Electrophoresis

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For the process of administering medicine, see Iontophoresis.
For other uses, see Electrophoresis (disambiguation).
1. Illustration of electrophoresis

2. Illustration of electrophoresis retardation

In

cations) is sometimes called cataphoresis, while electrophoresis of negatively charged particles or molecules (anions) is sometimes called anaphoresis.[1][2][3][4][5][6][7][8]

Electrophoresis is the basis for analytical techniques used in biochemistry for separating particles, molecules, or ions by size, charge, or binding affinity.[9]

Biochemist Arne Tiselius won the Nobel Prize in Chemistry in 1948 "for his research on electrophoresis and adsorption analysis, especially for his discoveries concerning the complex nature of the serum proteins."[10]

In principle, electrophoresis is used in laboratories to separate macromolecules based on charge.[11] The technique normally applies a negative charge so proteins move towards a positive charge called anode. It is used extensively in DNA, RNA and protein analysis.[12]

History

This section is an excerpt from History of electrophoresis.[edit]
The
zone electrophoresis methods in the 1940s and 1950s, which used filter paper or gels as supporting media. By the 1960s, increasingly sophisticated gel electrophoresis methods made it possible to separate biological molecules based on minute physical and chemical differences, helping to drive the rise of molecular biology. Gel electrophoresis and related techniques became the basis for a wide range of biochemical methods, such as protein fingerprinting, Southern blot, other blotting procedures, DNA sequencing, and many more.[15]

Theory

Suspended particles have an

stress
. This part of the force is also called electrophoretic retardation force, or ERF in short. When the electric field is applied and the charged particle to be analyzed is at steady movement through the diffuse layer, the total resulting force is zero:

Considering the drag on the moving particles due to the viscosity of the dispersant, in the case of low Reynolds number and moderate electric field strength E, the drift velocity of a dispersed particle v is simply proportional to the applied field, which leaves the electrophoretic mobility μe defined as:[17]

The most well known and widely used theory of electrophoresis was developed in 1903 by Marian Smoluchowski:[18]

,

where εr is the

double layer
, units mV or V).

The Smoluchowski theory is very powerful because it works for

dispersed particles of any shape at any concentration. It has limitations on its validity. For instance, it does not include Debye length
κ−1 (units m). However, Debye length must be important for electrophoresis, as follows immediately from Figure 2, "Illustration of electrophoresis retardation". Increasing thickness of the double layer (DL) leads to removing the point of retardation force further from the particle surface. The thicker the DL, the smaller the retardation force must be.

Detailed theoretical analysis proved that the Smoluchowski theory is valid only for sufficiently thin DL, when particle radius a is much greater than the Debye length:

.

This model of "thin double layer" offers tremendous simplifications not only for electrophoresis theory but for many other electrokinetic theories. This model is valid for most

nanometers. It only breaks for nano-colloids in solution with ionic strength
close to water.

The Smoluchowski theory also neglects the contributions from surface conductivity. This is expressed in modern theory as condition of small Dukhin number:

In the effort of expanding the range of validity of electrophoretic theories, the opposite asymptotic case was considered, when Debye length is larger than particle radius:

.

Under this condition of a "thick double layer", Erich Hückel[19] predicted the following relation for electrophoretic mobility:

.

This model can be useful for some nanoparticles and non-polar fluids, where Debye length is much larger than in the usual cases.

There are several analytical theories that incorporate surface conductivity and eliminate the restriction of a small Dukhin number, pioneered by Theodoor Overbeek[20] and F. Booth.[21] Modern, rigorous theories valid for any Zeta potential and often any aκ stem mostly from Dukhin–Semenikhin theory.[22]

In the thin double layer limit, these theories confirm the numerical solution to the problem provided by Richard W. O'Brien and Lee R. White.[23]

For modeling more complex scenarios, these simplifications become inaccurate, and the electric field must be modeled spatially, tracking its magnitude and direction. Poisson's equation can be used to model this spatially-varying electric field. Its influence on fluid flow can be modeled with the Stokes law,[24] while transport of different ions can be modeled using the Nernst–Planck equation. This combined approach is referred to as the Poisson-Nernst-Planck-Stokes equations.[25] This approach has been validated the electrophoresis of particles.[25]

See also

References

  1. ^ Lyklema, J. (1995). Fundamentals of Interface and Colloid Science. Vol. 2. p. 3.208.
  2. ^ Hunter, R.J. (1989). Foundations of Colloid Science. Oxford University Press.
  3. ^ Dukhin, S.S.; Derjaguin, B.V. (1974). Electrokinetic Phenomena. J. Wiley and Sons.
  4. ISBN 9780521341882
    .
  5. ^ Kruyt, H.R. (1952). Colloid Science. Vol. 1, Irreversible systems. Elsevier.
  6. ISBN 978-0-444-63908-0
    .
  7. ISSN 0066-4189
    .
  8. ISBN 9783110761641
    .
  9. ^ Malhotra, P. (2023). Analytical Chemistry: Basic Techniques and Methods. Springer, ISBN 9783031267567. p. 346.
  10. ^ "The Nobel Prize in Chemistry 1948". NobelPrize.org. Retrieved 2023-11-03.
  11. doi:10.1002/elan.200403344
    .
  12. doi:10.1016/B978-012286230-4/50003-1
    .
  13. ^ Malhotra, P. (2023). Analytical Chemistry: Basic Techniques and Methods. Springer, ISBN 9783031267567. p. 346.
  14. doi:10.1039/TF9373300524
    .
  15. ^ Michov, B. (1995). Elektrophorese: Theorie und Praxis. De Gruyter, ISBN 9783110149944. p. 405.
  16. ^ Hanaor, D.A.H.; Michelazzi, M.; Leonelli, C.; Sorrell, C.C. (2012). "The effects of carboxylic acids on the aqueous dispersion and electrophoretic deposition of ZrO2". Journal of the European Ceramic Society. 32 (1): 235–244.
    S2CID 98812224
    .
  17. S2CID 98781292
    .
  18. ^ von Smoluchowski, M. (1903). "Contribution à la théorie de l'endosmose électrique et de quelques phénomènes corrélatifs". Bull. Int. Acad. Sci. Cracovie. 184.
  19. ^ Hückel, E. (1924). "Die kataphorese der kugel". Phys. Z. 25: 204.
  20. ^ Overbeek, J.Th.G (1943). "Theory of electrophoresis — The relaxation effect". Koll. Bith.: 287.
  21. S2CID 4115758
    .
  22. ^ Dukhin, S.S. and Semenikhin N.V. "Theory of double layer polarization and its effect on electrophoresis", Koll.Zhur. USSR, volume 32, page 366, 1970.
  23. doi:10.1039/F29787401607
    .
  24. PMID 16052651
    .
  25. ^
    S2CID 100357810
    .

Further reading

External links

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