Istvan Banyai University of Debrecen Dept of Colloid and...

Electrokinetic phenomena Istvan Banyai University of Debrecen Dept of Colloid and Environmental Chemistry

Transcript of Istvan Banyai University of Debrecen Dept of Colloid and...

Electrokinetic phenomena

Istvan BanyaiUniversity of Debrecen

Dept of Colloid and Environmental Chemistry

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.



( )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= − −


xst or xd~ distance of Stern plane from the surface


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






Electrokinetic potential1



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

The value of zeta potential may differ significantly from ψ0but it has the same sign as ψSt



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



6 /



= = =

= =



el fric

F QEF fv


QE v Qv uf E fze zeu

r kT Dπη

( )0e C aεε ζμ κ


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.



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)

Isoelectric focusing (IEF)

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 1

Capillary electrophoresis 2.

Move in capillary

electrophoretic mobility:surface potential (zeta potential), size


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.


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 potential and Electrodeposition




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+ ↔ +

Water softener

Zeolit, clays, resins

It can be regenerated

A water softener reduces the dissolved calcium, magnesium, in hard water.