1. COLLOID: ZETA POTENTIAL
Physical Pharmacy 2
4/25/2017
COLLOID: ZETA POTENTIAL
Kausar Ahmad
Physical Pharmacy 2
KBA

2. CONTENTS Types of electrokinetic phenomena Measuring zeta potential
Traditional microelectrophoresis
Laser Doppler velocimetry
Applications
Physical Pharmacy 2

3. ELECTROKINETIC PHENOMENA
Electrophoresis
The movement of charged colloidal particles in electric field.
Most practical.
Electroosmosis
When the charged solid surface is fixed, the electric field causes a movement of the liquid
Streaming potential
Forcing a liquid through a capillary or porous plug induces a difference of electric potentials
Sedimentation potential
Forced movement of charged solid particles in a liquid, e.g., due to gravitation induces a difference of electric potentials
Physical Pharmacy 2

4. ELECTRICAL POTENTIALS
0 - potential at charged surface
s - potential at Stern layer
 - potential at plane of shear – ZETA POTENTIAL
Only zeta potential can be determined experimentally.
Both 0 and s are thermodynamic and theoretical quantities - calculated from theory only.
Physical Pharmacy 2

5. ZETA POTENTIAL
The shear plane separates the thin layer of liquid bound to the particle surface (elastic behavior) from the rest of the continuous phase (normal viscous behavior)
Physical Pharmacy 2

6. DETERMINATION OF ZETA POTENTIAL
Measure electrophoretic mobility
the electrophoretic mobility is the ratio of the velocity of particles to the field strength
= v/E
v - Particle velocity
E - strength of electrical field
Physical Pharmacy 2

7. TRADITIONAL MEASUREMENT OF ELECTROPHORETIC MOBILITY
An electrophoresis system consists of a capillary cell with electrodes at either end to which a potential is applied.
Observe individual particles using a microscope and time their transit across a graticule.
Physical Pharmacy 2

8. CALCULATION OF ZETA POTENTIAL SMOLUCHOWSKI EQUATION
Physical Pharmacy 2
4/25/2017
CALCULATION OF ZETA POTENTIAL SMOLUCHOWSKI EQUATION
From Marian Smoluchowski:
m=ez/h
m electrophoretic mobility, e electric permittivity of the liquid, is the viscosity
applicable for
thin double layer
when the zeta potential is not too high
large colloidal particles and high ionic strengths
thin double layers (as compared with the particle radius): ka>>100
Physical Pharmacy 2
KBA

9. CALCULATION OF ZETA POTENTIAL HUCKEL EQUATION
Physical Pharmacy 2
4/25/2017
CALCULATION OF ZETA POTENTIAL HUCKEL EQUATION
IF size of particle is small compared to EDL (or thick EDL)
Use Huckel equation: m=ez/1.5h
Thickness of EDL, 1/κ
When the size of the particle is smaller compared to 1/k (ka<<1)
Physical Pharmacy 2
KBA

10. LASER DOPPLER VELOCIMETRY
Physical Pharmacy 2
4/25/2017
LASER DOPPLER VELOCIMETRY
Young’s interference fringes formed at stationary level
Two coherent beams derived from the output of a low power helium-neon laser, are aligned at the stationary layer in the cell.
At the crossing point of the beams (at the stationary layer), Young's interference fringes of known spacing are formed.
Particles that are moving through the fringes under the influence of the applied electric field, scatter light whose intensity fluctuates with a frequency that is related to the particles velocity.
The signals received from the scattered light are captured by a photomultiplier and analysed by a digital correlator.
Using Fourier Transform, the correlator produces a
frequency spectrum from which the mobility distribution and
hence zeta potential are calculated.
Physical Pharmacy 2
KBA

11. FLUCTUATION IN INTENSITY OF SCATTERED LIGHT
Physical Pharmacy 2
4/25/2017
FLUCTUATION IN INTENSITY OF SCATTERED LIGHT
Fluctuation in intensity of scattered light is related to particle velocity.
The higher the fluctuation, the higher the velocity, the higher the charge.
From:
Physical Pharmacy 2
KBA

12. ADVANTAGES OF LASER TECHNIQUE
Physical Pharmacy 2
4/25/2017
ADVANTAGES OF LASER TECHNIQUE
High speed
Applicable for non-aqueous, highly conductive, high ionic strength environment
Can detect particles
< 100 nm
Does not differentiate particles i.e. even small ones are detected.
Measures over thousands of particles
Physical Pharmacy 2
KBA

13. EXAMPLES OF APPLICATION
Physical Pharmacy 2
4/25/2017
EXAMPLES OF APPLICATION
Investigate electrophoretic properties of blood lipid particles in connection to potential heart problems.
Control of size and zeta potential of droplets of artificial blood
The relationship between the zeta potential of certain cells in amniotic fluid, and lung maturity (suggested by a study in Holland).
Determination of IEPs are carried out to e.g. confirm the pH for flocculation/coagulation.
Determination of CRC can be used to identify the nature of surface groupings e.g. sulfates, carboxylates etc
Physical Pharmacy 2
KBA

14. ISOELECTRIC POINT (IEP)
Physical Pharmacy 2
4/25/2017
ISOELECTRIC POINT (IEP)
If pH<4 or pH>8 there is sufficient charge to confer stability
If 4<pH<8 dispersion may be unstable.
Most unstable at around pH 6 (IEP)
From:
Physical Pharmacy 2
KBA

15. Physical Pharmacy 2
4/25/2017
ZETA POTENTIAL OF CHLORAMPHENICOL AND GLASS PARTICLES IN BENZETHONIUM (+) CHLORIDE SOLUTION (ADSORBING CATION)
Charge reversal concentration
Irrespective of the charge of the particles, adsorbing ions will cover the surface of the particles and will influence the zeta potential.
Non-adsorbing ions such as simple cations will change the zeta potential by changing the thickness of the double layer
For adsorbing ions, depending on the surface groupings, the charge may reverse at a certain concentration.
This is known as the charge reversal concentration (CRC).
From: Florence & Attwood
Physical Pharmacy 2
KBA

16. Physical Pharmacy 2
4/25/2017
REFERENCES
RJ Hunter, Zeta Potential in Colloid Science, Academic Press (1988) RJ Hunter, Foundations of Colloid Science Volume 2, Clarendon Press Oxford (1989) ID Morrison & S Ross, Colloidal Dispersions, Wiley-Interscience, New York (2002)
Physical Pharmacy 2
KBA