Kinetic stability of disperse systems and the general stabilization mechanisms

2 Kinetic stability of disperse systems and the general stabilization mechanisms 

As mentioned above disperse systems lack thermodynamic stability and they tend to reduce their surface energy by aggregation and/or coalescence of the particles or droplets. The main driving force for the aggregation process is the universal van der Waals attraction which will be discussed in subsequent sections. To overcome the aggregation and/or coalescence processes, one must overcome the van der Waals attraction by some repulsive mechanism and this will give the system kinetic stability with an adequate shelf life. Normally one requires a shelf life of 2-3 years under various storage conditions (e.g. temperature variation). Several stabilization mechanisms are encountered with disperse systems and these are summarized below. 

For two particles or droplets with equal radius R, and surface-to-surface separation h, the van der Waals attractive energy, G, (when h R) is given by [2] 

The Hamaker constant of any material depends on the number of atoms per unit volume q and the London dispersion constant p. 

It can be seen that G increases very sharply with decreasing h when the latter reaches small values. In the absence of any repulsion between the particles or droplets, the latter will aggregate (flocculate) by simple diffusion through the medium. This leads to fast flocculation kinetics and the rate constant for the process k has been calculated by Smolulokowski [3], 

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