W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
W. Richard Bowen and Nidal Hilal 4
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62 2. MEASUREMENT OF PARTICLE ANd SURFACE INTERACTIONS<br />
with previous measurements undertaken with the surface force apparatus<br />
[118]. The authors concluded that the forces scaled along with the surface<br />
interaction dimensions between the mesoscale <strong>and</strong> nanoscale [117].<br />
2.3.5 Steric Interaction Forces<br />
For molecules attached to a solid surface in a liquid environment,<br />
chains with a degree of freedom to move will tend to dangle out into the<br />
solution where they remain thermally mobile. On approach of two polymer-covered<br />
surfaces, the entropy of confining these dangling chains<br />
results in a repulsive entropic force, which, for overlapping polymer<br />
molecules, is known as the steric or overlap repulsion. In ancient Egypt,<br />
people already knew how to keep ink stabilised by dispersing soot particles<br />
in water, incubated with gum arabicum or egg-adsorbed polymers,<br />
which, in the first case, is a mixture of polysaccharide <strong>and</strong> glycoprotein<br />
<strong>and</strong> in the second mainly the protein albumin, which works through this<br />
steric repulsion. However, steric repulsion does not necessarily have to<br />
be due to polymeric molecules; layers of small molecules can have the<br />
same effect, albeit at a much shorter range.<br />
Steric stabilisation of dispersions is very important in many industrial<br />
processes. This is because colloidal particles that normally coagulate in<br />
a solvent can often be stabilised by adding a small amount of polymer<br />
to the dispersing medium. Such polymer additives are known as protectives<br />
against coagulation <strong>and</strong> they lead to the steric stabilisation of a colloid.<br />
Both synthetic polymers <strong>and</strong> biopolymers (e.g. protein, gelatine) are<br />
widely used in both non-polar <strong>and</strong> polar solvents (e.g. in paints, toners,<br />
emulsions, cosmetics, pharmaceuticals, processed food, soils, lubricants).<br />
Theories of steric interactions are not well developed. There is no simple,<br />
comprehensive theory available as steric forces are complicated <strong>and</strong><br />
difficult to describe [119–121]. The magnitude of the force between surfaces<br />
coated with polymers depends on the quantity or coverage of polymer on<br />
each surface, on whether the polymer is simply adsorbed from solution (a<br />
reversible process) or irreversibly grafted onto the surfaces <strong>and</strong> finally on<br />
the quality of the solvent [119, 122]. Different components contribute to the<br />
force, <strong>and</strong> which component dominates the total force is situation specific.<br />
For interactions in poor <strong>and</strong> theta solvents, there are some theories<br />
available for low <strong>and</strong> high surface coverage. In the case of the low coverage<br />
where there is no overlap or entanglement of neighbouring chains,<br />
the repulsive energy per unit area is a complex series <strong>and</strong> roughly exponential<br />
[123–126]. As for the high coverage of end-grafted chains, the<br />
thickness of the brush layer increases linearly with the length of the polymer<br />
molecule. Once two brush-bearing surfaces are close enough to each<br />
other, there is a repulsive pressure between them, <strong>and</strong> this force can be<br />
approximated by the Alex<strong>and</strong>er–de Gennes theory [119, 127, 128].