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Zeta Potential Determination by Electrophoretic Light Scattering NanoPlus Operator’s Manual
E-8 Mar 2014
Zeta Potential Determination by Electrophoretic Light Scattering
Electrical Double Layer
Most particles dispersed in a liquid have positive or negative charge. In a liquid, the ions that have
opposite charge to the particle surface gather close to the particle to keep an electric neutrality.
Since the particle surface is surrounded by such ion clusters, ionized layers with opposite charge
surround the layer on the surface of a particle (Figure 3). This phenomenon is expressed as an
electrical double layer .
Figure 3. Schematic Diagram of Electrical Double Layer
Since the ion in a liquid moves by thermal diffusion, concentration of the counter ion is high in the
area near the particles surface and gradually decreases with distance from the surface. Equal
numbers of positive and negative ions exist in the area far from the particle’s surface so that
electric neutrality is maintained. This ion distribution is called a
diffuse electrical double layer and
can be divided into two layers:
The layer of ions near the particle surface is called the Stern layer . The ions in the Stern layer are
strongly attracted to the surface of the particle.
The layer outside the Stern layer is called the diffuse layer ; in this layer, the ions are diffused.
Zeta Potential
The stability of the dispersing particles is influenced by their surface charge. Zeta potential is used
as the index of the surface charge of the particles. It is assumed that the particles undergoing
Brownian motion in a liquid move not only with the ions in the Stern layer where the ions are
attracted strongly near the particle surface, but also with part of the diffuse layer. The field from
which this movement takes place is called the
slipping plane . Zeta potential is considered to be the
potential at the slipping plane and the potential at the position far from the particle surface is
defined as zero (Figure 4).