On the diffuse double layer

1. The formulae derived by Gouy for the diffuse double layer hold only in the case of a single surface in an infinite amount of medium. In practice they are not very suitable, for very frequently we have to deal either with capillaries, as in streaming potential measurements, or with colloidal particles which may be near enough to influence one another. In these cases it is too difficult to calculate the effect of the mutual influence of two double layers, though in less complicated systems it is possible. Suppose we have two plane parallel surfaces separated by a distance 2 h and charged both to the same potential, in a solution of an electrolyte. If the dimensions of the surfaces are large compared with the distance h there will be no drop of potential between the two. We therefore need to make the assumption (which was superfluous in Gouy’s case) of a specific adsorption by the surface of one or more of the ions in the solution, otherwise there will be no double layer at all; the same effect will also be obtained if ions of the surface dissolve.