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Zeta
Potential Theory
Zeta Potential is the electrical potential that exists
at the "shear plane" of a particle, which is some small distance from
its surface. Zeta Potential is derived from measuring the mobility
distribution of a dispersion of charged particles as they are subjected
to an electric field. Mobility is defined as the velocity of a particle
per electric field unit and is measured by applying an electric field
to the dispersion of particles and measuring their average velocity.
Depending on the concentration of ions in the diluent, either the
Smoluchowski (for high ionic strengths) or Huckel (for low ionic strengths)
equations are used to obtain the Zeta potential from the measured
mobilities.
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Zeta
Cuvette cell with Electrodes |
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Particles
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Scattered
Light
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(particles
and laser beam not to scale)
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CAPTION:
Charged particles in a liquid suspension can be made to move by
applying an electric field to the liquid through two electrodes.
By alternating the charge between the electrodes, the particles
move back and forth between the electrodes at a velocity relative
to their surface charge and the electrode potential. This velocity
can be determined by measuring the doppler shift of laser light
scattered off of the moving particles.
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The Nicomp 380/ZLS uses the method of Electrophoretic
Light Scattering (ELS) to measure Zeta potential. To make a measurement,
a small aliquot of sample is typically placed in a disposable plastic
cuvette. Then the platinum electrodes are inserted. The entire cell
is placed into the Nicomp 380. Because of the unique cell design,
there is no need to align the cell to the stationary plane. After
the cell is in place, a simple click of the mouse starts the measurement.
Since ELS requires the use of heterodyned light, the scattered light
must be properly mixed with a reference beam (split off from the incident
light beam) prior to entering the detector. The software will begin
a measurement by automatically adjusting the incident light intensity
to optimize the mixing between the scattered light and the reference
beam. Once this is completed, a reference power spectrum is measured
while the electric field is off. Then the electric field is applied
and another power spectrum is measured. The change in the frequency
of the peak in this power spectrum when compared to the reference
spectrum is the Doppler shift. The Doppler shift is used to calculate
the average mobility. Using the Smoluchowski equation, the zeta
potential is determined.
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