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The secret of Nature’s microscopic patterns 105

pressure difference results in solvent being pulled from the depletion zone
in essence, pulling the particles closer together. This is equivalent to an
attractive interparticle interaction. Interactions involving surface poly-
mers are of great interest in explaining biological microarchitectures as in
many cases, the likely components will be separated from the supporting
fluids by mixed polymeric membranes involving lipids, proteins and poly-
saccharides.
Another important interaction that needs to be considered is the
‘hydrophobic interaction’. This can be most easily thought of in terms of
two immiscible liquids such as oil and water being induced to mix by
adding surfactants, to form (micro) emulsions. The exact structure of the
phase formed depends heavily on the relative compositions of the various
phases and the structure of the surfactant (see Figure 6.4).
Below some critical surfactant concentration, the system is two-phase
with excess oil or water depending on the oil/water concentration. On
adding more surfactant, the system moves into a one-phase region with
normal micelles forming in water-rich systems. The water constitutes the
continuous phase, solvating the headgroups of the surfactant whose hydro-
phobic tails solubilise oil in the core of the micelle. In oil rich systems,
reverse-micelles form. With further increases in surfactant composition,

Figure 6.4. Schematic phase diagram for a three-component (oil, water, surfactant)
system showing some of the self-assembled structures which form in the various
regions.