Surface potentials and dispersion stability

lameubiquityMécanique

21 févr. 2014 (il y a 3 années et 7 mois)

86 vue(s)

SURFACE

POTENTIAL

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.

medium

liquid

liquid

liquid

gas

gas

solid

solid

solid

particle

solid

liquid



gas

solid

liquid

solid

liquid

gas

name

colloida
l sol

emulsio
n

foam

aerosol

aerosol

solid
suspensi
on

porous
material

solid
foam

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







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:



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
).

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.

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.

potential

stationary (wall)

moving (colloid
particles)

applied

electro
-
osmosis

electrophoresis

induced

streaming potential

sedimentation
potential

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.

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
.

Sedimentation potential:

an electrolyte is moved relative to a charged surface.
This applies to capillaries, membranes or powders.

Electroosmosis
:

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.

the principle is comparable to electroosmosis, but now
an external pressure difference drives an electrolyte
through the bundle/aggregate of

Streaming potential
:

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.

Research

work

Preparation and characterization of
ZnS:Mn


At first Zn (Ac)
2
, Mn (Ac)
2

and Cysteamin were dissolved in
H2O under stirring. Then Na
2
S
-
solution was added slowly (drop
wise). After this the solution was heated for 3 hours (100
°
C)
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


The particle size was measured in Malvern Instrument


Characterization:


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




References


D.Fennell Evans, H.Wennerström:
The
Colloidal Domain, 1994


J.Goodwin:
Colloids and Interfaces with
Surfactants and Polymers, 2004