Chemistry Reference
In-Depth Information
Therefore, before discussing significant developments in these fields, it is per-
haps useful to try and define more clearly the characteristic features of each
sub-discipline. The IUPAC Gold Book [
25
] provides the following definitions:
Colloidal
is a state of subdivision, implying that the molecules or polymolecular
particles dispersed in a medium have at least in one direction a dimension roughly
between 1 nm and 1
μ
m or that in a system discontinuities are found at distances of
that order. The following are characteristic features [
7
-
9
,
11
]:
1. Traditionally colloids had poorly defined compositions and
>
15% dispersion in
size, but there have been improvements in synthetic methods which have
improved their quality. The colloidal particle contains between 400 and 40,000
metal atoms and their agglomeration to bulk metal is prevented by the build-up
of charges on their surface. These charges may arise from surface metal atoms in
a different oxidation state, or ligands attached to the surface metal atoms. In the
solution around the colloid charged particles are present in higher concentrations
than the bulk of the solvent. The Stern layer shown in Fig.
1
is defined as those
charged species which migrate with the colloid particle under the influence of an
electric field. The presence of this charged layer means that the ionic strength of
the solution may alter the dimensions and properties of the colloid.
2. Electron microscopy studies have established that colloids may be synthesised
with ordered structures based on close-packed arrangements of metals even
when the particle diameters are in the nanometre range. This ordering manifests
itself either in single or twinned crystals and for smaller-sized particles struc-
tures with fivefold symmetry based on the icosahedrons and dodecahedron are
commonly observed for silver and gold. These particles are described as multi-
ply twinned [
2
,
3
,
26
,
27
].
3. Electron microscopy is not able to resolve the lighter ligating atoms on the metal
surface and consequently the colloids have an ill-defined quantity of surface
anions such as O
2
,X
,OH
and solvent molecules such as H
2
O or polar
organic molecules.
4. Although the colloids of particular interest in this review are based on metals a
wide range of inorganic and organic molecular species also form colloids.
5. They form optically clear solutions (
sols
) which have spectroscopic properties
which may be related to the sizes of the particles (see below).
6. Reproducible syntheses have been developed which can lead to a wide range and
specifically sized metal colloids and the dispersities of the colloid may be
reduced by more controlled synthetic procedures. This reproducibility means
that colloids with specified dimensions and dispersities are now commercially
available [
28
].
7. Traditional colloids are not readily isolated as solids or re-dissolved, but recently
made thiolato-based colloids have been made which do re-dissolve in organic
solvents.
8. Traditionally produced colloids did not have reproducible catalytic properties,
but improved synthetic methods and the availability of standard samples have
increased the reliability of the results.
