Zeta potential
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Zeta potential is an abbreviation for electrokinetic potential in colloidal systems. In the colloidal
chemistry literature, it is usually denoted using the Greek letter zeta, hence ζ-potential. From a
theoretical viewpoint, zeta potential is electric potential in the interfacial double layer (DL) at the
location of the slipping plane versus 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.
A value of 25 mV (positive or negative) can be taken as the arbitrary value that separates lowcharged surfaces from highly-charged surfaces.
The significance of zeta potential is that its value can be related to the stability of colloidal
dispersions (e.g. a multivitamin syrup). The zeta potential indicates the degree of repulsion
between adjacent, similarly charged particles (the vitamins) 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 low, attraction exceeds repulsion and the
dispersion will break and flocculate. So, colloids with high zeta potential (negative or positive)
are electrically stabilized while colloids with low zeta potentials tend to coagulate or flocculate
as outlined in the table.[1]
Zeta Potential [mV] Stability behavior of the colloid
from 0 to ±5,
Rapid coagulation or flocculation
from ±10 to ±30
Incipient instability
from ±30 to ±40
Moderate stability
from ±40 to ±60
Good stability
more than ±61
Excellent stability
Zeta potential is widely used for quantification of the magnitude of the electrical charge at the
double layer. However, zeta potential is not equal to the Stern potential or electric surface
potential in the double layer. Such assumptions of equality should be applied with caution.
Nevertheless, zeta potential is often the only available path for characterization of double-layer
properties. Zeta potential should not be confused with electrode potential or electrochemical
potential (because electrochemical reactions are generally not involved in the development of
zeta potential).
Contents
[hide]




1 Methods for experimental determination of zeta potential
o 1.1 Electrokinetic phenomena
 1.1.1 Electrophoresis
o 1.2 Electroacoustic phenomena
2 Theory for Zeta potential calculation
3 References
4 External links
[edit] Methods for experimental determination 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.
[edit] Electrokinetic phenomena
Main article: 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 (see
below) [2][3] .
[edit] Electrophoresis
Main article: 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,[4],[5],[6],[7].[8][9] There is an IUPAC Technical Report[10] prepared by a group of
world experts on the electrokinetic phenomena.
From the instrumental viewpoint, there are two different experimental techniques:

Microelectrophoresis. It 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. It 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 (as is the case for a chemical formulation), additional diluent can be prepared. If the
diluent is unknown, equilibrium supernatant is readily obtained by centrifugation.
[edit] Electroacoustic phenomena
Main article: Electroacoustic phenomena
There are two electroacoustic effects that are widely used for characterizing zeta potential:
Colloid Vibration Current and Electric Sonic Amplitude, see reference.[9] 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. However, in practice these theories require significant
knowledge about the sample, including the particle size and size distribution along with the
mechanical properties of both the particles of interest and the diluent.[9] Often these properties
are unknown or poorly known.
[edit] Theory for Zeta potential calculation
The most known and widely-used theory for calculating zeta potential from experimental data is
that developed by Smoluchowski in 1903.[11] This theory was originally developed for
electrophoresis; however, an extension to electroacoustics is now also available.[9]
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 is valid only for a
sufficiently thin DL, 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 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[12] and Booth.[13]
Modern, rigorous electrokinetic theories that are valid for any zeta potential and often any κa,
stem mostly from the Ukrainian (Dukhin, Shilov and others) and Australian (O'Brien, White,
Hunter and others) schools. Historically, the first one was Dukhin-Semenikhin theory.[14] A
similar theory was created 10 years later by O'Brien and Hunter.[15] Assuming a thin double
layer, these theories would yield results that are very close to the numerical solution provided by
O'Brien and White.[16] There are also general electroacoustic theories that are valid for any
values of Debye length and Dukhin number.[9][5]
All these theories predict electrophoretic mobility and zeta potential to be equal in sign. Recent
molecular dynamics simulations, though, suggest that the main contribution to the zeta potential
can arise from anisotropic water dipole at the interface not included in the traditional continuum
theories and that electrophoretic mobility and zeta potential may in fact be opposite in sign.