introduction to dispersed systems
DESCRIPTION
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. - PowerPoint PPT PresentationTRANSCRIPT
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Introduction to Dispersed Systems
FDSC40009/28/2001
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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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COLLOIDAL SCALE
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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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Solid Liquid Gas
Solid Some glasses
Sol Smoke
Liquid Emulsion Aerosol
Gas Solid foam
Foam
Dis
pers
ed p
hase
Continuous 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..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
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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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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
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Surface Tension-molecular scale-
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Surface Tension-bulk scale-
Area, A
Force,
Inte
rfac
ial e
nerg
y
Interfacial area
Slope
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Curved Surface
Highly curved surface
Slightly curved surface
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Curved Surfaces
Molecules at highly deformed surfaces are less well anchored into their phase
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Laplace Pressure
Surface pressure pulls inwards increasing pressure on dispersed phase pressure
rPL
2
Increased pressure
Surface tension
radius
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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
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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
ln Bulk concentration
Surf
ace
conc
entra
tion
/mg
m-2
Surface saturation
No binding below a certain concentration
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Surface Tension Lowering
Bare surface (tension 0)
Interface partly “hidden”(tension )
Surface pressure – the ability of a surfactant to lower surface tension