Electrophoresis

Electrophoresis

Overview
Electrophoresis, also called cataphoresis, is the motion of dispersed particles
Interface and colloid science
Interface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with colloids, heterogeneous systems consisting of a mechanical mixture of particles between 1 nm and 1000 nm dispersed in a continuous...

 relative to a fluid under the influence of a spatially uniform electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

. This electrokinetic phenomenon
Electrokinetic phenomena
Electrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids or in porous bodies filled with fluid. The term heterogeneous here means a fluid containing particles...

 was observed for the first time in 1807 by Reuss (Moscow State University
Moscow State University
Lomonosov Moscow State University , previously known as Lomonosov University or MSU , is the largest university in Russia. Founded in 1755, it also claims to be one of the oldest university in Russia and to have the tallest educational building in the world. Its current rector is Viktor Sadovnichiy...

), who noticed that the application of a constant electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 caused clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 particles dispersed in water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 to migrate. It is ultimately caused by the presence of a charged interface between the particle surface and the surrounding fluid.

The dispersed particles have an electric surface charge
Electric surface charge
Electric surface charge practically always appears on an object surface when it is placed into a fluid. All fluids contain ions, positive and negative . These ions interact with the object surface. This interaction might lead to the adsorption of some of them on the surface...

, on which an external electric field exerts an electrostatic Coulomb force.
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Encyclopedia
Electrophoresis, also called cataphoresis, is the motion of dispersed particles
Interface and colloid science
Interface and colloid science is an interdisciplinary intersection of branches of chemistry, physics, nanoscience and other fields dealing with colloids, heterogeneous systems consisting of a mechanical mixture of particles between 1 nm and 1000 nm dispersed in a continuous...

 relative to a fluid under the influence of a spatially uniform electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

. This electrokinetic phenomenon
Electrokinetic phenomena
Electrokinetic phenomena are a family of several different effects that occur in heterogeneous fluids or in porous bodies filled with fluid. The term heterogeneous here means a fluid containing particles...

 was observed for the first time in 1807 by Reuss (Moscow State University
Moscow State University
Lomonosov Moscow State University , previously known as Lomonosov University or MSU , is the largest university in Russia. Founded in 1755, it also claims to be one of the oldest university in Russia and to have the tallest educational building in the world. Its current rector is Viktor Sadovnichiy...

), who noticed that the application of a constant electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 caused clay
Clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

 particles dispersed in water
Water
Water is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...

 to migrate. It is ultimately caused by the presence of a charged interface between the particle surface and the surrounding fluid.

Theory


The dispersed particles have an electric surface charge
Electric surface charge
Electric surface charge practically always appears on an object surface when it is placed into a fluid. All fluids contain ions, positive and negative . These ions interact with the object surface. This interaction might lead to the adsorption of some of them on the surface...

, on which an external electric field exerts an electrostatic Coulomb force. According to the double layer
Double layer (interfacial)
A double layer is a structure that appears on the surface of an object when it is placed into a liquid. The object might be a solid particle, a gas bubble, a liquid droplet, or a porous body. The DL refers to two parallel layers of charge surrounding the object...

 theory, all surface charges in fluids are screened by a diffuse layer of ions, which has the same absolute charge but opposite sign with respect to that of the surface charge. The electric field also exerts a force on the ions in the diffuse layer which has direction opposite to that acting on the surface charge
Surface charge
Surface charge is the electric charge present at an interface. There are many different processes which can lead to a surface being charged, including adsorption of ions, protonation/deprotonation, and the application of an external electric field...

. This latter force is not actually applied to the particle, but to the ions in the diffuse layer located at some distance from the particle surface, and part of it is transferred all the way to the particle surface through viscous
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 stress
Stress (physics)
In continuum mechanics, stress is a measure of the internal forces acting within a deformable body. Quantitatively, it is a measure of the average force per unit area of a surface within the body on which internal forces act. These internal forces are a reaction to external forces applied on the body...

. This part of the force is also called electrophoretic retardation force.
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
Drag (physics)
In fluid dynamics, drag refers to forces which act on a solid object in the direction of the relative fluid flow velocity...

 on the moving particles due to the viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...

 of the dispersant, in the case of low Reynolds number and moderate electric field
Electric field
In physics, an electric field surrounds electrically charged particles and time-varying magnetic fields. The electric field depicts the force exerted on other electrically charged objects by the electrically charged particle the field is surrounding...

 strength E, the velocity of a dispersed particle v is simply proportional to the applied field, which leaves the electrophoretic mobility
Electrical mobility
Electrical mobility is the ability of charged particles to move through a medium in response to an electric field that is pulling them...

 μe defined as:


The most known and widely used theory of electrophoresis was developed in 1903 by Smoluchowski
Marian Smoluchowski
Marian Smoluchowski was an ethnic Polish scientist in the Austro-Hungarian Empire. He was a pioneer of statistical physics and an avid mountaineer.-Life:...


,

where εr is the dielectric constant
Dielectric constant
The relative permittivity of a material under given conditions reflects the extent to which it concentrates electrostatic lines of flux. In technical terms, it is the ratio of the amount of electrical energy stored in a material by an applied voltage, relative to that stored in a vacuum...

 of the dispersion medium, ε0 is the permittivity of free space (C² N−1 m−2), η is dynamic viscosity of the dispersion medium (Pa s), and ζ is zeta potential
Zeta potential
Zeta potential is a scientific term for electrokinetic potential in colloidal systems. In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta, hence ζ-potential...

 (i.e., the electrokinetic potential of the slipping plane in the double layer
Double layer
Double layer may refer to:* Double layer , a structure in a plasma and consists of two parallel layers with opposite electrical charge* Double layer , a structure that appears on the surface of an object when it is placed into a liquid...

).

The Smoluchowski theory is very powerful because it works for dispersed particles of any shape
Shape
The shape of an object located in some space is a geometrical description of the part of that space occupied by the object, as determined by its external boundary – abstracting from location and orientation in space, size, and other properties such as colour, content, and material...

 at any concentration
Concentration
In chemistry, concentration is defined as the abundance of a constituent divided by the total volume of a mixture. Four types can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration...

. Unfortunately, it has limitations on its validity. It follows, for instance, from the fact that it does not include Debye length
Debye length
In plasma physics, the Debye length , named after the Dutch physicist and physical chemist Peter Debye, is the scale over which mobile charge carriers screen out electric fields in plasmas and other conductors. In other words, the Debye length is the distance over which significant charge...

 κ−1. However, Debye length must be important for electrophoresis, as follows immediately from the Figure on the right. Increasing thickness of the double layer (DL) leads to removing point of retardation force further from the particle surface. The thicker DL, the smaller 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
Debye length
In plasma physics, the Debye length , named after the Dutch physicist and physical chemist Peter Debye, is the scale over which mobile charge carriers screen out electric fields in plasmas and other conductors. In other words, the Debye length is the distance over which significant charge...

 :
.

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 aqueous systems because the Debye length is only a few nanometers there. It breaks only for nano-colloids in solution with ionic strength
Ionic strength
The ionic strength of a solution is a measure of the concentration of ions in that solution. Ionic compounds, when dissolved in water, dissociate into ions. The total electrolyte concentration in solution will affect important properties such as the dissociation or the solubility of different salts...

 close to water.

The Smoluchowski theory also neglects contribution of surface conductivity
Surface conductivity
Surface conductivity is an additional conductivity of an electrolyte in the vicinity of charged surfaces. Close to charged surfaces a layer of counter ions of opposite polarity exists which is attracted by the surface charges. This layer of higher ionic concentration is a part of the interfacial ...

. This is expressed in modern theory as condition of small Dukhin number
Dukhin number
Dukhin number is a dimensionless quantity that characterizes the contribution of the surface conductivity to various electrokinetic and electroacoustic effects, as well as to electrical conductivity and permittivity of fluid heterogeneous systems....

:


In the effort of expanding the range of validity of electrophoretic theories, the opposite asymptotic case was considered, when Debye length
Debye length
In plasma physics, the Debye length , named after the Dutch physicist and physical chemist Peter Debye, is the scale over which mobile charge carriers screen out electric fields in plasmas and other conductors. In other words, the Debye length is the distance over which significant charge...

 is larger than particle radius:
.

Under this condition of a "thick Double Layer", Hückel
Erich Hückel
Erich Armand Arthur Joseph Hückel was a German physicist and physical chemist. He is known for two major contributions:*The Debye–Hückel theory of electrolytic solutions...

 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
Surface conductivity
Surface conductivity is an additional conductivity of an electrolyte in the vicinity of charged surfaces. Close to charged surfaces a layer of counter ions of opposite polarity exists which is attracted by the surface charges. This layer of higher ionic concentration is a part of the interfacial ...

 and eliminate the restriction of a small Dukhin number
Dukhin number
Dukhin number is a dimensionless quantity that characterizes the contribution of the surface conductivity to various electrokinetic and electroacoustic effects, as well as to electrical conductivity and permittivity of fluid heterogeneous systems....

, pioneered by Overbeek and Booth. Modern, rigorous theories valid for any Zeta potential
Zeta potential
Zeta potential is a scientific term for electrokinetic potential in colloidal systems. In the colloidal chemistry literature, it is usually denoted using the Greek letter zeta, hence ζ-potential...

 and often any stem mostly from Dukhin-Semenikhin theory. In the thin Double Layer limit, these theories confirm the numerical solution to the problem provided by O'Brien and White.

A recent set of molecular dynamics
Molecular dynamics
Molecular dynamics is a computer simulation of physical movements of atoms and molecules. The atoms and molecules are allowed to interact for a period of time, giving a view of the motion of the atoms...

 simulations have shown that even neutral particles can migrate in an electric field due to the molecular structure of water at the interface.

See also

  • Affinity electrophoresis
    Affinity electrophoresis
    Affinity electrophoresis is a general name for many analytical methods used in biochemistry and biotechnology. Both qualitative and quantitative information may be obtained through affinity electrophoresis. The methods include the so-called mobility shift electrophoresis, charge shift...

  • Capillary electrophoresis
    Capillary electrophoresis
    Capillary electrophoresis , also known as capillary zone electrophoresis , can be used to separate ionic species by their charge and frictional forces and hydrodynamic radius. In traditional electrophoresis, electrically charged analytes move in a conductive liquid medium under the influence of an...

  • Dielectrophoresis
    Dielectrophoresis
    Dielectrophoresis is a phenomenon in which a force is exerted on a dielectric particle when it is subjected to a non-uniform electric field. This force does not require the particle to be charged. All particles exhibit dielectrophoretic activity in the presence of electric fields...

  • DNA electrophoresis
    DNA electrophoresis
    Nucleic acid electrophoresis is an analytical technique used to separate DNA or RNA fragments by size and reactivity. Nucleic acid molecules which are to be analyzed are set upon a viscous medium, the gel, where an electric field induces the nucleic acids to migrate toward the anode, due to the net...

  • Electroblotting
    Electroblotting
    Electroblotting is a method in molecular biology/biochemistry/immunogenetics to transfer proteins or nucleic acids onto a membrane by using PVDF or nitrocellulose, after gel electrophoresis. The protein or nucleic acid can then be further analyzed using probes such as specific antibodies, ligands...

  • Electrofocusing
  • Gel electrophoresis
    Gel electrophoresis
    Gel electrophoresis is a method used in clinical chemistry to separate proteins by charge and or size and in biochemistry and molecular biology to separate a mixed population of DNA and RNA fragments by length, to estimate the size of DNA and RNA fragments or to separate proteins by charge...

  • Immunoelectrophoresis
    Immunoelectrophoresis
    Immunoelectrophoresis is a general name for a number of biochemical methods for separation and characterization of proteins based on electrophoresis and reaction with antibodies. All variants of immunoelectrophoresis require immunoglobulins, also known as antibodies reacting with the proteins to be...

  • Isotachophoresis
    Isotachophoresis
    Isotachophoresis is a technique in analytical chemistry used to separate charged particles. It is a further development of electrophoresis. It is a powerful separation technique using a discontinuous electrical field to create sharp boundaries between the sample constituents.In conventional...

  • Protein electrophoresis
    Protein electrophoresis
    Protein electrophoresis is a method for analysing the proteins in a fluid or an extract. The electrophoresis may be performed with a small volume of sample in a number of alternative ways with or without a supporting medium: SDS polyacrylamide gel electrophoresis Protein electrophoresis is a method...

  • Pulsed field gel electrophoresis
    Pulsed field gel electrophoresis
    Pulsed field gel electrophoresis is a technique used for the separation of large deoxyribonucleic acid molecules by applying an electric field that periodically changes direction to a gel matrix.-Historical background:...


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