Repulsion - Electrosteric

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Electrosteric stabilization is a combination of electrostatic and steric stabilization. Specifically, this type of stabilization features adsorbed polymers like in steric stabilization, however the adsorbed polymers have non-negligible electrostatic charge. Therefore, we get significant double-layer repulsion. A common way to electrosterically stabilize a colloid is by adding a polymer (called a polyelectrolyte) with an ionizable group that is dissociated in the solvent to create charged polymers. For more details, see Ceramic Processing and Sintering, by Rahaman.

Electrosteric stabilization is adding the effect of an adsorbed layer on the surface the energy diagram. If we assume that the layer is repulsive, that is, it interacts strongly with the solvent, any compression comes in energetically unfavorable.

A simple model, then, is to add a short-range replusive barrier to the DLVO equation.

It this example: 200 nm particles, A121 = 7x10-20 J, -100 mV zeta potential, 4 mM ionic strength, 1 nm polymer layer:



Morrison, Fig. 21.7


What is immediately obvious is that even a thin polymer layer (in this case 1nm) eliminates the deep potential minimum. That means that the particles are still kinetically unstable, but the depth of the well is much less and therefore the particles are more easily redispersed.

Furthermore, it does not require a particularly high molecular weight to extend the repulsion to the electrostatic maximum. That would results in a thermodynamically stable dispersion.

The stability being a combination of short-range steric repulsion and long range electrostatic repulsion - hence the name electrosteric stabilization.


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