An Illustrative Example Colloidal Gold

In later chapters we shall discuss the properties of a variety of colloidal systems. However, as a preparation for our consideration of the general properties and stability of dispersions, it will be useful to use one simple example to outline some of their more important characteristics. It so happens that one of the first colloidal dispersions to have been examined systematically will suit our purpose admirably. 

Faraday concluded that the change from ruby to blue resulted from an increase in particle size. Of the particles in the ruby liquid he said, Whether the particles be considered as mutually repulsive, or else as molecules of gold with associated envelopes of water, they differ from those particles which by the application of salt or other substances are rendered mutually adhesive, and so fall and clot together.  

His observations on the jellied samples implied, he believed, a like association (of the gold particles) with that animal substance which explained their stability in the ruby form. 

It is perhaps surprising that Faraday did not examine the effect of an electric current on his gold sols. Had he done so, he would have discovered the one additional factor which in due course provided the clue to many of their properties, namely that colloidal particles in an aqueous medium (except under special circumstances) move under the influence of an electric field. The 

With this brief account of the properties of one typical colloidal dispersion we are now in a position to examine in the next two chapters the factors that are responsible for the stability of dispersions. 

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