Mini-Encyclopedia of Papermaking Wet-End
Part Two: Definitions and Concepts
The words "double layer" refer to displacement of electrical charges associated with most surfaces exposed to aqueous solution. The layer is "double" because it involves two parts - the net charge on the surface, and the cloud of excess counter-ions in the adjacent solution. There have been many studies aimed at understanding how ions are distributed within the counter-ion field. Most colloidal phenomena are described quite well by a model in which the counter-ions are distributed between two regions. In the Stern layer (condensed layer) the ions are pressed against the surface. Out beyond the Stern layer the ions are distributed in a diffuse atmosphere, the Gouy layer. The density of ions within this layer is given by the Poisson-Boltzmann relationship. At distances of much more than 10 nm from surfaces (under typical electrolyte conditions) there is an equal density of positive and negative ions.
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