Electrokinetic phenomena

Electrokinetic phenomena
István Bányai
University of Debrecen
Dept of Colloid and Environmental Chemistry
http://kolloid.unideb.hu
Non-equivalent ion adsorption
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
The electrical double layer at a charged surface
A solid surface in contact with a solution of an electrolyte
usually carries an electric charge, s0. 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
   St exp   ( x  xSt ) 
xSt
Plane of shear
xSt
Electrokinetic potential
   St exp   ( 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/surface with
negative ion atmosphere
   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 d= 1/
within the slipping plane the
particle acts as a single entity
bulk
2<1
1
2
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 When a particle
moves (e.g. due to gravity), ions within the boundary move with it, but any ions beyond the boundary do not travel with the particle. This
boundary is called the surface of hydrodynamic shear or slipping plane. The potential that exists at this boundary is known as the Zeta
potential.
Electrokinetic potential: relation to surface and
Stern-potential
1
0 
St
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 0
but it has the same sign as St
3
1 = 2 = 3
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 in practice
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 = gDr
1. electrophoresis
The movement of a charged particle relative to the liquid it is suspended in under
the influence of an applied electric field.
2. electroosmosis
The movement of a liquid relative to a stationary charged surface under the
influence of an applied electric field.
3. Streaming potential
The electric field generated when a liquid is forced to flow past a stationary
charged surface.
4. Sedimentation potential
The electric field generated when charged particles move relative to a stationary
liquid.
Electrophoretic mobility
Fel  QE

Ffric  fv
since Fel  Ffric
QE
v
mobility
f
ze
ze
u

6 r kT / D
a
v Q
u 
E f
 0
e 
C  a 

C(a) Henry function 1.5-1
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.
Electrophoresis (principle)
gélben
Electrophoretic mobility
Biochemical proof of proteinDNA 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)
e 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 (electro osmoric flow)
Capillary electrophoresis 2.
The walls of the capillary cell carry a surface charge so the application of the electric field causes the
liquid adjacent to the walls to undergo electroosmotic flow. Colloidal particles will be subject to this
flow superimposed on their electrophoretic mobility.
Move in capillary
Neutral
electrophoretic mobility:
surface potential (zeta potential), size
Electro osmotic flow
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
lD. . 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: change in direction
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
Fal szárítása
Summary in cartoons
Electroosmosis
electrophoresis
Streaming potential
http://membranes.nist.gov/ACSchapter/toddPAGE.html
http://zeta-potential.sourceforge.net/zeta-potential.shtml
Sedimentation potential and Electrodeposition
Application in cosmetics: hair (Anton Paar
slide)
• sampoo
• conditioner
• color
If you check the ingredients list of a hair detangler, you'll likely see one or more of
these ingredients:
Silicone (e.g., dimethicone or cyclomethicone) - polymer that adds gloss to hair by
kondícionáló
binding to its surface.
Acidifier - chemical that lowers the pH of the detangler, strengthening the hydrogen
bonds between keratin molecules in hair, smoothing and tightening each strand.
Hydrolyzed Protein - helps to repair damaged keratin, smoothing the broken edges
so strands of hair don't catch on each other as much.
öblítés
Cationic Surfactants - Bind sampon
to the negatively-charged
keratin, becoming the new
smoother surface of the hair.
Oil - Oils fill in the pores of dry or damaged hair, making it softer, more pliable, and
öblítés
less likely to tangle.
adsorption on Caucasian virgin hair
0.001 mol/l KCl
0.4% shampoo
0.4% conditioner
http://chemistry.about.com/od/healthbeauty/a/Hair-Detangler.htm
Colloid stability
Thermodynamically stable, unstable
Solutions are thermodynamically stable
Dispersions are thermodynamically unstable
Kinetically stable system
Direction of spontaneous changes toward
states with lower Gibbs energy
Protect dispersion particles against aggregation /
Delta G (constant P,T) <0
coagulation / flocculation/ coalescence by repulsive
interactions
Gsol > Ginitial Sols , because of large specific surface
area
A most important physical property of colloid dispersions is
the tendency of particles to aggregate.
STABLE, UNSTABLE
Solutions: mixing is spontaneous
Mixtures are thermodynamically
stable.
Inhomogeneities on molecular levels
Mixing is reversible
Properties of solutions
independent on the way is
prepared
Dispersions: mixing is nonspontaneous (requires mechanical
energy) Thermodynamically unstable
(requires stabilizing agent) i.e. unmix
spontaneously. Inhomogeneities on
length scales large compared to
molecular dimensions Mixing is
irreversible
Properties are strongly dependent
on the way is prepared
Empirical preparations procedures
Kinetically stable* or unstable systems
In an unstable system the particles
may adhere to one another and form
aggregates of increasing size that
may settle out under the influence of
gravity. An initially formed
aggregate is called a floc and the
process of its formation flocculation.
The floc may or may not separate out.
If the aggregate changes to a much
denser form, it is said to undergo
coagulation. An aggregate usually
separates out either by sedimentation
(if it is more dense than the medium)
or by creaming (if it less dense than
the medium). The term’s flocculation
and coagulation have often been used
interchangeably. Usually coagulation
is irreversible whereas flocculation
can be reversed by the process of
deflocculation.
*In a kinetically stable system dispersity, morphology and spatial distribution are
unchanged within the examination. This will depend upon the balance of the repulsive
and attractive forces that exist between particles as they approach one another.
Coagulation or flocculation
much denser
form,
irreversible
a floc
loosely adhering
mass, it can be
reversed by
deflocculation
a cake
a) coagulated, b) flocculated particles
b) The suspended particles form light, fluffy agglomerates held together
by strong van der Waals forces. The flocculated particles settle rapidly
forming a loosely adhering mass with a large sediment height instead of a
cake. Gentle agitation will easily resuspend the particles. Weak
flocculation requires strong adhesion and a zeta potential of almost
zero.
thistle's flowerheads with sharp prickles give a loosely adhering mass
Strength of interparticle forces
Encounters between particles occur as
a result of Brownian motion and
stability of a suspension is determined
by the interaction between particles
during these encounters
Stability depends on the balance of
attractive and repulsive interactions
There is no repulsion
Attraction from van der Waals forces
between particles.
Repulsion is a consequence of
interaction between similar charged
electric double layers and/or particle
-solvent affinity. Repulsion prevents
particles to get close enough and
attach
There is repulsion
Stability of lyophobic* colloids
• DLVO theory (Derjaguin, Landau and Verwey, Overbeek):
stability of lyophobic colloids
– The double layer, zeta potential, (repulsive)
- Wan der Waals interactions (attractive)
- the balance of repulsion & attraction, the energy barrier.
– Rates of flocculation
*Lyophobic colloid includes charged particles of weak solvent affinity
This will depend upon the balance of the repulsive and attractive forces that exist between
particles as they approach one another. If all the particles have a mutual repulsion then the
dispersion will remain stable. However, if the particles have little or no repulsive force then
some instability mechanism will eventually take place e.g. flocculation, aggregation etc.