SURFACE POTENTIAL

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SURFACE POTENTIAL
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Why should we be interested in colloidal
system?1. First reason PURELY EPISTEMOLOGICAL
If we consider the three states of matter- gas,
liquid and solid- we can observe colloidal
system in all possible combination.
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2. Second reason ANTHROPOLOGICAL Life processes
involve the control and transformation of
colloidal assemblies and many diseases are
associated with malfunction at the colloidal
level
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DISPERSION COLLOIDS Dispersion colloids may be
stabilized by charging the surfaces leading to
electrostatic repulsion particles then will keep
distances of at least several nanometers. Charging
occurs by adsorption of ions or surface
hydrolysis. The counter ions are loosely bound in
a diffuse cloud
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The thickness of the counter ion cloud can be
estimated by an adopted Debye-Hückel -theory .
The ion clouds with their net charge stabilize
the colloid against agglomeration however,
van-der-Waals forces always lead to an additional
attractive interaction. The combination of both
effects is basis of the DLVO -theory ( Derjaguin,
Landau, Verwey and Overbeek).
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ZETA POTENTIAL
To use the DLVO theory we need to know the
potential energy of attraction and repulsion
between colloidal particles. In many practical
situations, it is difficult to obtain a reliable
estimate of a colloidal particles surface
potential. An alternative strategy is to use
electrokinetic measurement which we can interpret
in terms of the zeta potential.
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ZETA POTENTIAL
Experience has shown that we can correlate
colloid stability with this readily accessible
experimental quantity table correlates critical
coagulation concentrations with zeta potentials.
Consequently, it is important to see how
electrokinetic techniques can be used to
determine the zeta potential.
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Elektrophoresis
Movement of charged particles in an electrical
field. The background medium does not need to be
a simple liquid, it may be also highly viscous
elelectrophoresis. The charged particles may be
ordinary colloids or charged macromolecules (e.g.
proteins, biochemistry !).
From the observation of individual particle, the
electrophoretic velocity v W can be measured.
Under the assumption of non deformable, spherical
and nonconducting particles, v W can be
correlated to the surface charge z.
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Sedimentation potential
The effect is inverse to electrophoresis charged
particles are moving in the gravity field. Since
the mass of the particles is much larger than the
mass of the individual ions in the surrounding
ion cloud, the system of particle and ion is
deformed, forms a dipole and thus gives rise to
an electric potential difference along the
sedimentation path.
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Electroosmosis
an electrolyte is moved relative to a charged
surface. This applies to capillaries, membranes
or powders.
The effect relies on the fact, that the
electrical field supplied exerts a force on the
electrochemical double layer. The mobile layer
drags on the electrolyte which results in a
liquid stream through the apparatus.
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Streaming potential
the principle is comparable to electroosmosis,
but now an external pressure difference drives an
electrolyte through the bundle/aggregate of
immobile charged surfaces. The effect is caused
by the retardation of a part of the electrolyte
in the double layer the resulting charge
separation sums up to a measurable potential
difference.
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Research work
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Preparation and characterization of ZnSMn

• At first Zn (Ac)2, Mn (Ac)2 and Cysteamin were
  dissolved in H2O under stirring. Then
  Na2S-solution was added slowly (drop wise). After
  this the solution was heated for 3 hours (100C)
  under reflux. After that the crude solution was
  reduced to about 50ml, and the particles were
  precipitated by addition of ethanol. The
  particles were isolated by centrifugation and
  redissolved in a determined amount of water. The
  pH was adjusted to 4.8 by addition of acetic acid

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Characterization

• The particle size was measured in Malvern
  Instrument

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Results

• Influence of pH
• pH 5 small particles our
  sample about 5 nm
• pH 5 bigger particles
• Nanoparticles have good orange luminescence
  dopping of Mn

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References

• D.Fennell Evans, H.Wennerström The Colloidal
  Domain, 1994
• J.Goodwin Colloids and Interfaces with
  Surfactants and Polymers, 2004