Zeta potential is a scientific term for electrokinetic potential in colloidal dispersions. In the
colloidal chemistry literature, it is usually denoted using the Greek letter zeta, hence
ζ-potential. From a theoretical viewpoint, the zeta potential is the electric potential
in the interfacial double layer at the location of the slipping plane relative to a point
in the bulk fluid away from the interface. In other words, zeta potential is the potential
difference between the dispersion medium and the stationary layer of fluid attached to
the dispersed particle. The zeta potential is caused by the net electrical
charge contained within the region bounded by the slipping plane, and also depends on
the location of that plane. Thus it is widely used for quantification of the magnitude of
the charge. However, zeta potential is not equal to the Stern potential or electric surface
potential in the double layer, because these are defined at different locations. Such assumptions
of equality should be applied with caution. Nevertheless, zeta potential is often the
only available path for characterization of double-layer properties.
The zeta potential is a key indicator of the stability of colloidal dispersions. The magnitude
of the zeta potential indicates the degree of electrostatic repulsion between adjacent,
similarly charged particles in a dispersion. For molecules and particles that are small
enough, a high zeta potential will confer stability, i.e., the solution or dispersion
will resist aggregation. When the potential is small, attractive forces may exceed this
repulsion and the dispersion may break and flocculate. So, colloids with high zeta potential
are electrically stabilized while colloids with low zeta potentials tend to coagulate
or flocculate as outlined in the table.
Measurement of zeta potential Zeta potential is not measurable directly
but it can be calculated using theoretical models and an experimentally-determined electrophoretic
mobility or dynamic electrophoretic mobility. Electrokinetic phenomena and electroacoustic
phenomena are the usual sources of data for calculation of zeta potential.
Electrokinetic phenomena
Electrophoresis is used for estimating zeta potential of particulates, whereas streaming
potential/current is used for porous bodies and flat surfaces. In practice, the Zeta potential
of dispersion is measured by applying an electric field across the dispersion. Particles within
the dispersion with a zeta potential will migrate toward the electrode of opposite charge
with a velocity proportional to the magnitude of the zeta potential.
This velocity is measured using the technique of the Laser Doppler Anemometer. The frequency
shift or phase shift of an incident laser beam caused by these moving particles is measured
as the particle mobility, and this mobility is converted to the zeta potential by inputting
the dispersant viscosity and dielectric permittivity, and the application of the Smoluchowski theories.
Electrophoresis
Electrophoretic velocity is proportional to electrophoretic mobility, which is the measurable
parameter. There are several theories that link electrophoretic mobility with zeta potential.
They are briefly described in the article on electrophoresis and in details in many
books on colloid and interface science. There is an IUPAC Technical Report prepared by a
group of world experts on the electrokinetic phenomena.
From the instrumental viewpoint, there are two different experimental techniques: microelectrophoresis
and electrophoretic light scattering. Microelectrophoresis has the advantage of yielding an image of
the moving particles. On the other hand, it is complicated by electro-osmosis at the walls
of the sample cell. Electrophoretic light scattering is based on dynamic light scattering.
It allows measurement in an open cell which eliminates the problem of electro-osmotic
flow for the case of an Uzgiris, but not a capillary cell. And, it can be used to characterize
very small particles, but at the price of the lost ability to display images of moving
particles. Both these measuring techniques may require
dilution of the sample. Sometimes this dilution might affect properties of the sample and
change zeta potential. There is only one justified way to perform this dilution - by using equilibrium
supernatant. In this case the interfacial equilibrium between the surface and the bulk
liquid would be maintained and zeta potential would be the same for all volume fractions
of particles in the suspension. When the diluent is known, additional diluent can be prepared.
If the diluent is unknown, equilibrium supernatant is readily obtained by centrifugation.
Electroacoustic phenomena
There are two electroacoustic effects that are widely used for characterizing zeta potential:
colloid vibration current and electric sonic amplitude, see reference. There are commercially
available instruments that exploit these effects for measuring dynamic electrophoretic mobility,
which depends on zeta potential. Electroacoustic techniques have the advantage
of being able to perform measurements in intact samples, without dilution. Published and well-verified
theories allow such measurements at volume fractions up to 50%, see reference. Calculation
of zeta potential from the dynamic electrophoretic mobility requires information on the densities
for particles and liquid. In addition, for larger particles exceeding roughly 300 nm
in size information on the particle size required as well.
Calculation of zeta potential The most known and widely used theory for
calculating zeta potential from experimental data is that developed by Marian Smoluchowski
in 1903. This theory was originally developed for electrophoresis; however, an extension
to electroacoustics is now also available. Smoluchowski's theory is powerful because
it is valid for dispersed particles of any shape and any concentration. However, it has
its limitations: Detailed theoretical analysis proved that
Smoluchowski's theory are valid only for a sufficiently thin double layer, when the Debye
length, 1/κ, is much smaller than the particle radius a:
The model of the "thin double layer" offers tremendous simplifications not only for electrophoresis
theory but for many other electrokinetic and electroacoustic theories. This model is valid
for most aqueous systems because the Debye length is typically only a few nanometers
in water. The model breaks only for nano-colloids in a solution with ionic strength approaching
that of pure water. Smoluchowski's theory neglects the contribution
of surface conductivity. This is expressed in modern theories as the condition of a small
Dukhin number:
The development of electrophoretic and electroacoustic theories with a wider range of validity was
a purpose of many studies during the 20th century. There are several analytical theories
that incorporate surface conductivity and eliminate the restriction of the small Dukhin
number for both the electrokinetic and electroacoustic applications.
Early pioneering work in that direction dates back to Overbeek and Booth.
Modern, rigorous electrokinetic theories that are valid for any zeta potential and often
any κa, stem mostly from Soviet Ukrainian and Australian schools. Historically, the
first one was Dukhin-Semenikhin theory. A similar theory was created 10 years later
by O'Brien and Hunter. Assuming a thin double layer, these theories would yield results
that are very close to the numerical solution provided by O'Brien and White. There are also
general electroacoustic theories that are valid for any values of Debye length and Dukhin
number. References