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 nm:
 fiber length of 1273 km
1 liter of a 0.1 M surfactant solution:
 interfacial area of m2

4. Surface atoms [in %] in dependence on the particle size [in 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 interactionsd
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 – Theory Van der Waals attractive energy
a) between two plates:
b) between two spheres:

11. Double layer models
Helmholtz
Gouy Chapman
Stern

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

13. I distribution of ions in the diffuse double layer
Electrolyte theory
I distribution of ions in the diffuse double layer
(Boltzmann equation)
II equation for the room charge density
III Poisson relation
Aus I, II und III folgt:
Poisson – Boltzmann - relation

14. Solution of the P-B equation( ) x e k d y - × = 2
For small potentials (< 25 mV) :
Integrable form

15. DLVO – Theory Electrostatic repulsive energy
Resulting repulsive overlap energy
Between two plates:
c° – volume concentration of the
z – valent electrolyte
b) Between two spheres

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)
Vtot / dd = Vtot = 0
 for two spheres:

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
CCC of a
Arsensulfid -Dispersion
Schulze-Hardy-ratio
NaCl
5,1 10-2
1000
KCl
5,0 10-2
1000
MgCl2
7,2 10-4 13
CaCl2
6,5 10-4 13
AlCl3
9,3 10-5
1,7

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