CHEM-E2150 Biobased Systems - Aalto

4. Emulsions and foams

Juan José Valle-DelgadoJanuary 2019

CHEM-E2150Interfacial Phenomena in

Biobased Systems

Learning outcomesAt the end of this teaching session you should be able to:

Explain what emulsions, foams and emulsifiers are.

Distinguish between O/W and W/O emulsions.

Predict how surfactants adsorb at emulsions’ and at foams’ interfaces.

Interpret SWO diagrams.

Know what factors affect emulsion characteristics.

Understand different mechanisms of emulsion stabilization by surfactants.

Explain different mechanisms that affect foam stability

Contents Emulsions

Foams

Summary

- O/W and W/O emulsions

- Preparation and factors affecting emulsion characteristics- Emulsifiers and emulsion stability

- Microemulsions- SWO diagrams

- Foam formation

- Solid foams- Foam stability- Bubble arrangement in foams

Emulsions

MilkMayonnaise

Butter

PaintsCosmetics

Pharmaceuticals

What do they have in common?

Asphalts

They are emulsions.

EmulsionsAn emulsion is a mixture

of two or more liquids that are normally immiscible.

Applications: foods (milk, mayonnaise,margarine, cream, sauces…), paints,cosmetics, pharmaceuticals, asphaltproducts…

An emulsion consists of liquid drops (the disperse or internal

phase) dispersed in another liquid (the continuous or external phase).

EmulsionsGenerally in emulsions one liquid is water and the

other is a water-insoluble organic liquid (oil).

Water

Oil

Oil

Water

Oil in water (O/W) emulsion Water in oil (W/O) emulsion

Normal or simple emulsions

Emulsions

Biemulsion Oil 1 and Oil 2 in water

O1/W/O2 emulsion

Multiple emulsion

Water

Oil 1

Oil 2

Oil 2Water

Oil 1

Other types of emulsions:

EmulsionsEmulsions are thermodynamically unstable and

quickly separate into different phases.

The surface tension at the oil-water interface

is usually high.

The system evolves to minimize the interfacial area.

Coalescence of drops.

A third component is needed to stabilize the emulsion: the emulsifier (= emulgent).

EmulsionsEmulsifiers are molecules that adsorb at the oil-water interface and stabilize emulsions.

Emulsifiers are usually surfactants.

Pickering emulsions: emulsions stabilized by particles

Other amphiphilic molecules (proteins, polymers) or particles can be emulsifiers too.

O/W emulsion stabilized by lignin nanoparticles

Ago et al., ACS Appl. Mater. Interfaces 8, 23302−23310 (2016)

O/W emulsion stabilized by CTAB-silica nanoparticles

Destribats et al., Langmuir 26, 1734−1742 (2010)

100 µm 20 µm

Emulsions

Water

Oil

Oil

Water

O/W emulsion W/O emulsion

How do surfactants adsorb at the oil-water interface in O/W and W/O emulsions?

Water

Oil

Oil

Water

Stability of emulsions

Emulsions

Emulsions

Stability mechanisms: Reduction of interfacial tension. Electrostatic interactions. Steric interactions.

Emulsion stability

Mechanical dispersion.

Agitation of the mixture oil, water and emulsifier.

Condensation methods.

Addition of an oil-in-ethanolsolution to water in the

presence of an emulsifier.wateremulsifier

oilethanol

The final characteristics of an emulsion depend on several factors: oil/water volume ratio, oil nature, type of emulsifier, presence of

electrolytes, temperature, method of preparation...

Emulsion preparation

Emulsions

Surfactants act as dispersants for oil spills, facilitating the

biodegradation by microorganisms.

Example of applicationsEmulsions

O/W emulsion

Emulsion inversion:change from one emulsion

type to the other.

O/W and W/O emulsions have different conductivities

and viscosities.

conductivity O/W > conductivity W/O

viscosity O/W < viscosity W/O

EmulsionsO/W and W/O emulsions

Example: cream butter

stirring

Surfactant emulsifier O/W or W/O emulsion?

Winsor’s ratio:

ELO

EHW

(L)

(H)

R < 1 O/W emulsionWater

Oil

R > 1 W/O emulsion

(Winsor I emulsion)

(Winsor II emulsion)


ELO = affinity, interaction energy L-OEHW = affinity, interaction energy H-W

Surfactant emulsifier O/W or W/O emulsion?

Hydrophilic-Lipophilic Balance (HLB)

Griffin: HLB = 20 MH/M

8 < HLB < 18 O/W emulsion3.5 < HLB < 6 W/O emulsion

Davies: HLB = 7 + m HH - n HL

HH , HL

(L)

(H)


MH = molecular mass of hydrophilic partM = molecular mass of whole molecule

m = number of hydrophilic groupsn = number of lipophilic groups

Griffin, J. Soc. Cosmet. Chem. 5, 249-256 (1954)

(non-ionic surfactants)

Davies, Proc. Int. Congress Surf. Activity, 426-438 (1957)

Diameter of drops < 60 nm

Microemulsions are clear. Normal emulsions are white or opaque because the light is scattered by bigger drops.

Unlike normal emulsions, microemulsions are thermodynamically stable. They are formed spontaneously after mixing the components (oil, water,

surfactant, cosurfactant), without need of strong shake.

EmulsionsMicroemulsions

SWO Diagrams

Water Oil

Surfactant

0.5

0.50.5

P XSXS

XW

XW

XO

XO

XW + XO + XS = 1 (or 100%)

Surfactant-Water-Oildiagram X = mole fraction

(or mass percentage)

SWO Diagrams

Different systems have different SWO diagrams. The SWO diagram of a given system depends on the temperature and salt concentration.

Lawrence & Rees, Adv. Drug Deliv. Rev. 64, 175-193 (2012)

(1 phase)

(2 phases)

SWO Diagrams

OW+S+O

(W/O emulsion)

O

W W

O+S+W(O/W emulsion)

W+S+O(W/O or O/W

emulsion)

W+S+O(W/O or O/W

emulsion)

2 phases 2 phases 3 phases 1 phase

Winsor type I Winsor type II Winsor type III Winsor type IV

Different emulsion systems:

SWO Diagrams

3 phases

SWO Diagrams

Inverse Micelle

Foams

FoamsDetergents, soaps, shaving cream, tooth paste,foods (ice cream, beer, capuccino, meringue…),cosmetics, firefighting foams, mineral flotation…

Foams

A foam is a coarse dispersion of gas in a liquid where the volume fraction

of the gas is larger.

Liquid

Gas

Gas

Liquid

Foam

A foam is a random package of bubbles in a relatively small amount of liquid.

FoamsFoams are thermodynamically unstable.

Foams are not formed in pure liquids.

The surface tension at the gas-liquid (water)

interface is usually high.

The system evolves to minimize the interfacial area.

Coalescenceof bubbles

+Coalescence

Rupture of bubbles at the

interface

+

FoamsFoams are stabilized by molecules that

adsorb at the gas-liquid interface.

Foams can be stabilized by surfactants, proteins or other amphiphilic molecules.

Soap film Different surfactants will produce different amounts of foam under identical conditions.

Shake, stir, agitation.

Injection of gas, blow bubbles.

Mineral flotation

FoamsFoam formation

Release or production of gas.

Turbulent mixing of thin liquid jet with gas.

FoamsFoam formation

gas phase

lamella

Plateauborder

FoamsBubble arrangement

Low gas/liquid ratio High gas/liquid ratio

How space can be partitioned into cells of equal volume with the leastsurface area between them? What is the most efficient bubble foam?

Lord Kelvin (1887): Kelvin structure

Bitruncated cubic honeycomb, formed by truncated octahedrons (tetradecahedrons

with 6 square faces and 8 hexagonal faces).

D. Weaire, R. Phelan (1993): Weaire-Phelan structure

Arrangement of two kinds of cells, with equal volume though. One cell is an irregular dodecahedron with

pentagonal faces. The second cell is a tetradecahedronwith two hexagonal and twelve pentagonal faces.


Plateau border:- Scalloped-triangular channel where 3 films meet (at an angle of 120º).- The edge shared by 3 neighbouring bubbles.

Vertex:- Region where 4 Plateau borders meet (at an angle of 109.5º).- The point shared by 4 neighbouring bubbles.

Plateau’s laws


Configurations that do not fulfill Plateau's laws are unstable, and the bubbles will quickly tend to rearrange themselves to conform to those laws.

Foam stabilityDifferent processes compete to stabilize or destabilize a liquid foam:

Drainage: liquid tend to flow from the lamellae into the Plateau borderspromoting film drainage, thinning the soap films, and reducing thestability of the foam.

Laplace equation:

Colours due to optical interference, which depends on

soap film thickness. Different colours mean different

thicknesses because of drainage.


Coarsening: gas difusses between bubbles (some grow while othersshrink and disappear). The average bubble size increases in time.

Film rupture: if a foam film gets too thin and weak, it will rupture. Eventually, the foam will collapse and vanish.

Anti-foaming agents: they can displace surfactants from bubblesurfaces, thinning and breaking foam films.


Hydration repulsion: repulsions due to overlap of hydration layers onbubble surfaces (surfactant head groups) will oppose film thinning.

Steric repulsions: repulsions due tosteric hindrance between bubbles(nonionic surfactants) will opposefilm thinning.

Electrostatic repulsions: repulsions dueto surface charge on bubbles (ionicsurfactants) will oppose film thinning.


Gibbs-Marangoni effect: changes in surface concentration of surfactant due to film perturbation will provoke differences in surface tension that will oppose film thinning.

Gibbs-Marangoni effect attenuates perturbation of local film thickness, moderates water drainage, and increases foam stability.

More general definition: foam is a substance that is formed by trapping many gas bubbles in a liquid or a solid.

Liquid foams

Solid foams

Sponges, bakery products, materials used in construction or insulation…

(presented before)

(bubbles pores)

Solid foams

Solid foamsThey can be classified according to the type of porosity:

Open-cell foams

Closed-cell foams

The pores are interconnected.

The pores are not interconnected.

Summary Emulsions are mixture of two or more liquids (oil-water) that are normally immiscible. Emulsifiers are molecules that adsorb

at the oil-water interface and stabilize emulsions.

Surfactants can be used as emulsifiers. They stabilize emulsions by reducing the oil-water surface tension. Electrostatic and/or steric

repulsions between surfactants can also stabilize emulsions.

The final characteristics of an emulsion depend on different factors: oil/water volume ratio, oil nature, type of emulsifier, presence of

electrolytes, temperature, method of preparation.

SWO diagrams give information about the different phase states of surfactant-water-oil mixtures at different proportion of their components.

Depending on the preparation conditions, O/W or W/O emulsions can be obtained. The type of surfactant used as emulsifier plays an important role.

Summary Foams are coarse dispersions of gas in a liquid where the volume

fraction of the gas is larger. Surfactants can stabilize foams.

There are different methods to produce foams: agitation, injection of gas, release of gas dissolved in a liquid, turbulent mixing of liquid and gas.

Spherical gas bubbles can eventually adopt polyhedral shapes in a foam, where Plateau borders, vertexes and lamellae can be distinguished.

Some porous materials can be considered as solid foams.

Different mechanisms compete to stabilize or destabilize foams: drainage, coarsening, film rupture, anti-foaming agents, electrostatic

repulsions, steric repulsions, hydration repulsions, Gibbs-Marangoni effect.

Additional reading

https://ebookcentral.proquest.com/lib/aalto-ebooks/reader.action?ppg=19&docID=1791862&tm=1534860301784

B. Kronberg, K. Holmberg, B. Lindman. Surface Chemistry of Surfactants and Polymers. John Wiley & Sons.

Feedback

Surfactant has been added to water.Draw the distribution of surfactant

molecules in the system.Water

Air

• What was especially interesting?

• What was especially uninteresting?

• Any suggestions for improvements?

• Is there anything that you did not understand?

• Final exercise:

Oil

Documents

CHEM-E2150 Biobased Systems - Aalto