Introduction to Dispersed Systems FDSC400 09/28/2001.
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Transcript of Introduction to Dispersed Systems FDSC400 09/28/2001.
Introduction to Dispersed Systems
FDSC400
09/28/2001
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
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