Introduction to Electrokinetics - Electrokinetics for Petroleum and Environmental Engineers - page 3

The proportionality constant, D , has been verified by several investiga-
tors for various types of liquid-solid interfaces. However, extreme sensitiv-
ity and complexity of these phenomena have lead to reports of discrepancies
in the relative constancy of this term. Probstein and Hicks (1993) have
shown the effects of concentration of ionic species within the pore fluid,
electric potential, and pH on the zeta potential ( z ). Thus, it doesn't remain
constant throughout the electrically-induced transport in soils that are
governed by zeta potential, which is defined below.
1.2 Zeta Potential and the Electric Double
Layer Interaction
As pointed out by Donaldson and Alam (2008), there is a region at the
surface of solids that has a difference in electrical potential across just a
few molecular diameters. If a liquid and solid are brought together, an elec-
trical potential develops across a distance of a few molecular diameters at
the interface. The changes that are established are characteristic of specific
phases and are the underlying cause of many natural phenomena such as
electroosmosis, electrophoresis, colloid stability, fluid flow behavior, adsorp-
tion, catalysis, corrosion, and crystal growth (Donaldson and Alam, 2008).
The separation of charges is known as the interfacial electrical double
layer. It is a complex association of charges illustrated schematically in
Fig. 1.2. There is a potential charge (negative or positive) at one or two
molecular distances from the surface. This charge may originate from
several sources such as: (1) inclusions of extraneous atoms in the lattice
structure, (2) dissolution of slightly soluble atoms at the surface of water,
(3) chemical reaction (chemisorption) of ions in water with surface atoms
forming complex polar molecules on the surface, or (4) exposure of metal-
lic oxides at the surface which react with water to form surface ions. These
are some of the major causes of surface charges; others are recognized in
suspensions of particles and flocculants in water (Hunter, 1981).
Counterions from the water solution balance the charges at the solid sur-
face and form the immobile Stern layer (Fig. 1.2). The thickness of the Stern
layer is only one or two molecular diameters consisting of ions that are
adsorbed strongly enough to form an immobile layer. The outer edge of the
Stern layer where the ions are mobile is known as the shear plane. There is
a linear potential drop across the width of the Stern layer (ψ s - ψ ζ ) , followed
by an exponential potential difference across the diffuse layer between the
shear plane and the bulk solution (ψ ζ - ψ ∞ ) . The bulk solution is designated
as the reference zero potential. The potential difference between the shear