12.4 OTHER INTERACTIONS
Table 3.1 lists some other kinds of intermolecular forces, and most of these
can also act between particles.
Hydrogen bondingcan certainly occur, but it only acts at a very short
distance.
Solvation, i.e., mostly hydration, of groups at a particle surface may
cause repulsion. Especially charged groups are hydrated, but these cause
electrostatic repulsion anyway. However, dipoles may play a part. Their
dehydration is needed for close contact, and that costs free energy. The
range over which such a hydration force acts is quite short, however, at most
2 nm. The importance of hydration in stabilizing particles against
aggregation is unclear, and probably fairly small.
Apolar particles in water may be subject to attraction caused by the
hydrophobic effect. Again, the range over which such a force acts is generally
small.
If particles are already quite close to each other, hydrogen bonds or
hydrophobic interaction may lead tobond strengthening. Also the kind of
cross-linking discussed in relation to Figure 12.8b may be enhanced after
particles become aggregated. The theories of colloidal interaction, as
discussed in the previous sections, may well predict whether aggregation will
occur, but they tell little about the force needed to break the link between
particles. In other words, links that are assumed to be more or less reversible
(a few timeskBT) may become irreversible after a while.
Figure 12.11 illustrates some other mechanisms of bond strengthen-
ing. Theparticle rearrangementshown increases the coordination number.
Two particles being attracted by a net free energy of 1kBTwill not stay
together for a long time, but a particle attached to 6 other ones by the
same energy will not readily come loose. Solid particles often have an
uneven surface. If aggregated, such particles may move a little until they
have obtained the closest fit, implying the strongest van der Waals
attraction. Soft or fluid particles mayflattenupon attraction, whereby the
attractive force is generally enhanced. Finally, Ostwald ripening (see
Section 10.5.3) may lead to local sinteringof solid particles, especially
crystals. At sharp edges the solubility of the particle material is enhanced;
at a gap between particles it will be decreased. This then leads to
diffusion of material to the gap and thereby to growth of the junction
between particles, as depicted.
Junctions. This brings up another point. It is fairly common to
speak of thebondbetween two aggregated particles. However, it rarely
concerns one bond on the atomic or molecular scale. Actually, it often