gaber zeta potential
DESCRIPTION
Zeta potentialTRANSCRIPT
Zeta Potential and particle size
By
Professor Mohamed H. Gaber
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 surface-
functionalized/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 DPPC)
Small ions:
e.g. Ca++, Mg++, etc.
(For example in certain physiological processes)
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
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
Particle surface
Characteristics of Surface Charge: Definitions
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 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.
UE = 2 z f(Ka)/3
= dielectric constant (of electrolyte)
= viscosity (of electrolyte)
f(Ka) = Henry’s function
= ~1.5
for particles >~ 200 nm and electrolyte ~> 1 x 10-3 M
= ~1.0
for smaller particles and/or dilute/non-aqueous dispersions
z = Zeta potential
+ - +
-
This velocity is the partlcle’s electrophoretic mobility, UE
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
Determination of Zeta Potential
Principles of PALS:
Similar to particle sizing by dynamic light scattering
I.e. what is measured is temporal fluctuations in intensity of light
scattered by the particles in the dispersion.
In light scattering, the fluctuations are related to
Brownian motion of particles.
In PALS for ZP, the fluctuations are related to the
movement of the particle in the applied field, i.e. to UE;
The ZP is then calculated from the UE that is
determined by the PALS measurement.
(As in light scattering, the instrument’s autocorrelator
and software take care of the data reduction.)
Zeta Potential vs pH
pH dependency of ZP
is very important!
Remember, dispersion
stability (or
conversely, ability of
particles to approach
each other) is
determined by ZP, with
~ 30 mV being the
approximate cutoff.
[In this example, the dispersion is stable below pH ~4 and above pH ~7.5]
At ZP=0, net charge on particle is 0.
This is called the isoelectric point
Typical plot of Zeta Potential vs
pH.
Ze
ta P
ote
nti
al,
mV
pH
Example: Zeta Potential Measurements
Particle diameter Zeta potential
At low values of
Zeta potential
(near pH 6), the
dispersion de-
stabilizes and
the particles
agglomerate
Optimizing a
process for
preparing human
serum albumin
nanoparticles
(from the assigned
paper, K.Langer et al.)
