Electrokinetic Theory

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Electrokinetic Theory
Study of moving charged particles and their
interactions with electric fields. 4 general
types of phenomenon Electrophoresis applying a
field moves a charged particle Sedimentation
potential opposite effect moving particles
create the field
2
Electro-osmosis movement of a liquid (containing
aqueous ions) with respect to a charged
surface in a field. Streaming Potential opposite
effect the moving liquid now creates the field
3
What can we measure? Mobility (u) how fast the
particle moves (v) in an applied field, E.
uv/E Need to relate this to properties of
the colloidal particles charge, surface
potential, and size.
For a point particle qze For a colloid though,
a large particle does not act like a point
particle, so we must consider the surface
potential.
4
Zeta Potential Potential at the surface of shear
between charged surface and the liquid layer
static moving
We assume that z and the Stern Potential Ys are
approximately the same
5
No longer are we concerned with a simple surface
either we assume that the charged particles are
spheres
kRs small Huckel limit
kRs large Helmholtz- Smoluchowski limit
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Huckel Limit (kRS small) (see Hiemenz and
Rajagopalan)
Want a relationship between q and z. A general
expression for the electric field is given by
Coulombs law
z
q
q
-

z
k

pe
pe
)
/
1
R
(
4
R
4
s
s
q

z
k

pe
)
R
1
(
R
4
s
s
q

z
pe
R
4
s
Since...
Huckel Equation
valid for kRS lt 0.1
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Helmholtz-Smoluchowski Equation (kRS large)
8
Helmholtz-Smoluchowski Equation
valid for kRS gt 100
9
Henrys Equation
10

• Assumptions
• spherical particles
• low field conditions
• (D-H approximation)
• non-conducting particles
• double layer undistorted by flow
• (relaxation effect)

E
-

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12
Surface Charge and Zeta Potential
Recall from our discussion of the Huckel limit
that...

• Again
• spherical particles
• low field limit

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Experimental Techniques
Particle Electrophoresis
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Electro-osmotic effects can also
cause difficulties in interpretation.