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NanoPlus Operator’s Manual Zeta Potential Determination by Electrophoretic Light Scattering
Mar 2014 E-13
Zeta Potential Measurement Using Electrophoretic Light Scattering
Most colloidal particles have an electrical charge on their surface when dispersed in liquids. The
particles move towards the electrode that has an opposite charge if an electric field is applied to the
cell that contains the particle suspension. Besides the particle movement, electroosmotic flow is
also induced in the cell due to the surface charge of the cell wall. Because the cell is typically a
closed system, electroosmotic flow occurring at the position close to the cell wall moves towards
the opposite electrode, then hits the side wall of the cell, and flows back to the center of the cell.
When measuring the zeta potential of the particles, an apparent mobility of the particles, which is
equal to the sum of the electroosmotic flow and particle true mobility, is observed (Figure 10).
Electroosmotic flow
Electroosmotic flow and
particle true mobility
Figure 10. Schematic Diagram of the Electroosmotic Flow Occurring in a Close Cell
Electroosmotic flow has a symmetric parabolic profile when the particle concentration in the cell
is homogeneous and the electrical charges on the upper and lower surfaces of the cell are equal.
However, electroosmotic flow is asymmetric in many cases due to the sedimentation of particles,
differences in the charges of the upper and lower cell surfaces, or for other reasons. Mori and
Okamoto expanded Komaga's model to generate an equation that can be applied to both symmetric
and asymmetric osmotic flows.
Assume the velocity of the electroosmotic flow at upper and lower surfaces of the cell is W
1
and
W
0
, respectively. The cross-sectional dimensions of the rectangular cell as 2a (X axis) and 2b (Z
axis), with a>b, as shown in Figure 11.
Figure 11. Boundary Condition of the Electroosmotic Flow in the Cell
The electrophoretic mobility of the particles is the ratio of velocity of the particles to the electric
field strength. The apparent mobility

(x, z) of the particles can be represented as the sum of true
mobility and electroosmotic flow, where the velocity of electroosmotic flow changes linearly from