Page 226 - Introduction to Colloid and Surface Chemistry

Colloid stability 215

increasing K (i.e. by increasing electrolyte concentration and/or
counter-ion charge number). Specific effects may also influence
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F R . Counter-ion adsorption in the Stern layer may cause a reversal
of charge (see page 183)> so that V R for a pair of identical particles
will be zero at the reversal of charge concentration and positive
(repulsion) at both below and above this concentration. In contrast to
the effect of electrolyte on the diffuse part of the electric double
layer, the amount of added electrolyte required to produce such a
specific effect will depend on the total surface area of the particles,
The nature of the electric double layer (and of V R) may also be
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influenced by ion hydrolysis and/or complexation reactions °- .
An interesting example of electrostatic attraction of oppositely
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charged surfaces is that exhibited by kaolinite clay particles . The
faces of the plate-like particles tend to be negatively charged and the
edges positively charged. This can be demonstrated by introducing
negatively charged colloidal gold particles into the clay suspension,
then subsequently taking an electron micrograph, which shows the
small gold particles adhering to the edges (but not to the faces) of the
clay platelets. Edge-to-face attraction between the clay platelets can
lead to the formation of a 'cardhouse' structure with a relatively low
particle density.

van der Waals forces between colloidal particles

The forces of attraction between neutral, chemically saturated
molecules, postulated by van der Waals to explain non-ideal gas
behaviour, also originate from electrical interactions. Three types of
such intermolecular attraction are recognised:

1. Two molecules with permanent dipoles mutually orientate each
other in such a way that, on average, attraction results.
2. Dipolar molecules induce dipoles in other molecules so that
attraction results.
3. Attractive forces are also operative between non-polar molecules,
as is evident from the liquefaction of hydrogen, helium, etc. These
universal attractive forces (known as dispersion forces) were first
explained by London (1930) and are due to the polarisation of one
molecule by fluctuations in the charge distribution in a second
molecule, and vice versa.