Download Electrokinetic phenomena

Electrokinetic phenomena
Istvan Banyai
University of Debrecen
Dept of Colloid and Environmental Chemistry
http://dragon.unideb.hu/~kolloid/
The electrical double layer at a charged surface
A 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.
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
ψ = ψ St exp ( −κ ( x − xSt ) )
xSt
Plane of shear
xSt
Electrokinetic potential
ψ = ψ St exp ( −κ ( x − xst ) )
ψSt
ζ
Shear plane
xst or xd~ distance of Stern plane from the
surface
Electrokinetic potential or zeta
potential is the electrostatic potential
in the plane of shear
Positive particle with negative
ion atmosphere
ζ ≈ ψ St
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
Thickness of diffuse layer δ= 1/κ
within the slipping plane the
particle acts as a single entity
bulk
ζ2<ζ1
ζ1
ζ2
Stern plane
interface
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
1
ψ0 ψ
St
Electrokinetic potential
Shear plane
Iron oxide pH PZC ~6.5
1. Iron oxide 0,01 M KCl pH 4
2. Iron oxide 0.0001 M KCl pH 5
2
distance
3. Iron oxide 0.001 MKCl pH 8.5 +
cationic tenzid
The value of zeta potential may differ significantly from ψ0
but it has the same sign as ψSt
3
ζ1 = ζ2 = ζ3
Stern plane
1. A high positive surface potential with a low to moderate adsorption of an ionic solute at the
Stern plane but with supporting electrolyte concentration to yield a thin diffuse layer.
2. Lower surface potential but still positive, little Stern layer adsorption and low
concentration of electrolyte so that there is considerable extension of diffuse layer.
3. Negative but small surface potential, strong super-equivalent adsorption in the Stern plane
and moderate extension of diffuse layer, i.e. moderate concentration of
supporting electrolyte
Electrokinetic phenomena
Technique
What Is measured
What Moves
What Causes
Movement
Electrophoresis
Velocity
particles move
applied electric field
Electroosmosis
Velocity
liquid moves in
capillary
applied electric field
Streaming Potential
Potential
liquid moves
pressure gradient
Sedimentation
Potential
Potential
particles move
gravity = gΔρ
1. electrophoresis
Particles move
2. electroosmosis
Liquid moves in capillary
3. Streaming potential
The moving liquid generates potential (reverse of electroosmosis)
4. Sedimentation potential
Moving particles generate potential
Electrophoretic mobility
Fel = QE
κ
Ffric = fv
since Fel = Ffric
QE
mobility
v=
f
ze
ze
=
u=
6πη r kT / D
a
v Q
u= =
E f
εε0ζ
μe =
C(κa)
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.
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)
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).
http://www.biochem.arizona.edu/classes/bioc462/462a/N
OTES/Protein_Properties/protein_purification.htm
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 positive
and negative charges of a zwitteric surface are balanced.
Capillary electrophoresis 1
Capillary electrophoresis 2.
http://www.chemsoc.org/ExemplarChem/entries/2003/leeds_chromatography/chromatography/eof.htm
Move in capillary
Neutral
electrophoretic mobility:
surface potential (zeta potential), size
Electroosmotic Flow
Schematic illustrating electroosmosis in a capillary.
The circles indicate molecules and ions of the indicated
charges, as well as their migration speed vector
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.
Electroosmosis
http://www.chemsoc.org/ExemplarChem/entries/2003/leeds_chromatography/chromatography/eof.htm
The capillary wall can be pretreated
with a cationic surfactant and
the EOF will be reversed, that is,
toward the anode
(LB layers)
Another way to control the EOF
(electro osmotic flow) is to
modify the wall with coatings
So far
–The charged surface stands liquid
moves
Summary
Electroosmosis
electrophoresis
Streaming potential
http://membranes.nist.gov/ACSchapter/toddPAGE.html
http://zeta-potential.sourceforge.net/zeta-potential.shtml
Sedimentation potential and Electrodeposition
Non-stoichiometric or ionic exchange
The 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.
XR + KA ↔ KR + XA
RY + KA ↔ RA + KY
Cation, anion exchange, acid exchange,
amphoteric surfaces
Water softener
A water softener reduces the dissolved calcium, magnesium, in hard
water.
Zeolit, clays, resins
It can be regenerated