W. Richard Bowen and Nidal Hilal 4

62 2. MEASUREMENT OF PARTICLE ANd SURFACE INTERACTIONS
with previous measurements undertaken with the surface force apparatus
[118]. The authors concluded that the forces scaled along with the surface
interaction dimensions between the mesoscale and nanoscale [117].
2.3.5 Steric Interaction Forces
For molecules attached to a solid surface in a liquid environment,
chains with a degree of freedom to move will tend to dangle out into the
solution where they remain thermally mobile. On approach of two polymer-covered
surfaces, the entropy of confining these dangling chains
results in a repulsive entropic force, which, for overlapping polymer
molecules, is known as the steric or overlap repulsion. In ancient Egypt,
people already knew how to keep ink stabilised by dispersing soot particles
in water, incubated with gum arabicum or egg-adsorbed polymers,
which, in the first case, is a mixture of polysaccharide and glycoprotein
and in the second mainly the protein albumin, which works through this
steric repulsion. However, steric repulsion does not necessarily have to
be due to polymeric molecules; layers of small molecules can have the
same effect, albeit at a much shorter range.
Steric stabilisation of dispersions is very important in many industrial
processes. This is because colloidal particles that normally coagulate in
a solvent can often be stabilised by adding a small amount of polymer
to the dispersing medium. Such polymer additives are known as protectives
against coagulation and they lead to the steric stabilisation of a colloid.
Both synthetic polymers and biopolymers (e.g. protein, gelatine) are
widely used in both non-polar and polar solvents (e.g. in paints, toners,
emulsions, cosmetics, pharmaceuticals, processed food, soils, lubricants).
Theories of steric interactions are not well developed. There is no simple,
comprehensive theory available as steric forces are complicated and
difficult to describe [119–121]. The magnitude of the force between surfaces
coated with polymers depends on the quantity or coverage of polymer on
each surface, on whether the polymer is simply adsorbed from solution (a
reversible process) or irreversibly grafted onto the surfaces and finally on
the quality of the solvent [119, 122]. Different components contribute to the
force, and which component dominates the total force is situation specific.
For interactions in poor and theta solvents, there are some theories
available for low and high surface coverage. In the case of the low coverage
where there is no overlap or entanglement of neighbouring chains,
the repulsive energy per unit area is a complex series and roughly exponential
[123–126]. As for the high coverage of end-grafted chains, the
thickness of the brush layer increases linearly with the length of the polymer
molecule. Once two brush-bearing surfaces are close enough to each
other, there is a repulsive pressure between them, and this force can be
approximated by the Alexander–de Gennes theory [119, 127, 128].