Definition of Zeta Potential

Zeta potential describes the electrokinetic potential between the solid particle and liquid phase of a colloid, such as this ferrofluid.
Zeta potential describes the electrokinetic potential between the solid particle and liquid phase of a colloid, such as this ferrofluid. PASIEKA, Getty Images

The zeta potential (ζ-potential) is the potential difference across phase boundaries between solids and liquids. It's a measure of the electrical charge of particles are that are suspended in liquid. Since zeta potential is not equal to the electric surface potential in a double layer or to the Stern potential, it is often the only value that can be used to describe double-layer properties of a colloidal dispersion.

 Zeta potential, also known as electrokinetic potential, is measured in millivolts (mV).

In colloids, zeta potential is the electric potential difference across the ionic layer around a charged colloid ion. Put another way, it's the potential in the interface double layer at the slipping plane. Typically, the higher the zeta-potential, the more stable the colloid. Zeta potential that are less negative than -15 mV typically represents the beginnings of agglomeration of particles. When the zeta-potential equals zero, the colloid will precipitate into a solid.

Measuring Zeta Potential

Zeta potential cannot be directly measured. It is calculated from theoretical models or estimated experimentally, often based on electrophoretic mobility. Basically, to determine zeta potential, one tracks that rate at which a charged particle moves in response to an electric field. Particles that possess a zeta potential will migrate toward the opposite-charged electrode.

The rate of migration is proportional to zeta potential. Velocity typically is measured using a Laser Doppler Anemometer. The calculation is based on a theory described in 1903 by Marian Smoluchowski. Smoluchowski's theory is valid for any concentration or shape of dispersed particles.  However, it assumes a sufficiently thin double layer and it ignores any contribution of surface conductivity.

Newer theories are used to perform electroacoustic and electrokinetic analyses under these conditions.

There is a device called a zeta meter -- it's expensive, but a trained operator can interpret the estimated values that it produces. Zeta meters typically rely on one of two electroacoustic effects: electric sonic amplitude and colloid vibration current. The advantage of using an electroacoustic method to characterize zeta potential is that the sample does not need to be diluted.

Applications of Zeta Potential

Since the physical properties of suspensions and colloids largely depend on the properties of the particle-liquid interface, knowing the zeta potential has practical applications.

Zeta potential measurements are used to:

  • Prepare colloidal dispersions for cosmetics, inks, dyes, foams, and other chemicals
  • Destroy undesirable colloidal dispersions during water and sewage treatment, preparation of beer and wine, and dispersing aerosol products
  • Reduce cost of additives by calculating the minimum amount needed to achieve a desired effect, such as amount of flocculant added to water during water treatment 
  • Incorporate colloidal dispersion during manufacturing, as in cements, pottery, coatings, etc.
  • Utilize desirable properties of colloids, which include capillary action and detergency. Properties may be applied for mineral flotation, impurity absorption, separating petroleum from reservoir rock, wetting phenomena, and electrophoretic deposition of paints or coatings
  • Microelectrophoresis to characterize blood, bacteria, and other biological surfaces
  • Characterize the properties of clay-water systems
  • Many other uses in mineral processing, ceramics manufacturing, electronics manufacturing, pharmaceutical production, etc.

References

American Filtration and Separations Society, "What Is Zeta Potential?"

Brookhaven Instruments, "Zeta Potential Applications".

Colloidal Dynamics, Electroacoustic Tutorials, "The Zeta Potential" (1999). 

M. von Smoluchowski, Bull. Int. Acad. Sci. Cracovie, 184 (1903).

Dukhin, S.S.

and Semenikhin, N.M. Koll. Zhur., 32, 366 (1970).