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Colloid Stability ?

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Colloid Stability ? 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 ... – PowerPoint PPT presentation

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Title: Colloid Stability ?


1
Colloid Stability ?
2
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

3
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

4
Surface atoms in in dependence on the
particle size in nm

nm
5
Colloidal systems
  • have large surface areas
  • surface atoms become dominant

6
Colloid stability
  • Colloidal gold stabilized against coagulation !
  • Creme stabilized against coagulation !
  • Milk stabilized against coagulation !

7
Particle Particle interactions
d
  • Interaction Energy ( Vtot) Distance of
    Separation (d) Relationship

8
Vtot(d) Vattr(d) Vrep(d)
  • - Van der Waals attraction - Electrostatic
    repulsion

  • - Steric repulsion

9
DLVO - Theory
  • 1940 Derjaguin Landau Verwey Overbeek
  • Long range attractive van der Waals forces
  • Long range repulsive electrostatic forces

10
DLVO TheoryVan der Waals attractive energy
11
Double layer models
  • Helmholtz
  • Gouy Chapman
  • Stern

12
Gouy Chapman model
  • planar double layer
  • Ions as point charges

13
Electrolyte theory
14
Solution of the P-B equation
15
DLVO TheoryElectrostatic repulsive energy
16
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

17
Electrostatic stabilization
  • stabilized against coagulation
  • ? Kinetically stable state
  • energetic metastable state in the
  • secondary minimum
  • with an energy barrier

18
Critical coagulation concentration (CCC)
  • The energy barrier disappears by adding a
    critical amount of low molecular salts

19
DLVO Theory (CCC)
20
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

21
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
22
Steric stabilization
  • What will be happen when we add polymers to a
    colloidal dispersion ?

23
Particle Particle interactions
Polymer adsorption layer
24
Particle Particle interactions
Overlap of the polymer adsorption layer
25
Overlap of the adsorption layer
  • Osmotic repulsion
  • Entropic repulsion
  • Enthalpic repulsion

26
Sterically stabilized systems can be controlled
by
  • The thickness of the adsorption layer
  • The density of the adsorption layer
  • The temperature

27
Stabilization and destabilization in dependence
on the molecular weight of the added polymer
28
Stabilization and destabilization in dependence
on the polymer-concentration
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