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

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**Surface atoms [in %] in dependence on the particle size [in 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 – Theory Van der Waals attractive energy**

a) between two plates: b) between two spheres:

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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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**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

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**Solution of the P-B equation**

( ) x e k d y - × = 2 For small potentials (< 25 mV) : Integrable form

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**DLVO – Theory Electrostatic repulsive energy**

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

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

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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 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

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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**

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