Introduction to Dispersed Systems

Dispersed Systems

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

• Dispersed phase

• 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, 

• Interfacial energy

• 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

• Surface concentration /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

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

• Number

• Very few large droplets contain most of the oil

• Median

• Polydispersity

• Large droplets often contribute most to instability

• (Volume in class Total volume measured)

• Note log scale

Volume Fraction

• =Total volume of the dispersed phase
•  Total volume of the system

• Close packing, max
• Monodisperse
• Ideal ~0.69
• Random ~0.5
• Polydisperse
• Much greater

Emulsion Viscosity

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

• Viscosity of emulsion

• Continuous phase viscosity

• Dispersed phase
• volume fraction

Emulsion Destabilization

• Creaming
• Flocculation
• Coalescence
• Combined methods

Creaming

• Buoyancy
• (Archimedes)

• Friction
• (Stokes-Einstein)

•  Continuous phase viscosity
•  density difference
• g Acceleration due to gravity
• ddroplet diameter
• v droplet terminal velocity
• vs Stokes velocity

Flocculation and Coalescence

• Film rupture

• Rehomogenization

• 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