Dispersed Systems

FDSC400

2004 Version

Goals

• Scales and Types of Structure in Food

• Surface Tension

• Curved Surfaces

• Surface Active Materials

• Charged Surfaces

COLLOIDAL SCALE

Dispersed Systems

A kinetically stable mixture of one phase in another largely

immiscible phase. Usually at least one length scale is in the

colloidal range.

Dispersed Systems

Dispersed phase

Continuous phase

Interface

Solid Liquid Gas

Solid Some glasses

Sol Smoke

Liquid Emulsion Aerosol

Gas Solid foam

Foam

Dis

pers

ed p

hase

Continuous phase

Properties of Dispersed Systems

• Too small to see

• Affected by both gravitational forces and thermal diffusion

• Large interfacial area– SURFACE EFFECTS ARE IMPORTANT

Increased Surface Area

The same oil is split into 0.1 cm radius droplets, each has a volume of 0.004 cm3 and a surface area 0.125 cm2.

As we need about 5000 droplets we would have a total area of 625 cm2

We have 20 cm3 of oil in 1 cm radius droplets. Each has a volume of (4/3..r3) 5.5 cm3

and a surface area of (4..r2) 12.5 cm2.As we need about 3.6 droplets we would have a total area of 45.5 cm2

For a Fixed COMPOSITION

• Decrease size, increase number of particles

• Increase AREA of interfacial contact

decrease area

Tendency to break

• LYOPHOBIC• Weak interfacial

tension• Little to be gained by

breaking• e.g., gums

• LYOPHILIC• Strong interfacial

tension• Strong energetic

pressure to reduce area• e.g., emulsions

Surface Tension-molecular scale-

Surface Tension-bulk scale-

Area, A

Force,

Inte

rfac

ial e

nerg

y

Interfacial area

Slope

Surface Active Material

• Types of surfactant

• Surface accumulation

• Surface tension lowering

Types of Surfactant-small molecule-

Hydrophilic head group (charged or polar)

Hydrophobic tail (non-polar)

Types of Surfactant-polymeric-

Polymer backbone

Sequence of more water soluble subunits

Sequence of less water soluble subunits

Surface Binding

Equilibrium

ENTHALPY COST ENTROPY COST

Surface Binding Isotherm

ln Bulk concentration

Sur

face

con

cent

rati

on /m

g m

-2

Surface saturation

No binding below a certain concentration

Surface Tension Lowering

Bare surface (tension 0)

Interface partly “hidden”(tension )

Surface pressure – the ability of a surfactant to lower surface tension

Summary

• Small particles have a large surface area• Surfaces have energy associated with them (i.e.,

they are unstable) because of their interfacial tension

• Dispersions will tend to aggregate to reduce the interfacial area

• Proteins and small molecule surfactants will adsorb to the surface to reduce surface tension and increase stability.

Example Dispersion: Emulsions

Emulsion

A fine dispersion of one liquid in a second, largely immiscible liquid. In

foods the liquids are inevitably oil and an aqueous solution.

Types of Emulsion

Oil-in-water emulsion Water-in-oil emulsion

Water

Oil

m

Chemical Composition

Interfacial layer. Essential to stabilizing the emulsion

Oil Phase. Limited effects on the properties of the emulsion

Aqueous Phase. Aqueous chemical reactions affect the interface and hence emulsion stability

Emulsion Size

• < 0.5 m

• 0.5-1.5 m

• 1.5-3 m

• >3 m

Number Distributions

•<

0.5

m

•0.

5-1.

5 m

•1.

5-3

m•

>3

m

Num

ber Very few large

droplets contain most of the oil

Median

Pol

ydis

pers

ity

Large droplets often contribute most to instability

(Vol

ume

in c

lass

T

otal

vol

ume

mea

sure

d)

Note log scale

Volume Fraction=Total volume of the dispersed phase Total volume of the system

Close packing, max

MonodisperseIdeal ~0.69

Random ~0.5

PolydisperseMuch greater

Emulsion Viscosity

Emulsion droplets Emulsion droplets disrupt streamlines disrupt streamlines and require more and require more effort to get the effort to get the same flow ratesame flow rate

5.210

Viscosity of emulsion

Continuous phase viscosity

Dispersed phase volume fraction

Emulsion Destabilization

• Creaming

• Flocculation

• Coalescence

• Combined methods

CreamingBuoyancy(Archimedes)

Friction(Stokes-Einstein)

dv3

cs

gdv

18

2

Continuous phase viscosity density differenceg Acceleration due to gravityddroplet diameterv droplet terminal velocityvs Stokes velocity

6

3gdFb

Flocculation and Coalescence

Film ru

pture

Rehom

ogenization

Collision and sticking (reaction)

Stir or change chemical conditions FLOCCULATION

COALESCENCE

Aggregation Kinetics

• Droplets diffuse around and will collide often• In fact only a tiny proportion of collisions are

reactive

2P

P2

G

G

kslow=kfast/W

Function of energy barrier

Interaction Potential

• Non-covalent attractive and repulsive forces will act to pull droplets together (increase flocculation rate) or push them apart (decrease flocculation rate)

Van der Waals Attraction

• Always attractive

• Very short range

Electrostatic Repulsion

• Repulsive or attractive depending on sign of charges

• Magnitude depends on magnitude of the charge

• Gets weaker with distance but reasonably long range

Steric Repulsion

Droplets approach each other

Protein layers overlap

Proteins repel each other mechanically & by osmotic dehydration

What happens when protein molecules on different droplets are reactive?

Rheology of Flocculated Emulsions

• Flocculation leads to an increase in viscosity

• Water is trapped within the floc and must flow with the floc

• Effective volume fraction increased

rg

Gelled Emulsions

Thin liquid Viscous liquid Gelled solid