Instability mechanisms

Question 1

Processes affecting emulsion stability:

Answer 1

1. Flocculation (aggregation)
2. Coalescence
3. Creaming and sedimentation
4. Ostwald ripening ( in foams disproportionation)

Question 2

1. Flocculation, +++ more detailed explanation

Answer 2

Aggregation of two or multiple oil droplets. The droplets stay intact and keep their original shape, they just cluster together. Two different options:
a) leads to the formation of larger cluster/aggregates/ or flocs. Low volume fraction
b) High volume fraction of the dispersed oil droplets, formation of a network, 3D space network–>gel.
+++++
Is controlled by the balance between attractive and repulsive interactions between droplets. If oil droplets are positive charge, the electrostatic attraction will lead to aggregation or clustering of the droplets. When the droplets have the same charge, there will be electrostatic repulsion, flocculation is prevented. When there are more attractive interactions(hydrophobic, Van der Waals) flocculation occurs.
++++++
Low volume fractions: accelerated creaming
(large clusters rise faster than small individual droplets).
High volume fractions: network formation Emulsions gets solid properties

Question 3

2. Coalescence:+ more detailed explanation

Answer 3

Droplets are able to get into close contact. Merging of the oil droplets, they will merge into one droplet. Growth of the dispersed oil droplets.
The droplets are able to como close, the surfaces of the droplets are only separated by a very thin film, when is smaller than the critical thickness the film ruptures, a one larger spherical droplet is made.
++++++
FACTORS:
- Properties of the droplets and their interfaces.
- Properties of the film between the droplets.
*Interactions between droplets (repulsion–>prevent coalescence)
*Surface tension)larger surface tension, reduce surface area)
*Surface rheology (elastic or viscous, coalescence reduce)
*Size droplets (smaller droplets, faster coalescence)
* Continuous phase (when the viscosity of the continuous phase is high, this will prevent the droplet from coming closer, less draining)
DECREASE IN THE NUMBER OF DROPLETS

Question 4

3. Creaming:

Answer 4

Is the upwards movement of the oil droplets due to a density difference. Oil droplets are lighter than the continuous aqueous phase, the oil droplets will cream. In the case of heavier dispersed particles, will move down, sedimentation.

Question 5

4. Ostwald ripening:

Answer 5

Droplets do no merge, but the large droplets gros larger and the small droplets become smaller. This happens since there is a difference in the Laplace pressure for droplets of different size. This pressure depends on the size of the oil droplet. For smaller droplets, Laplace pressure will be larger than for larger droplets. The differences in the pressure, creates a diffusion of the oil. The flow of liquids will go from high pressure to low pressure. So the oil from small droplets to larger droplets. The smaller ones can even disappear. This will lead to coarsening of emulsion.
The amount of the droplets will stay the SAME.
ONLY POSSIBLE:
*emulsions with water soluble oils–> diffusion
*emulsion with a wide droplet size distribution.
FOAMS–>disproportionation, diffusion of air from small to large bubbles. ONLY POSSIBLE:
*solubility of air in water.
*Gas with different solubility (nitrogen) decrease.