SECTION
1.1
Properties of Water
3
compounds (e.g., sugars and alcohols). Ionic compounds
are soluble because water can overcome the electrostatic
attraction between ions through solvation of the ions.
Nonionic polar compounds are soluble because water
molecules can form hydrogen bonds to polar groups (e.g.,
-OH).
Amphipathic compounds,
which contain both large
nonpolar hydrocarbon chains (hydrophobic groups) and
polar or ionic groups (hydrophilic groups) may associate
with each other in submicroscopic aggregations called
micelles.
Micelles have hydrophilic (water-liking) groups
on their exterior (bonding with solvent water), and hy-
drophobic (water-disliking) groups clustered in their in-
terior (Figure 1-3). They occur in spherical, cylindrical,
or ellipsoidal shapes. Micelle structures are stabilized
by hydrogen bonding with water, by van der Waals at-
tractive forces between hydrocarbon groups in the inte-
rior, and by energy of hydrophobic reactions. The last
is the stabilization energy that would be lost if each hy-
drocarbon group were transferred from the hydrophobic
medium to the polar aqueous solvent. As with hydrogen
bonds, each hydrophobic interaction is very weak, but
many such interactions result in formation of large, stable
structures.
A micelle may contain many hundreds of thousands of
amphipathic molecules. The interior molecular organiza-
tion of micelles has been likened to a “liquid hydrocar-
bon droplet.” However, a recent model departs from this
conventional concept and suggests that because of severe
constraints in the space-filling requirements of hydrocar-
bon chains in the interior as well as because of micellar
Hydrophobic chain
Hydrophilic group
FIGURE 1-3
A geometrical representation of a spherical micelle showing the
hydrocarbon chains in the micelle. The hydrophilic groups are attracted to
water, and the hydrophobic chains are within the micelle. The ends of the
hydrocarbon chains are nonuniformly distributed, and many are located in
the middle. The degree of disorder is much higher in the periphery than in
the center of the micelle.
geometry, the chain ends are not uniformly distributed
throughout the micelle but tend to be clustered between the
center of the micelle and the outer surface, implying that
many of the hydrocarbon side chains are bent back upon
themselves (Figure 1-3). In this model, there appears to be
a progression from ordered (as in crystals) to disordered
(as in liquids) structures proceeding from the center of the
micelle to the periphery.
Hydrophobic interaction plays a major role in main-
taining the structure and function of cell membranes,
the activity of proteins, the anesthetic action of nonpo-
lar compounds such as chloroform and nitrous oxide, the
absorption of digested fats, and the circulation of hy-
drophobic molecules in the interior of micelles in blood
plasma.
Colligative Properties
The colligative properties of a solvent depend upon the
concentration of solute particles. These properties include
freezing point depression, vapor pressure depression, os-
motic pressure, and boiling point elevation. The freezing
point of water is depressed by 1,
8 6
°C when 1 mol of non-
volatile solute, which neither dissociates nor associates
in solution, is dissolved in 1 kg of water. The same con-
centration of solute elevates the boiling point by 0.543°C.
Osmotic pressure is a measure of the tendency of water
molecules to migrate from a dilute to a concentrated solu-
tion through a semipermeable membrane. This migration
of water molecules is termed
osmosis.
A solution contain-
ing
1
mol of solute particles in
1
kg of water is a
1
-osmolal
solution. When 1 mol of a solute (such as NaCl) that dis-
sociates into two ions (Na+ and Cl- ) is dissolved in 1 kg
of water, the solution is
2
-osmolal.
Measurement of colligative properties is useful in esti-
mating solute concentrations in biological fluids. For ex-
ample, in blood plasma, the normal total concentration
of solutes is remarkably constant (275-295 milliosmolal).
Pathological conditions (e.g., dehydration, renal failure)
involving abnormal plasma osmolality are discussed in
Chapter 39.
Dissociation of Water and the pH Scale
Water dissociates to yield a hydrogen ion (H+) and a hy-
droxyl ion (OH- ).
H
2
O ^ H + + OH-
(1.1)
The H+ bonds to the oxygen atom of an undissociated
H20 molecule to form a hydronium ion (H
3
O"1").
H20 + H20
H
3
0 + + OH-
