Nanomaterials: Properties, Preparation and Applications - Environmental and Human Health Impacts of Nanotechnology - page 40

Furthermore, it can be seen that the distances travelled due to sedimentation are
much less than the average random walk due to diffusion. This means that diffusion
due to Brownian motion will overcome the sedimentation process. It is also clear
that for particles with high densities such as titania or gold, sedimentation will
overcome Brownian motion and therefore sedimentation will occur.
The implication of this is that at very low concentrations of small particles, sedi-
mentation is not very probable without some other interaction such as attachment
to another colloidal particle in suspension. So whilst in principle many very small
nanoparticles should form stable dispersions without surface modifi cation, their
high surface energy generally results in aggregation, and rapid precipitation.
Another reason to consider the surface of a nanoparticle is because the surfaces
of nanoparticles are often modifi ed in order to improve their compatibility with
the matrix into which they are placed or to improve their dispersion stability. Often
the surface of a nanoparticle will be covered with a layer of other molecules that
improves the processing properties of the particles. For example, silver nanoparti-
cles are regularly prepared with a layer of citrate ions on the surface. These ions
are bound to the surface of the particle but also carry a charge, which therefore
imparts a charge to the surface of the particle and results in a stable dispersion of
particles once they are dispersed in water.
There are essentially two methods for generating stabilised dispersions of
nanoparticles. Both methods are the same as those used to generate stable disper-
sions of much larger colloids and rely on the principle of creating an energy barrier
for the close contact of two suspended particles. Here a simplistic description of
the dispersed system we will be considered briefl y. Chapters 3 and 4 deal in more
depth with the chemistry at the surface of the nanoparticle in terms of DVLO
theory, zeta potentials and so on in relation to their environmental behaviour.

2.3.2 Charge Stabilisation

It is well known that like charges repel. Therefore, if a charge is placed on the
surface of the particle an energy barrier will exist which, if large enough, will
prevent aggregation. A particle is prepared so that the surface of the particle has
a charge associated with it. This charge may have been deliberately attached to the
surface, but in some cases it is possible that a particle may serendipitously attain
charge as a result of the adsorption of molecules or ions to its surface. The charge
on the surface must be neutralised by a suitable counter ion which will form a
strongly associated layer at the surface of the particle. This layer usually consists of
both ions and solvent. This strongly associated layer is called the Stern Layer. There
is a second so-called diffuse layer beyond the Stern Layer which is less tightly
bound. If a single charged point P + at a distance r from the particle surface and
distance r -

r from the Stern Layer is considered (Figure 2.8), then the net force on
the point charge P will be the difference between the attraction of the negative
surface and the repulsion of the positively charged Stern Layer; this may be calcu-
lated using Coulomb's Law showing that the repulsion will be inversely propor-
tional to both the dielectric constant of the medium and the square of the thickness
of the Stern Layer. This is, in fact, a large simplifi cation as it fails to take into account

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