mostly weak in this case. It appears that water molecules adjacent
to an -OH group may have either a somewhat shorter or a
somewhat longer residence time than water molecules in the bulk,
according to the conformation of the OH group in relation to the
rest of the solute molecule.- Nonpolar or hydrophobic groups. The water molecules cannot
make H bonds with these groups. Bringing an apolar molecule or
group in water then leads to some breaking of H bonds, which will
cause an increase in enthalpy. However, the system tries to make
as many H bonds as possible; this leads to a locally altered water
structure and thereby to a decrease in entropy. Anyway, the free
energy is increased, which implies negative solvation. Similar
changes presumably occur at the surface of larger molecules and
particles.
The Hydrophobic Effect. If two hydrophobic molecules or groups
in water come close together, negative solvation is diminished, which implies
a decrease in free energy. This works as if an attractive force is acting
between these groups, and this is calledhydrophobic bonding. Such bonds
especially act between aliphatic chains or between aromatic groups. They
are largely responsible for the micellization of amphiphilic molecules in
water and for the formation of vesicles and membranes of lipid bilayers.
They are also important for the conformation of globular proteins (Section
7.2.1). For a large hydrophobic group, the bond free energy is about
proportional to the surface area involved, and equals about
4 kBTð10 kJ?mol^1 Þper nm^2.
The explanation of the hydrophobic effect and the resulting
hydrophobic bonding is still a matter of some dispute. For instance,
attraction due to dispersion forces may provide a considerable part of the
interaction free energy of a hydrophobic bond, varying with the chemical
constitution of the groups involved.
The explanation of the temperature dependence of hydrophobic
bonding is especially intricate and controversial. By and large, at low
temperature (near 0 8 C),DHfor bond formation is positive;DSis relatively
large and positive. The result is a relatively small negativeDG, i.e., bonds
are formed. This would all be in agreement with an overriding effect of
water entropy. Above a certain temperature, however, DS starts to
decrease, but hydrophobic bonding nevertheless increases in strength,
becauseDHalso increases. A hypothetical result is depicted in Figure 3.4,
merely to illustrate trends. It may be concluded, that hydrophobic bonding
strongly increases with temperature, especially in the range from 0 to about
608 C. It may further be noted that we have here a clear exception to the