Chemistry Reference
In-Depth Information
which is the opposite of streaming potential. The reason for investigating
electrokinetic properties of a system is to determine a quantity known as
the
zeta potential
.
Electrophoresis
is the movement of an electrically charged substance under the
influence of an electric field. This movement may be related to fundamental electri-
cal properties of the body under study and the ambient electrical conditions by the
following equation. F is the force, q is the charge carried by the body, and E is the
electric field:
F
e
= q E
(7.13)
The resulting electrophoretic migration is countered by forces of friction such that
the rate of migration is constant in a constant and homogeneous electric field:
Ff =
v
f
r
(7.14)
where
v
is the velocity, and fr is the frictional coefficient.
Q E = v fr
(7.15)
The electrophoretic mobility μ is defined as
μ =
v/E
=
q
/
fr
(7.16)
The foregoing expression applies only to ions at a concentration approaching 0 and
in a nonconductive solvent. Polyionic molecules are surrounded by a cloud of coun-
terions that alter the effective electric field applied on the ions to be separated. This
renders the previous expression a poor approximation of what really happens in an
electrophoretic apparatus.
The mobility depends on both the particle properties (e.g., surface charge density
and size) and solution properties (e.g., ionic strength, electric permittivity, and pH).
For high ionic strengths, an approximate expression for the
electrophoretic mobility
,
μ
e
, is given by the Smoluchowski equation:
μ
e
= ε εo η/n
(7.17)
where ε is the dielectric constant of the liquid, εo is the permittivity of free space,
η is the viscosity of the liquid, and η is the zeta potential (i.e., surface potential) of
the particle.
7.2.3 S
T a b I l I T y
o f
l
y o p h o b I c
S
u S p e n S I o n S
Particles in all kinds of suspensions or dispersions interact with two different kinds of
forces (e.g., attractive forces and repulsive forces). One observes that lyophobic sus-
pensions (sols) must exhibit a maximum in repulsion energy in order to have a stable
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