Zeta potential



Zeta potential is an abbreviation for dispersed particle.

Zeta potential units are mV. The value of 25 mV can be taken as the boundary that separates low-charged surfaces from highly-charged surfaces.

The significance of zeta potential is that its value can be related to the stability of colloidal dispersions. Colloids with high zeta potential (negative or positive) are electrically stabilized. Colloids with low zeta potentials tend to flocculate as outlined in the table[1].

Zeta Potential [mV] Stablity behaviour 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 electrochemical potential (because electrochemical reactions are generally not involved in the development of zeta potential).

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 dynamic electrophoretic mobility.

electroacoustic phenomena are the usual sources of data for calculation of zeta potential.

Electrokinetic phenomena

Main article: Electrokinetic phenomena

streaming potential/current is used for porous bodies and flat surfaces.

Electrophoresis

Main article: Electrophoresis

Electrophoretic velocity is proportional to electrokinetic phenomena.

From the instrumental viewpoint, there are two different experimental techniques:

  • dynamic light scattering. It allows measurement in an open cell, which eliminates the problem of electro-osmotic flow, but at the price of the lost ability to display images of moving particles.

Both these measuring techniques require extreme dilution of the sample. 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 supernate. Only 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.

Streaming potential/current

Electroacoustic phenomena

There are two electroacoustic effects that are widely used for characterizing zeta potential : 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-verfied theories allow such measurements at volume fractions up to 50%, see reference[7].

On the other hand, electroacoustic methods yield only a single average value for zeta potential, whereas the two other methods mentioned above provide information on the distribution of zeta potential.

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 [9]. This theory was originally developed for concentration. However, it has its limitations:

  • Detailed theoretical analysis proved that Smoluchowski theory is valid only for a sufficiently thin DL, when the Debye length, 1/κ, is much smaller than the particle radius a:
{\kappa} \cdot a \gg 1
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-ionic strength approaching that of pure water.
  • Smoluchowski's theory neglects the contribution of Dukhin number:
Du \ll 1

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 Dukhin number for both the electrokinetic and electroacoustic applications.

Early pioneering work in that direction dates back to Overbeek [10] and Booth [11].

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 [12]. A similar theory was created 10 years later by O'Brien and Hunter [13]. Assuming a thin double layer, these theories would yield results that are very close to the numerical solution provided by O'Brien and White [14].

There are also general electroacoustic theories that are valid for any values of Dukhin number[7][3]. Modern instruments for determining zeta potential are expected to have an option for selecting between the possible algorithms (including those based on the most modern theories).

References

  1. ^ "Zeta Potential of Colloids in Water and Waste Water", ASTM Standard D 4187-82, American Society for Testing and Materials, 1985
  2. ^ Lyklema, J. “Fundamentals of Interface and Colloid Science”, vol.2, page.3.208, 1995
  3. ^ a b Hunter, R.J. "Foundations of Colloid Science", Oxford University Press, 1989
  4. ^ Dukhin, S.S. & Derjaguin, B.V. "Electrokinetic Phenomena", J.Willey and Sons, 1974
  5. ^ Russel, W.B., Saville, D.A. and Schowalter, W.R. “Colloidal Dispersions”, Cambridge University Press,1989
  6. ^ Kruyt, H.R. “Colloid Science”, Elsevier: Volume 1, Irreversible systems, (1952)
  7. ^ a b c d e Dukhin, A.S. and Goetz, P.J. "Ultrasound for characterizing colloids", Elsevier, 2002.
  8. ^ ”Measurement and Interpretation of Electrokinetic Phenomena”, International Union of Pure and Applied Chemistry, Technical Report, published in Pure Appl.Chem., vol 77, 10, pp.1753-1805, 2005
  9. ^ M. von Smoluchowski, Bull. Int. Acad. Sci. Cracovie, 184 (1903)
  10. ^ Overbeek, J.Th.G., Koll.Bith, 287 (1943)
  11. ^ Booth, F. Nature, 161, 83 (1948)
  12. ^ Dukhin, S.S. and Semenikhin, N.M. Koll.Zhur., 32, 366 (1970)
  13. ^ O'Brien, R.W. and Hunter, R.J. Can.J.Chem., 59, 1878 (1981)
  14. ^ O'Brien, R.W. and White, L.R. J.Chem.Soc.Faraday Trans. 2, 74, 1607, (1978)
 
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