Charged interfaces 185iodide-aqueous electrolyte interfaces^181. If the double layer is
treated as two capacitors in series, then
t 1 1The capacity C 2 of the diffuse part of the double layer can be
calculated. At low potentials (see equation (7.9)):
( -z^2 \*- 2.28 - - — - F m~^2 for aqueous electrolyte at 25° C
^ mol dimf )
The capacity of the Stern layer (C! = e'/d) does not depend on
electrolyte concentration except in so far as e' is affected. In the case
of the silver iodide-aqueous electrolyte interface, Stern layer
capacities of c. 0.1 to 0.2 F m~^2 have been calculated; taking 8 = 5 x
10~^10 m, this corresponds to a dielectric constant in the Stern layer of
c. 5-10, which, compared with the normal value of c. 80 for water,
r.uggests considerable ordering of water molecules close to the
surface.
Stern potentials and electrokinetic (zeta) potentialsij/d can be estimated from electrokinetic measurements. Electrokinetic
behaviour (discussed in the following sections of this chapter)
depends on the potential at the surface of shear between the charged
surface and the electrolyte solution. This potential is called the
electrokinetic or £ (zeta) potential. The exact location of the shear
plane (which, in reality, is a region of rapidly changing viscosity) is
another unknown feature of the electric double layer. In addition to
ions in the Stern layer, a certain amount of solvent will probably be
bound to the charged surface and form a part of the electrokinetic
unit. It is, therefore, reasonable to suppose that the shear plane is
usually located at a small distance further out from the surface than
the Stern plane and that £ is, in general, marginally smaller in
magnitude than tf/d (see Figures 7.2 and 7.3). In tests of double-layer
theory it is customary to assume identity of i^d and £, and the bulk of
experimental evidence suggests that errors introduced through this
assumption are usually small, especially at lyophobic surfaces. Any