1. Colloid: Electrokinetic properties
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Colloid: Electrokinetic properties
Kausar Ahmad
Kulliyyah of Pharmacy
Physical Pharmacy 2
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2. Contents Types of electrokinetic phenomena Measuring zeta potential
Traditional microelectrophoresis
Laser Doppler velocimetry
Applications
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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
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4. Potentials 0 - potential at charged surface
s - potential at Stern layer
 - potential at plane of shear.
Only zeta potential can be determined experimentally.
Both 0 and s are thermodynamic and theoretical quantities and are calculated from theory only.
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5. Zeta potential
The slipping/ shear plane separates the thin layer of liquid bound to the solid surface (elastic behavior) from the rest of liquid (normal viscous behavior).
The electric potential at the shear plane is called zeta potential.
From
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6. Determination of zeta potential
Measure electrophoretic mobility
the electrophoretic mobility is the ratio of the velocity of particles to the field strength
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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.
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8. Smoluchowski equation
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Smoluchowski equation
From Marian Smoluchowski:
m=ez/h
m electrophoretic mobility
e electric permittivity of the liquid
is the viscosity
applies 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
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9. Huckel Equation m=ez/1.5h
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Huckel Equation
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)
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10. Complications in zeta potential determination
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Complications in zeta potential determination
Electrophoretic retardation
Relaxation effect
Under electrical field, the particle and the charged cloud move in opposite direction.
Thus becoming unsymmetrical and not spherical anymore.
This non-uniform charge density results in electrophoretic retardation i.e. a reduction in the velocity of the migrating particle.
This effect is accounted by Henry’s equation.
Following electrophoretic retardation, the process whereby the moving particle reverts to a symmetrical spherical particle is known as relaxation.
The relaxation effect is the distortion of the field induced by the movement of the particle. The extent depends on mobility and charge of counter ions.
The effect was accounted for by Overbeek & Booth and later by Wiersema, Loeb & Overbeek
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11. Laser Doppler Velocimetry
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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.
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12. Fluctuation in intensity of scattered light
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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:
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13. Advantages of laser technique
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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
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14. Examples of Application
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Examples of Application
To investigate the electrophoretic properties of blood lipid particles in connection to potential heart problems.
Control of size and zeta potential of droplets of artificial blood, which is vital for its safe use.
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
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15. Isoelectric point (IEP)
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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:
Tutorials/Intro/pcs18.html
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16. Physical Pharmacy 2
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Zeta potential of chloramphenicol and glass particles in benzethonium (+) chloride solution. - an 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
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17. Physical Pharmacy 2
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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)
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