Electrokinetic phenomena include Electrophoresis,
electrosmosis, sedimentation potential and Streaming potential, whereby a
potential is created when charged particles move with respect to a stationary
liquid or particles move upon application of a potential. The phenomena are
evidence that particles are charged.
Electrophoresis:
When a dc electric field is
applied to a dispersion, the particles move towards the electrode of a charge
opposite to that on their surface. The counterions located inside their hydration
shell are dragged along. The counterions in the diffuse layer outside the plane
of shear will move towards the other electrode. This phenomenon is called
electrophoresis. The rate of migration of particles is observed by means of an
ultramicroscopic and is a function of the charge on particle. As mentioned
earlier, the potential at the plane of shear is the Zeta potential. The sign
and magnitude of zeta potential can be measured from a knowledge of the
direction and rate of migration.
ζ = ν
x 4πη x (9 x 104)
E ε
Where ν is the velocity of
migration (cm/sec)
η = viscosity of the medium
(poise), ε is the dielectric constant
E is potential gradient
(volts/cm).
ζ is in volts.
The expression v/E is known as mobility.
Sedimentation potential:
The reverse of electrophoresis,
that is a potential is created when particles undergo sedimentation (against a
stationary liquid).
Electrosmosis:
If a charged surface is immobile
(e.g. a packed bed of particles), application of an electric field causes the counterions
in the free water to move towards the opposite electrode, dragging the solvent
with them. Thus the liquid moves relative to the stationary charged surface,
and the phenomenon is known as electrosmosis. The pressure produced is the
electroosmotic pressure. By determining the rate of flow of a liquid under
standard conditions, Zeta potential can be measured.
Streaming potential
If the liquid is made to flow
past a charged surface in a capillary tube by applying hydrostatic pressure,
the displacement of the counterions in the free water produces a potential
difference between the two ends of the tube or bed. This potential is known as
Streaming Potential. Measuring this potential, Es, and knowing the applied
pressure, P, we can obtain from zeta potential from the following equation:
ζ = 4πηκΕl
εP
