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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.

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Dispersed systems l.jpg

Dispersed Systems

FDSC400

2004 Version


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Goals

  • Scales and Types of Structure in Food

  • Surface Tension

  • Curved Surfaces

  • Surface Active Materials

  • Charged Surfaces



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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.


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Dispersed Systems

Dispersed phase

Continuous phase

Interface


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Continuous phase

Dispersed phase


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Properties of Dispersed Systems

  • Too small to see

  • Affected by both gravitational forces and thermal diffusion

  • Large interfacial area

    • SURFACE EFFECTS ARE IMPORTANT


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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.p.r3) 5.5 cm3 and a surface area of (4.p.r2) 12.5 cm2.

As we need about 3.6 droplets we would have a total area of 45.5 cm2


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For a Fixed COMPOSITION

  • Decrease size, increase number of particles

  • Increase AREA of interfacial contact

decrease area


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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

Tendency to break


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Surface Tension-molecular scale-


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Surface Tension-bulk scale-

Force, g

Slope g

Interfacial energy

Area, A

Interfacial area


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Surface Active Material

  • Types of surfactant

  • Surface accumulation

  • Surface tension lowering


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Types of Surfactant-small molecule-

Hydrophilic head group (charged or polar)

Hydrophobic tail (non-polar)


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Types of Surfactant-polymeric-

Polymer backbone

Sequence of more water soluble subunits

Sequence of less water soluble subunits


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Surface Binding

Equilibrium

ENTHALPY COST

ENTROPY COST


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Surface Binding Isotherm

Surface saturation

Surface concentration /mg m-2

No binding below a certain concentration

ln Bulk concentration


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Surface Tension Lowering

Bare surface

(tension g0)

Interface partly “hidden”

(tension g)

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

p = g-g0


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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.



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Emulsion

A fine dispersion of one liquid in a second, largely immiscible liquid. In foods the liquids are inevitably oil and an aqueous solution.


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Types of Emulsion

mm

Water

Oil

Oil-in-water emulsion

Water-in-oil emulsion


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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


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< 0.5 mm

0.5-1.5 mm

1.5-3 mm

>3 mm

Emulsion Size


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Very few large droplets contain most of the oil

Number Distributions

Number

  • < 0.5 mm

  • 0.5-1.5 mm

  • 1.5-3 mm

  • >3 mm


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Large droplets often contribute most to instability

Median

(Volume in class Total volume measured)

Polydispersity

Note log scale


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Volume Fraction

f=Total volume of the dispersed phase

 Total volume of the system

Close packing, fmax

Monodisperse

Ideal ~0.69

Random ~0.5

Polydisperse

Much greater


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

Emulsion Viscosity

Dispersed phase

volume fraction

Viscosity of emulsion

Continuous phase viscosity


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Emulsion Destabilization effort to get the same flow rate

  • Creaming

  • Flocculation

  • Coalescence

  • Combined methods


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Creaming effort to get the same flow rate

Buoyancy

(Archimedes)

h Continuous phase viscosity

Dr density difference

g Acceleration due to gravity

ddroplet diameter

v droplet terminal velocity

vs Stokes velocity

Friction

(Stokes-Einstein)


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Flocculation and Coalescence effort to get the same flow rate

Collision and

sticking (reaction)

Stir or change chemical conditions

FLOCCULATION

Rehomogenization

Film rupture

COALESCENCE


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Aggregation Kinetics effort to get the same flow rate

  • Droplets diffuse around and will collide often

  • In fact only a tiny proportion of collisions are reactive

DG

2P

G

kslow=kfast/W

P2

Function of energy barrier


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Interaction Potential effort to get the same flow rate

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


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Van der Waals Attraction effort to get the same flow rate

  • Always attractive

  • Very short range


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Electrostatic Repulsion effort to get the same flow rate

  • Repulsive or attractive depending on sign of charges

  • Magnitude depends on magnitude of the charge

  • Gets weaker with distance but reasonably long range


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Steric Repulsion effort to get the same flow rate

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?


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Flocculation leads to an increase in viscosity effort to get the same flow rate

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

Effective volume fraction increased

Rheology of Flocculated Emulsions

rg


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Gelled Emulsions effort to get the same flow rate

Thin liquid

Viscous liquid

Gelled solid