Hydrophobically associated networks - Physical Gels from Biological and Synthetic Polymers - page 157

hydrocarbon chains driven by their mutual attraction. The free energy gained in associ-
ation was equal to the energy difference between liquid and gaseous hydrocarbons, but
this mutual attraction of non-polar groups plays only a minor role in the overall hydro-
phobic effect.
Understanding of the hydrophobic effect starts with a more precise description of
aspects of water structure related to the formation of hydrogen bonds. Hydrogen bonds
are strong and orientation-dependent bonds involving a particularly strong type of
directional dipole

dipole interaction. A hydrogen bond between two groups XH and Y
is usually denoted XH

-

Y, and its strength lies between 10 and 40 kJ mol − 1 , larger than
that of a van der Waals bond (1 kJ mol − 1 ) but still much weaker than a typical covalent or
ionic bond (500 kJ mol − 1 ).
Hydrogen bonds can form weak three-dimensional structures in liquids, where they
create a short-range order that is still signi

⋯

cantly longer lived than in simple liquids;

consequently the term is used. Hydrogen bonds play a crucial role in

liquid water structure, since each oxygen and its two hydrogen atoms can participate in
four linkages with other water molecules: two bonds involving its own H atoms and two
involving its unshared lone-pair electrons with other H atoms. For this reason, liquid
water has a tendency to retain an ice-like tetrahedral network structure, but the long-
distance arrangement in the liquid is disordered and labile. This tetrahedral coordination
of a water molecule is the origin of its unusual properties, including a maximum density
at 4°C with solid ice less dense than liquid water, a low compressibility and a high
dielectric constant.
Non-polar molecules such as alkanes cannot form hydrogen bonds with water
(Israelachvili, 1992 ). If a non-polar molecule is not too large, it is possible for water
molecules to organize around the dissolved non-polar solute in so-called

' associated liquids '

' clathrate

cages , forming labile structures in which water molecules are more ordered than in the

bulk liquid. Thus, when water molecules come in contact with a non-polar molecule the
main effect is to induce reorientation of the water molecules in such a way that they can
participate in hydrogen-bond formation with the bulk water, but without breaking any
hydrogen bonds, an effect called hydrophobic solvation. The reorientation of water
molecules around non-polar (hydrophobic) solutes is entropically unfavourable and,
surprisingly, the new structure of the surrounding water molecules is more ordered
than in the bulk liquid.
Closely related to hydrophobic solvation is the hydrophobic interaction or hydro-
phobic effect, which describes an unusually strong attraction between hydrophobic
molecules and surfaces in water, much stronger than their attraction in free space. Such
hydrophobic solutes then assemble spontaneously into larger structures, and as two
hydrophobic species come together there is a rearrangement of hydrogen-bond con-

'

figurations. The net result for the free energy change of all the molecules involved in
the spontaneous aggregation is always negative. Hydrophobic interactions play a
central role in molecular self-assembly in micelle formation of amphiphilic molecules
and in biological membrane formation. The thermodynamic principles of the self-
assembly of amphiphilic molecules are well understood, and so will only be summar-
ized here.