl + S + SO2 + H2O)

Iron(III) hydroxide sol (FeCl3 in water → Fe(OH)3 with hydrolysis)

LaMer diagram (1950): precipitation

Example: ceria nanoparticles

LaMer diagram (1950): precipitation

Yugang Sun, Chem. Soc. Rev. 42: 2497—2511 (2013)

Phenomena after preparation (changes in size)

Ageing of colloids; Ostwald-ripening (moving slowly to equilibrium)

lyophobic colloid systems are thermodynamically unstable → ageing (spontaneous slow, irreversible change) → coarsening

ln(pr

p)=

2 γV m

RT rln(

Lr

L)=

2γV m

RT r

Kelvin equation Ostwald equation

pr: vapor pressure over surface of radius r (N) Lr: pr, cr, or μr in the droplet of radius rp: saturation vapor pressure in gas phase (N) L: p, c, or μ in the mediumγ: surface tension (N/m)r: radius of curvature (m)VM: molar volume (m3/mol)

Gels

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

components forms a skeleton (network made with primary or secondary bonds) and contains a fluid dispersion medium

– State of transition between liquids (vapor pressure, conductivity) and solids (shape)

• Types– Porodin gels: consist of a skeleton of particles – Reticular gels: skeleton of fibers, coarse fibers, bunch of

fibers – Spongoid gels: skeleton of lamellae or films,

Definition by IUPAC (reading)• 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.

Typical gels

a) reversible polymer gelb) reversible porodin gelc) irreversible polymer geld) irreversible solid-gas xerogel

a) Ionic, b) hydrophobic, c) H-bridge, d) van der Waals, e) hairy micelles, f-g) coordination bond

pregel

gelapolarsolvent

Porodin gel (e.g. silica)

Silica gel (SiO2 · n H2O)

The size of silica gel particles is determined by the pH.

In acidic medium the hydrolysis is fasterThe condensation is slow: small particles form.

In alkaline medium: bigger particles, loose structure

TEM pictures

Porodin gel (e.g. clay)Example: (Na,Ca)0.33(Al,Mg)2Si4O10(OH)2·(H2O)n (montmorillonite)

Drilling mud:

1. Viscosity is high: takes up solids2. Cools and lubricates3. Increases pressure to keep away

liquids (density)4. Cover the pores of the wall5. Keeps the stability of the wall

Takes 4-5 times its weight of waterComposition: water + clay + baryte (for its weight) + xanthan or carboxymethyl cellulose (for their viscosity)

30

Sol-gel technology

Aerogel („frozen smoke”)

Aerogels are the lightest solid materials. They are very good insulators. Silica based aerogel was the first to make, but today Al, Cr, Zn or carbon are also used for synthesizing aerogels.

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

Structure of aerogel

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

Preparation of silica aerogels

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

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

Exchange the liquid to gas!

Si or Al are biocompatible

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.

Lyogels (solvent in the skeleton)

Polymer gels (e.g. “intelligent” gels) → reversible transformations(as a function of T, pH, salt content, etc.)

Disposable diapers

Example:

drug delivery

gel

solvent

syneresis swelling

Hydrogels

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

Poly (sodium propenoate): poly acrylic acid.

The monomer:

Randomly coiled molecules, swelling in water

Examples of hydrogels: gelled foods, fruit jellies, etc.

By addition of salt water flows out.

Disposable diapers

Solidification of liquid waste

• Easier to handle• Storage• Destruction is easier

Intelligent gels

PDMS: poly(dimethyl-siloxane) elastomers

Magnetic nanoparticles

Polyaspartic acid gel: artifical muscle

Non-ionized in acidic medium: shrinks

Temperature-sensitive gels (e.g. NIPA)

N-isopropylacrylamide gel: transition at 34 oC

PEM (proton exchange mebrane)

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Xerogel coating

Xerogel coating: applications,modern artificial opal

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

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

● Light interference (e.g. 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).

http://www.molecularexpressions.com/primer/lightandcolor/interferenceintro.html