Colloid Stability ?

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Colloid Stability ?
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Colloidal systems

• A state of subdivision in which the particles,
  droplets, or bubbles dispersed in another phase
  have at least one dimension between 1 and 1000 nm
  
• all combinations are possible between
• gas, liquid, and solid
• W. Ostwald

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Surface area of colloidal systems

• Cube (1cm 1cm 1cm)
• after size reduction to an edge length of 500
  nm
• ? surface area of 60 m2
• Spinning dope (1 cm3)
• after spinning to a fibre with diameter of
  1000 nm
• ? fiber length of 1273 km
• 1 liter of a 0.1 M surfactant solution
• ? interfacial area of 40000 m2

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Surface atoms in in dependence on the
particle size in nm

nm
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Colloidal systems

• have large surface areas
• surface atoms become dominant

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Colloid stability

• Colloidal gold stabilized against coagulation !
• Creme stabilized against coagulation !
• Milk stabilized against coagulation !

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Particle Particle interactions
d

• Interaction Energy ( Vtot) Distance of
  Separation (d) Relationship

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Vtot(d) Vattr(d) Vrep(d)

• - Van der Waals attraction - Electrostatic
  repulsion
• 
  - Steric repulsion

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DLVO - Theory

• 1940 Derjaguin Landau Verwey Overbeek
• Long range attractive van der Waals forces
• Long range repulsive electrostatic forces

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DLVO TheoryVan der Waals attractive energy
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Double layer models

• Helmholtz
• Gouy Chapman
• Stern

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Gouy Chapman model

• planar double layer
• Ions as point charges

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Electrolyte theory
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Solution of the P-B equation
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DLVO TheoryElectrostatic repulsive energy
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Vtot(d) Vattr(d) Vrep(d)

• Vvan der Waals - A a / 12 d
  Velectrost. k e-?d
• A Hamaker constant
• a particle radius
• d distance between the particles
• 1/? - thickness of the double-layer

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Electrostatic stabilization

• stabilized against coagulation
• ? Kinetically stable state

• energetic metastable state in the
• secondary minimum
• with an energy barrier

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Critical coagulation concentration (CCC)

• The energy barrier disappears by adding a
  critical amount of low molecular salts

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DLVO Theory (CCC)
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DLVO Theory (CCC)

• For two spheres the ccc should be related to the
  valency (1 2 3) of the counterions as
• 1000 16 1,3

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CCC of a colloidal dispersion as a function of
the salt concentration
electrolyte
Schulze-Hardy-ratio
CCC of a Arsensulfid -Dispersion
NaCl
1000
5,1 10-2
KCl
1000
5,0 10-2
MgCl2
13
7,2 10-4
CaCl2
13
6,5 10-4
AlCl3
1,7
9,3 10-5
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Steric stabilization

• What will be happen when we add polymers to a
  colloidal dispersion ?

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Particle Particle interactions
Polymer adsorption layer
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Particle Particle interactions
Overlap of the polymer adsorption layer
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Overlap of the adsorption layer

• Osmotic repulsion
• Entropic repulsion
• Enthalpic repulsion

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Sterically stabilized systems can be controlled
by

• The thickness of the adsorption layer
• The density of the adsorption layer
• The temperature

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Stabilization and destabilization in dependence
on the molecular weight of the added polymer
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Stabilization and destabilization in dependence
on the polymer-concentration