Electrostatic and Polymer-Induced Colloid Stability

who are blessed with shade as well as light, you who are gifted with two eyes, endowed with a knowledge of perspective, and charmed with the enjoyment of various colours, you, who can actually see an angle, and contemplate the complete circumference of a Circle in the happy region of Three Dimensions— how shall I make clear to you the extreme difficulty which we in Flatland experience in recognizing one another s configuration  

We saw in Chapter 10 that the van der Waals force between particles in a dispersion is usually attractive and is strong at short interparticle separations. Therefore, if there are no repulsive interactions between particles, the dispersion will be unstable and coagulate. The protection against van der Waals attraction is usually provided in one of two ways  

As we saw in Chapter 11, surfaces of colloidal particles typically acquire charges for a number of reasons. The electrostatic force that results when the electrical double layers of two particles overlap, if repulsive, serves to counteract the attraction due to van der Waals force. The stability in this case is known as electrostatic stability, and our task is to understand how it depends on the relevant parameters. 

In many practical instances (see Vignette 1.5), electrostatic repulsion is not a convenient option. In such cases, a suitable polymer that adsorbs on the particle surfaces may be added to the dispersion. The resulting polymer layer masks the attraction and may also provide a repulsive force, partly due to pure steric effect, when the polymer layers on two interacting particles attempt to overlap with each other. This is what is known as polymer-induced stability. Polymer-induced stability is often referred to as steric stability for the above 

Our primary focus in this chapter is on kinetic stability of dispersions arising due to either electrostatic forces or polymer-mediated forces. 

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