Mini-Encyclopedia of Papermaking Wet-End Chemistry
Part Two: Definitions and Concepts

Double Layer

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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