Table of contents 1 Why is particle size important?

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As shown in the top view, above, of the optical setup for zeta potential 
measurements in the SZ-100, the particles are illuminated with laser light and, 
therefore, the particles scatter light. A second beam of light (the reference beam) 
is mixed with the scattered beam in order to sensitively extract the frequency shift 
in the scattered light. The measured magnitude of the frequency shift is then used 
to determine the particle velocity. Equation 1 is used to calculate the electrophoretic 
mobility (µ) using the measured frequency shift.
figure 28
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OPTICAL DIAGRAM OF THE SZ-100 
 
CONFIGURATION FOR ZETA POTENTIAL
ZETA POTENTIAL
Zeta potential is a measure of the charge on a particle surface in a specific liquid
medium. This value of surface charge is useful for understanding and predicting 
interactions between particles in suspension. Manipulating zeta potential is a 
method of enhancing suspension stability for formulation work, or speeding particle 
flocculation in applications such as water treatment. Zeta potential is measured on
the SZ-100 using the technique of electrophoretic light scattering where particle 
motion is detected in an applied electric field.
The charge on the surface of a particle influences the ionic environment in the
region close to the particle surface. This ionic environment is typically described 
using a double layer model – the Stern layer of ions firmly attached adjacent to
the particle surface, and the diffuse layer further away from the particle surface, 
but still attracted to the particle such that these ions will move with the particle. 
The boundary between the electric double layer and the ions in equilibrium in the 
solution is called the slipping plane, as shown in Figure 27. Zeta potential is defined
as the potential measured in mV at the slipping plane distance from the particle 
surface. 
To measure zeta potential a small quantity of sample is injected into a cell 
containing two electrodes that are used to create an induced electric field. Once
the electric field is applied the particles move toward either the anode or cathode
depending on whether the surfaces are positively or negatively charged. The 
direction of the motion indicates positive vs. negative charge. The speed of the 

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