Page 214 - Introduction to Colloid and Surface Chemistry

Charged interfaces 203

are assumed to be simply superimposed. Mutual distortion of
these fields could affect electrophoretic mobility in two ways: (a)
through abnormal conductance (surface conductance) in the
vicinity of the charged surface, and (b) through loss of double-layer
symmetry (relaxation effect).
3. e and 17 are assumed to be constant throughout the mobile part of
the double layer.

Surface conductance

The distribution of ions in the diffuse part of the double layer gives
rise to a conductivity in this region which is in excess of that in the
bulk electrolyte medium. Surface conductance will affect the
distribution of electric field near to the surface of a charged particle
and so influence its electrokinetic behaviour. The effect of surface
conductance on electrophoretic behaviour can be neglected when KU
is small, since the applied electric field is hardly affected by the
particle in any case. When KO. is not small, calculated zeta potentials
may be significantly low, on account of surface conductance.
According to Booth and Henry 188 , the equation relating electro-
phoretic mobility with zeta potential for non-conducting spheres with
large K.a when corrected for surface conductance takes the form

= • (1.27)
u c
where k 0 is the conductivity of the bulk electrolyte medium and k s is
the surface conductivity.
Substituting £ a for r)u E/€ (i.e £ a is the apparent zeta potential
calculated from the Smoluchowski equation) gives
1 _ l f , . *s ) 7 28
I T l v J < - '

A plot of l/£ a against IIa should, therefore, give a straight line (if KU is
large and if /c s, k 0 and £ are constant) from which a zeta potential
corrected for surface conductance can be obtained by extrapolation.
Zeta potentials for oil droplets and protein-covered glass particles
189
have been determined in this way .
The importance of surface conductance at large Ka clearly depends
on the magnitude of kj(k^a) compared with unity. The surface