Stability criteria for emulsions

The two extreme cases of stabilisation of dispersed systems by electrostatic repulsion and steric hindrance are shown schematically in Fig. 1.14. When two droplets possessing electric charges of the same sign approach each other the electrostatic repulsion forces between them are 

The total force balance in the narrow gap between two particles is governed by the van der Waals attraction forces. Two plates made from the same material and separated by another medium develop attraction forces acting over small distances. 

and A2 are the so-called Hamaker constants of the particles and the solution respectively. The Hamaker constants of most materials are in the range of 5-100 kT, e.g. 10.6 kT for water. 

From the brief sketch of the Deijaguin, Landau, Verwey, Overbeek (DLVO)-theory the following three important practical conclusions, suitable for foams and emulsions, can be drawn  

The electrostatic repulsion between dispersed particles can be diminished by increasing the concentration of background electrolyte (e.g. NaCl, CaCl2)- Polyvalent ions are more effective than monovalent. There is a critical electrolyte concentration for every system at which flocculation or coalescence takes place. These principles must be taken into account when emulsions have to form in very hard water. 

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