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Colloidal Chemistry

Properties of metastable suspensions


Properties of colloids

A colloid is any object between 1 - 1000 nm that forms a suspension in a solvent. In more recent times, colloids have been called nanoparticles, but this is really marketing of science. Colloids have existed as a concept for hundreds of year (literally). In fact, colloid science is one of the oldest branches of science. One way that you can tell the difference between a solution (in which the solute is solvated by molecular interactions with the solvent) and a suspension (in which the solute is held in a meta-stable state by the opposing attractive and repulsive terms between the colloids themselves) is a scattering experiment. Colloids will scatter visible light since they are as large as the wavelength of visible light in some cases. This is known as Tyndall scattering and it is shown in the corner figure above that introduces this text. In this section, we shall study the potential energy terms that give rise to a colloidal suspension. These are collectively known as Derjaguin, Landau, Verwey, Overbeek (DLVO) theory.

DLVO theory

The DLVO theory combines both electrostatic repulsion and van der Waal's attraction between colloids to account for the stabilizing and destabilizing effects, respectively, of these opposing potential energy terms. The point of the theory is to explain how colloids can remain in suspension in a meta-stable state for long periods of time. The theory also explains ionic strength effects. As the ionic strength of the solution is increased the electrostatic repulsion decreases. Therefore, the colloids can approach more closely in a salt containing solution.

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Electrostatic repulsion

Electrostatic repulsion is a ubiquitous stabilizing potential term.

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Van der Waal's attraction

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Flory-Huggins theory

Debye-Huckel theory

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