ζ-potential
Origins of Surface Charge
1) Ionization of surface functional groups
Organic/molecular:
e.g. RCOOH <--> RCOO-, RNH2<--> RNH3+, etc
As in protein/peptide C-terminus, N-terminus,
certain side groups (aspartic acid, etc.)
Note: can be intrinsic to the particle and/or surfacefunctionalized/derivatized (biotin, etc.)
Inorganic/ionic:
e.g. SiOH <> SiO-)
(For example, glass beads, hydroxyapatite)
2) Adsorption of charged species
Charged/ionizable molecules:
e.g. surfactants, phospholipids
(For example: SDS, constituents of ECM)
Small ions:
e.g. Ca++, Mg++, etc.
(For example in certain physiological processes)
Particle dispersions
particles must be dispersed in the plating bath, no agglomeration
Stable dispersion
Electrostatic
repulsion forces
van der Waals and
other attraction forces.
For a stable dispersion the
particles must have a
large charge and a high
activation barrier
(affected by the zeta
potential, see next page)
Characteristics of Surface Charge: Definitions
Particle surface
Stern Layer: Rigid layer of ions
tightly bound to particle; ions travel
with the particle
Plane of hydrodynamic shear:
Also called Slipping Plane:
Boundary of the Stern layer:
ions beyond the shear plane do
not travel with the particle
Diffuse Layer:
Also called Electrical Double
Layer: Ionic concentration not the
same as in bulk; there is a gradient
in concentration of ions outward
from the particle until it matches
the bulk
When a colloidal particle moves
in the dispersion medium, a layer
of the surrounding liquid remains
attached to the particle. The
boundary of this layer is called
slipping plane (shear plane).
The value of the electric potential
at the slipping plane is called Zeta
potential, which is very important
parameter in the theory of
interaction of colloidal particles.
• Zeta potential is an abbreviation for electrokinetic potential in colloidal
systems. In the colloidal chemistry literature, it is usually denoted using
the Greek letter zeta, hence ζ-potential. From a theoretical viewpoint, zeta
potential is electric potential in the interfacial double layer (DL) at the
location of the slipping plane versus a point in the bulk fluid away from the
interface.
• In other words, zeta potential is the potential difference between the
dispersion medium and the stationary layer of fluid attached to the
dispersed particle.
• Zeta potential should not be confused with electrode potential or
electrochemical potential (because electrochemical reactions are generally
not involved in the development of zeta potential).
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Characteristics of Surface Charge: Definitions
Zeta potential:
The electrical
potential that
exists at the
slipping plane
The magnitude of the zeta potential gives an indication of the
potential stability of the colloidal system
* If all the particles have a large zeta potential they will repel each other
and there is dispersion stability
* If the particles have low zeta potential values then there is no force to
prevent the particles coming together and there is dispersion instability
A dividing line between stable and unstable aqueous dispersions is
generally taken at +30 or -30mV
Zeta Potential
• If all the particles have a large negative or positive zeta potential they will
repel each other and there is dispersion stability
• If the particles have low zeta potential values then there is no force to
prevent the particles coming together and there is dispersion instability
• A value of 25 mV (positive or negative) can be taken as the arbitrary value
that separates low-charged surfaces from highly-charged surfaces.
•
•
•
•
•
•
•
Zeta Potential [mV]
Stability behavior of the colloid:
from 0 to ±5, Rapid coagulation or flocculation
from ±10 to ±30 Incipient instability
from ±30 to ±40 Moderate stability
from ±40 to ±60 Good stability
more than ±61 Excellent stability
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The Influence of Zeta Potential
• Zeta Potential and pH
• The most important factor that affects zeta potential is pH
• A zeta potential value quoted without a definition of it's environment (pH,
ionic strength, concentration of any additives) is a meaningless number
• Imagine a particle in suspension with a negative zeta potential If more
alkali is added to this suspension then the particles tend to acquire more
negative charge
• If acid is added to this suspension then a point will be reached where the
charge will be neutralized
• Further addition of acid will cause a build up of positive charge
• In general, a zeta potential versus pH curve will be positive at low pH and
lower or negative at high pH
• There may be a point where the curve passes through zero zeta potential
• This point is called the isoelectric point and is very important from a
practical consideration
• It is normally the point where the colloidal system is least stable
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Zeta Potential vs pH
Typical plot of Zeta Potential vs
pH.
pH dependency of ZP
is very important!
Zeta Potential, mV
Remember, dispersion
stability (or
conversely, ability of
particles to approach
each other) is
determined by ZP, with
~ 30 mV being the
approximate cutoff.
pH
At ZP=0, net charge on particle is 0.
This is called the isoelectric point
[In this example, the
dispersion is stable
below pH ~4 and above
pH ~7.5]
In the above example it can be seen that if the dispersion pH is below 4 or
above 8 there is sufficient charge to confer stability. However if the pH of the
system is between 4 and 8 the dispersion may be unstable. This is most likely
to be the case at around pH 6 (the isoelectric point)
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Zeta Potential and Electrolyte Concentration
Zeta potential also depends on electrolyte
concentration! Remember that the ionic environment of
the particle exists as a gradient that that eventually
equilibrates with the bulk solution.
Too few ions: not enough charge to stabilize the particles
Too many ions: the double layer is compressed and
the particles can approach (“salting out”)
• Methods for experimental determination of
zeta potential
• Zeta potential is not measurable directly but it
can be calculated using theoretical models
and
an
experimentally-determined
electrophoretic
mobility
or
dynamic
electrophoretic mobility.
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• Electrophoretic velocity is proportional to
electrophoretic mobility, which is the
measurable parameter. There are several
theories that link electrophoretic mobility
with zeta potential. They are briefly described
in the article on electrophoresis and in details
in many books on Colloid and Interface
Science.
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• Electrophoresis: The movement of a charged
particle relative to the liquid it suspended in
under the influence of an applied electric field
• This technique finds application in the
measurements of zeta potentials of model
systems (like polystyrene latex dispersion) to
test colloidal stability theory
• To asses the stability of coarse dispersion
• In identification of charge groups
• The particles move with a characteristic
velocity which is dependent on the strength of
the electric field (measured by the
instrument), the dielectric constant and the
viscosity of the medium (known from
literature) and the zeta potential
• The velocity of a particle in a unit electric field
is referred to as its electrophoretic mobility
Determination of Zeta Potential
 Measure the Electrophoretic Mobility, UE
(and know viscosity, dielectric constant; and choose a Henry function)
 Solve Smoluchowski/Huckel Equation for
Zeta Potential
Predominant Methods:
 Laser Doppler Velocimetry
 Phase Analysis Light Scattering (PALS)
Method for particles with lower mobilities
Zeta Potential and Electrophoretic Mobility
In an applied electric field, charged particles travel
toward the electrode of opposite charge.
When attractive force of the electric field is balanced
by the viscous drag on the particle, the particle
travels with constant velocity.
+
+
-
-
This velocity is the partlcle’s electrophoretic mobility, UE
UE = 2  z f(Ka)/3 
z = Zeta potential
 = dielectric constant (of electrolyte)
= dielectric constant (of electrolyte)
f(Ka) = Henry’s function
= ~1.5 (Smoluchowski approximation)
for particles >~ 200 nm and electrolyte ~> 1 x 10-3 M
= ~1.0 (Huckel approximation)
for smaller particles and/or dilute/non-aqueous dispersions
• Zeta Potential (Smoluchowski’s Formula)
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Example: Zeta Potential Measurements
Zeta potential
Particle diameter
Optimizing a
process for
preparing human
serum albumin
nanoparticles
(from the assigned
paper, K.Langer et al.)
At low values of
Zeta potential
(near pH 6), the
dispersion destabilizes and
the particles
agglomerate