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Fig. 6.2 Supermolecular
structures of amphiphilic
molecules
3. Among the vast number of organic molecules the so-called amphiphilic
compounds are particularly important. These molecules represent a combination
of fragments with opposite properties—a hydrophilic group (e.g., an acid residue)
and a hydrophobic fragment, such as a long hydrocarbon chain. Due to their
structure amphiphilic molecules in a polar solvent (water) tend to be on the liquid-
gas boundary such that the hydrophilic “heads” are in the water, and the hydropho-
bic “tails” are pushed into the gas phase. A certain number of amphiphilic mole-
cules, which may vary depending on the structure of the molecule, are found in the
solution. In this case, there is a tendency to decrease the energy of interaction of the
molecules with the solvent due to spontaneous formation of structural aggregates
(Fig.
6.2
).
In spherical micelles hydrophilic groups are on the surface of the sphere, and the
hydrophobic ones are inside of the sphere. As a result, the repulsive interaction of
hydrophobic tails with the solvent is reduced. In a nonpolar solvent (oil) reverse
micelles appear in which the hydrophilic heads reside in the center of a spherical
formation. A large number of different supramolecular structures—bilayers, vesi-
cles, etc.—are known. Fundamentally important biological amphiphilic molecules
are lipids that form the basis of cell membranes.
4. Let us pour into a pan a thin layer of a viscous fluid (e.g., oil) and heat the pan
on fire, keeping the surface temperature constant. If the heating is weak, the fluid
remains stationary. If, however, the fire is made stronger, increasing the heat flow,
then suddenly and spontaneously the entire surface of the oil is broken into regular
hexagonal or cylindrical cells (Fig.
6.3a
). The structure on the pan resembles a
honeycomb.
