Introduction to Dispersed Systems

FDSC40009/28/2001

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

Curved Surface

Highly curved surface

Slightly curved surface

Curved Surfaces

Molecules at highly deformed surfaces are less well anchored into their phase

Laplace Pressure

Surface pressure pulls inwards increasing pressure on dispersed phase pressure

rPL

2

Increased pressure

Surface tension

radius

Curved Surfaces -Consequences-

• Dispersed phase structures tend to be round• Small fluid droplets behave as hard spheres• Solubility increases with pressure so…• Large droplets may grow at the expense of

small (Ostwald ripening)– Depends on the solubility of the dispersed

phase in the continuous

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

Surf

ace

conc

entra

tion

/mg

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