Szent Istvan Bazilika Karolina Kajari Szent Istvan Bazilika, Budapest.
Istvan Banyai University of Debrecen Dept of Colloid and...
Transcript of Istvan Banyai University of Debrecen Dept of Colloid and...
Electrokinetic phenomena
Istvan BanyaiUniversity of Debrecen
Dept of Colloid and Environmental Chemistry
http://dragon.unideb.hu/~kolloid/
The electrical double layer at a charged surfaceA solid surface in contact with a solution of an electrolyte
usually carries an electric charge, σ0. This gives rise an electric potential, ψ0, at the surface, and a decreasing potential, ψ, as we move through the liquid away from the surface, and in turn this effect the distribution of ions in the liquid.
Two regions: The Stern Layer immediately adjacent to the surface where ion size is important; and outside this is a diffuse layer.
xSt
xSt
( )exp ( )St Stx xψ ψ κ= − −Because of difference in charge between the diffuse layer
and the solid surface, movement of one relative to the other will cause charge separation and hence generate a potential difference, or alternatively, application of an electrical potential will cause movement of one relative to the other. The relative movement of the solid surface and the liquid occurs at a surface of shear.The potential at the shear plane is known as the zeta potential and its value can be determined by measurement of electrokinetic phenomena. Zeta potential is almost identical with the Stern potential thus gives a measure of the potential at the beginning of the diffuse layer
Plane of shear
Electrokinetic potential
Shear plane
Positive particle with negative ion atmosphere
( )exp ( )St stx xψ ψ κ= − −
ζ
xst or xd~ distance of Stern plane from the surface
ψSt
Stζ ψ≈
Electrokinetic potential or zeta potential is the electrostatic potential in the plane of shear The shear plane is located close the
outer edge of the Stern layer so Stern potential is close to the zeta potential at low electrolyte concentration
Electrokinetic potential of particles
An electrical double layer exists around each particle.
The liquid layer surrounding the particle exists as two parts; an inner region (Stern layer) where the ions are strongly bound and an outer (diffuse) region where they are less firmly associated
Within this diffuse layer is a notional boundary known as the slipping plane, within which the particle acts as a single entity
within the slipping plane the particle acts as a single entity
Stern plane
Thickness of diffuse layer δ= 1/κ
interface
ζ2<ζ1
ζ1
ζ2
bulk
Electrokinetic potential1
2
3
1. Iron oxide 0,01 M KCl pH 4
2. Iron oxide 0.0001 M KCl pH 5
3. Iron oxide 0.001 MKCl pH 8.5 + cationic tenzid
Iron oxide pH PZC ~6.5
Stern plane
Shear plane
ζ1 = ζ2 = ζ3
0ψ
The value of zeta potential may differ significantly from ψ0but it has the same sign as ψSt
Stψ
distance
1. A high positive surface potential with a low to moderate adsorption of an ionic solute at theStern plane but with supporting electrolyte concentration to yield a thin diffuse layer.
2. Lower surface potential but still positive, little Stern layer adsorption and lowconcentration of electrolyte so that there is considerable extension of diffuse layer.
3. Negative but small surface potential, strong super-equivalent adsorption in the Stern planeand moderate extension of diffuse layer, i.e. moderate concentration of supporting electrolyte
Electrokinetic phenomena
1. electrophoresisParticles move
2. electroosmosisLiquid moves in capillary
3. Streaming potentialThe moving liquid generates potential (reverse of electroosmosis)
4. Sedimentation potentialMoving particles generate potential
Technique What Is measured What Moves What CausesMovement
Electrophoresis Velocity particles move applied electric field
Electroosmosis Velocity liquid moves incapillary applied electric field
Streaming Potential Potential liquid moves pressure gradient
SedimentationPotential Potential particles move gravity = gΔρ
Electrophoretic mobility
since
mobility
6 /
==
=
= = =
= =
el
fric
el fric
F QEF fv
F F
QE v Qv uf E fze zeu
r kT Dπη
( )0e C aεε ζμ κ
η=
where Fel the direct electric force, E is the magnitude of the electric field, and Q is the particle charge, μe electrophoretic mobility V/m, ε is the dielectric constant of the dispersion medium, ε0 is the permittivity of free space (C² N m-2), η is dynamic viscosity of the dispersion medium (Pas), and ζ is zeta potential (i.e., the electrokinetic potential of the slipping plane in the double layer) in V.
a
κ
Electrophoretic mobility Biochemical proof of protein-DNA interactions using EMSA (electrophoretic mobility shift assay) The method bases on the property that unbound DNA in a non-denaturated gel exhibits a higher electrophoretical mobility than protein-bound DNA.
Gel Electrophoresis
Polyacrylamide Gel Electrophoresis (PAGE)
http://www.steve.gb.com/science/chromatography_electrophoresis.html
Isoelectric focusing (IEF)
http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/Protein_Properties/protein_purification.htm
Isoelectric focusing employs a pH gradient extending the length of an electrophoresis gel. A protein stops migrating when it enters the zone in which the surrounding pH equals its isoelectric point, pI. At any other point in the gradient, the protein acquires a charge which causes it to migrate toward its pI (green and blue arrows).
The stable pH gradient between the electrodes is formed by including a mixture of low molecular weight 'carrier ampholytes' in the inert support. These are synthetic, aliphatic polyaminopolycarboxylic acids available commercially whose individual pI values cover a preselected pH range
Isoelectric focusing (IEF)
μe is electrophoretic mobility (EPM)
It is important to avoid molecular sieving effects so that the protein separation occurs solely on the basis of charge
The isoelectric point is the pH at which the zeta potential is zero. It is usually determined by pH titration: measuring zeta potential as a function of pH. The point of zero charge is the pH at which the positiveand negative charges of a zwitteric surface are balanced.
Capillary electrophoresis 2.
http://www.chemsoc.org/ExemplarChem/entries/2003/leeds_chromatography/chromatography/eof.htm
Schematic illustrating electroosmosis in a capillary. The circles indicate molecules and ions of the indicated charges, as well as their migration speed vector
Electroosmotic Flow
Flow profiles in microchannels. (a) A pressure gradient, -∇P, along a channel generates a parabolic or Poiseuille flow profile in the channel. The velocity of the flow varies across the entire cross-sectional area of the channel. On the right is an experimental measurement of the distortion of a volume of fluid in a Poiseuille flow. The frames show the state of the volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule.
(b) In electroosmotic (EO) flow in a channel, motion is induced by an applied electric field E. The flow speed only varies within the so-called Debye screening layer, of thickness λD. . On the right is an experimental measurement of the distortion of a volume of fluid in an EO flow. The frames show the state of the fluorescent volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule.
Electroosmosishttp://www.chemsoc.org/ExemplarChem/entries/2003/leeds_chromatography/chromatography/eof.htm
Another way to control the EOF (electro osmotic flow) is to modify the wall with coatings
(LB layers)
The capillary wall can be pretreated with a cationic surfactant andthe EOF will be reversed, that is, toward the anode
–The charged surface stands liquid moves
So far
Streaming potential
http://membranes.nist.gov/ACSchapter/toddPAGE.html
http://zeta-potential.sourceforge.net/zeta-potential.shtmlSedimentation potential and Electrodeposition
electrophoresis
Electroosmosis
Summary
Non-stoichiometric or ionic exchangeThe exchange takes place in a "resin bed" made up of tiny bead-like material. The beads, having a negative charge, attract and hold positively charged ions such as sodium, but will exchange them whenever the beads encounter another positively charged ion, such as calcium or magnesium minerals.
Cation, anion exchange, acid exchange, amphoteric surfaces
XR KA KR XA+ ↔ +
RY KA RA KY+ ↔ +