92168.pdf

Charged interfaces 199

Reversible working

electrodes

Perforafed platinum

measuring

electrodes

Figure 7.11 An electro-osmosis apparatus^93 (By courtesy of Academic Press Inc.)

Electrokinetic theory

Electrokinetic phenomena are only directly related to the nature of
the mobile part of the electric double layer and may, therefore, be
interpreted only in terms of the zeta potential or the charge density at
the surface of shear. No direct information is given about the
potentials ift 0 and «/fd (although, as already discussed, the value of £

may not differ substantially from that of (^0) d), or about the charge
density at the surface of the material in question.
Electrokinetic theory involves both the theory of the electric
double layer and that of liquid flow, and is quite complicated. In this
section the relation between electrokinetically determined quantities
(particularly electrophoretic mobility) and the zeta potential will be
considered.
For curved surfaces the shape of the double layer can be described
in terms of the dimensionless quantity '/oz', which is the ratio of
radius of curvature to double-layer thickness. When KO is small, a
charged particle may be treated as a point charge; when KO is large,
the double layer is effectively flat and may be treated as such.