Introduction to Electrokinetics - Electrokinetics for Petroleum and Environmental Engineers - page 9

The qualitative rule presented by Coehn, seems to represent a very spe-
cial case where ionic liquid content is very small even when compared to
dilute aqueous solutions. The electrochemical behavior of the solid-liquid
interface greatly influences these electrokinetic phenomena. In the case
of relatively inert surfaces, such as quartz, the electrical charge density
depends primarily on the adsorbed electrolytes.
Many researchers, showed a linear logarithmic relationship between
zeta potential and concentration ( c ) (Adamson et al., 1963):

(1.8)

z =− AB log c

he zeta potential, ξ , goes through a maximum and then approaches
zero, which is explained by a combination of two processes: (1) adsorption
process of ions on the surface and (2) followed by a neutralization process
of the charged surface with opposite sign (Kruyt, 1952).
Rutger et al. (1945) showed the effects of the H + and OH - ions on zeta
potential at low concentrations. A small addition of the OH - increased the
negative zeta potential. In the case of larger concentrations, all electrolytes
decreased the zeta potential, especially pronounced in the case of polyva-
lent ions, whereas the addition of H + ions decreased the zeta potential.
Although electrolytes can strongly influence the zeta potential, they
have no effect on the total potential drop (Adamson et al., 1963.) The addi-
tion of multivalent ions may cause the reversal of the zeta potential sign.
This can be explained by the adsorbability for these ions in a layer bearing
a larger charge than is present on the wall. This will cause a reversal of the
charge and potential in the outer part of the double layer (Kruyt, 1952), in
order to maintain the electro-neutrality of the system.
The theory of the diffuse double layer leads to the conclusion that the con-
centration of the electrolyte varies inversely with the effective thickness of
the diffuse part of the double layer, (and zeta potential.) The larger the ionic
charges, the fewer the ions needed for charge compensation, whereas the
larger the ionic charges, the larger the electric forces between the diffuse layer
and the inner fixed layer. The fewer ions that are needed for charge compen-
sation, the larger the valences of the adsorbed ions (Adamson et al., 1963).

1.4 Combined Flow Rate Equation

In some of the experiments performed by researchers at USC (Chilingar
et al., 1970), an electric potential was applied across a core where oil was
already flowing hydrodynamically. When the imposed electrical potential