Environmental Engineering Reference
In-Depth Information
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Electrostatic
repulsion
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Particle
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Stern Layer
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δ
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Diffuse Layer
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P
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Figure 2.8
A representation of the stabilisation of a nanoparticle with a charged surface.
The Stern layer and diffuse layer are shown in the lower part of the diagram. The electrostatic
force between the particles and point P
+
will be a function of r-
δ
r.
the diffuse layer or nature of the surface which is clearly not a point charge; the
mathematics describing these phenomena are beyond the scope of this chapter.
However, even this simplifi cation has two important implications. Colloids based
on charge stabilisation will not remain stable if the dielectric constant is raised too
high, as might happen at high/low pH or with increasing ionic strength. Furthermore,
if a divalent, or greater, ion is added, it will tend to displace the ions at the surface
of the particle due to the entropic gain of this process. However, increasing the
charge density at the surface of the particle will only serve to make the Stern Layer
thinner and therefore reduce the repulsion between the particles. It is for these
reasons that the stability of charge stabilised colloids is very dependent on the exact
composition of the medium in which they are suspended. Furthermore, increasing
the concentration of the particles themselves is likely to result in signifi cant changes
to the dielectric constant of the medium due to the increased association of ions
with diffuse layer. It is, therefore, not easy to prepare concentrated suspension of
charge stabilised colloids.
The charge on the surface of these materials generally leads them to form stable
dispersions in water. However, they will readily interact with other oppositely
charged molecules and surfaces within the medium. The interaction of nanoparti-
cles with natural materials has already been briefl y discussed in Chapter 1 and is
discussed again in Chapter 4.
2.3.3
Steric Stabilisation
The use of charge stabilisation is viable in an aqueous environment. If particles are
to be suspended in a hydrophobic environment, where the dielectric constant is
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