Page 75 - Visions of the Future Chemistry and Life Science

64 D. J. MACQUARRIE

have searched for many years for materials with the same degree of unifor-
mity displayed by the zeolites, but with larger pores. This would allow the
concept of shape selectivity to be extended to larger molecules such as phar-
maceutical intermediates, and other highly functional compounds. Other
forms of selectivity will also benefit from a very regular structure.
The pore size of most zeolites is
1.5nm. This microporosity limits
their utility in most areas of chemistry, where the molecules used are
much larger, and for which mesoporous materials would be necessary.
Unfortunately, attempts to use larger template molecules in the zeolite
synthesis, an approach which should in theory lead to larger pore size zeo-
lites, have met with very little success. Indeed, some zeolitic materials
have been prepared which have mesopores – none of these has ever dis-
played any real stability and most collapse on attempts to use them. A new
methodology was thus required.

4.2 New mesoporous materials

In the past 20 years or so, the field of supramolecular chemistry has become
enormously important, with Jean-Marie Lehn, Donald Cram and Charles
Pedersen winning the Nobel Prize in 1987. The concept of supramolecular
chemistry is that molecules can self-organise into definite structures,
without forming covalent bonds, but rather through weaker interactions
such as hydrogen bonding. A hydrogen bond is a special type of weak chem-
ical bond, which holds water molecules together, giving water many
unique properties – the same bond is critical to the formation of the double
helix of DNA, and is often of extreme importance in biological systems.
Hydrophobic interactions, also important in self-assembly, are interac-
tions between oily molecules which minimise contact with water by
causing the oily parts to huddle together. One example of the latter,
although not at all new, is the ability of molecules containing a polar head
group and a long non-polar hydrocarbon tail (surfactants) to form micelles
in polar, aqueous environments. These micelles form because the water-
repelling hydrocarbon tails gather together in the centre of a sphere, or
sometimes a cylinder, to avoid contact with water. The polar head groups
then form a layer on the surface of the sphere or cylinder, forming a barrier
between the hydrocarbon tails and the water. The best-known example of
these micelle-forming materials are detergents.
The diameter of the micelles depends on the exact nature of the sur-