W. Richard Bowen and Nidal Hilal 4

Studies of forces between model alumina surfaces in electrolyte solutions
have noted the existence of steric interactions at basic (�8) pH,
most probably due to the formation of a hydrated gel layer at the solid/
liquid interface [77, 129] providing an extra resistance to hard contact
being made. At low pH values, the surface forces were described by classical
DLVO theory, but at high pH with the presence of the hydrated gel
layer, deviations from DLVO theory occurred at close separations. Polat
et al. [129] measured frictional (lateral) forces in addition to forces normal
to the surface. Differences in the normal forces at different pH values had
a significant effect on the frictional forces. At pH values where the additional
repulsive force due to hydration was present, the frictional forces
were significantly decreased. The authors concluded that such behaviour
was likely to have implications for the rheology stability and forming
behaviour of powders made from this material and potentially for some
other metal oxide materials.
The adsorption of electrolyte and surfactant molecules from solution
to solid surfaces is also likely to result in the addition of steric forces on
close approach between surfaces. Studies of the surface forces between
gold-coated silica spheres mounted on AFM cantilevers and plane gold
surfaces in aqueous solutions of gold chloride, sodium chloride and trisodium
citrate found that when the citrate and chloride anions were added
together a short-range steric barrier appeared [130]. The range of this steric
barrier was equal to the size of two citrate anions, suggesting that the
citrate was coating each opposing surface with a monolayer. Meagher and
colleagues studied the interactions between silica microspheres and �-alumina
plane surfaces in the presence of different combinations of electrolyte,
polyelectrolyte and surfactant [131]. In the presence of aqueous electrolyte
alone, long-range forces compared well with DLVO theory at all separations.
At high pH values, all forces measured were repulsive. As the pH
was decreased, the repulsion was reduced, eventually starting to show features
of attraction as pH was reduced to below the isoelectric point for the
alumina (pH 5.5). When the polyelectrolyte sodium poly(styrene sulfonate)
was added, with and without the presence of the cationic surfactant cetyltrimethylammonium
bromate (CTAB) (at concentrations below its critical
micelle concentration), at pH values equal to or higher than the isoelectric
point of alumina, an additional repulsive force was observed at approaches
of 3 nm or less. This deviation from the DLVO theory was observed as a
short-range repulsive force that varied in its magnitude and range.
2.3.6 Hydrophobic Interaction Forces
2.3 INTERACTION FORCES 63
A hydrophobic surface usually has no polar or ionic groups or hydrogen-bonding
sites so that there is no affinity for water and the surface
to bond together. Ordinary water in bulk is significantly structured