Transcript Introduction to Dispersed Systems

Introduction to Dispersed
Systems
FDSC400
09/28/2001
Goals
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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
Dispersed phase
Continuous phase
Solid
Solid
Liquid
Gas
Some
glasses
Sol
Smoke
Liquid
Gas
Emulsion Aerosol
Solid
foam
Foam
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
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
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
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 scaleForce, g
Area, A
Interfacial energy
Slope g
Interfacial area
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
Surface tension
Increased pressure
2g
PL 
r
radius
Curved Surfaces
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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 moleculeHydrophilic head group (charged or polar)
Hydrophobic tail (non-polar)
Types of Surfactant
-polymericPolymer backbone
Sequence of more water
soluble subunits
Sequence of less water
soluble subunits
Surface Binding
Equilibrium
ENTHALPY COST
ENTROPY COST
Surface concentration /mg m-2
Surface Binding Isotherm
Surface saturation
No binding below a certain
concentration
ln Bulk concentration
Surface Tension Lowering
Bare surface
(tension g0)
Surface pressure – the
ability of a surfactant to
lower surface tension
Interface partly “hidden”
(tension g)
p  g-g0