Colloid Stability ?

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

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

Surface atoms [in %] in dependence on the particle size [in nm]

Colloidal systems have large surface areas surface atoms become dominant

Colloid stability Colloidal gold: stabilized against coagulation ! Creme: stabilized against coagulation ! Milk: stabilized against coagulation !

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

Vtot(d) = Vattr(d) + Vrep(d) - Van der Waals attraction - Electrostatic repulsion - Steric repulsion

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

DLVO – Theory Van der Waals attractive energy a) between two plates: b) between two spheres:

Double layer models Helmholtz Gouy Chapman Stern

Gouy Chapman model planar double layer Ions as point charges

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

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

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

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

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

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

DLVO – Theory (CCC) Vtot / dd = 0 Vtot = 0  for two spheres:

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

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

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

Particle – Particle interactions Polymer adsorption layer

Particle – Particle interactions Overlap of the polymer adsorption layer

Overlap of the adsorption layer Osmotic repulsion Entropic repulsion Enthalpic repulsion

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

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

Stabilization and destabilization in dependence on the polymer-concentration