DLVO theory - a rapid overview

We start this chapter with an overview of the DLVO theory and the basic characteristics of the theory we will discuss later. 

The stability of colloids can be quantified with the DLVO theory which accounts (in an additive way) for the attractive van der Waals (vdW) and the repulsive electrostatic forces (or the corresponding potential energies). The DLVO theory does not account for other forces (the repulsive steric-polymeric and hydration ones, as well as hydrophobic and hydrodynamic forces). 

The electrostatic double-layer force dominates at relatively medium-large separations. However, when very far away from the surfaces (very large separations) and when the surfaces are brought very close to each other, the attractive van der Waals forces (may) overcome the repulsive forces and dominate the interactions. If vdW forces dominate the surfaces will be pulled into a strong adhesive contact (attraction -instability) whereas stability is obtained in the region where the repulsion forces dominate. 

If the maximum value of the potential energy is much higher than the natural kinetic energy of about kgT, about (15—25) 57 , then we may expect that our colloid is stable (metastable, to be precise). A secondary minimum can occur in some cases which correspond to a reversible flocculation. If the potential energy is lower than these values, and certainly when it is smaller than kgT, then the dispersion is unstable. 

Stable colloids are achieved if the Debye length (double layer thickness) is very high (i.e. low salt content, low valency ions), if the colloid particles are in a medium with high relative permittivity, if they have low (or even negative) Hamaker constants and high values of the surface or zeta potential. Control of the ionic concentration and surface charge are crucial. 

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