The biological importance of water arises in part from the polarity
of its molecules. One reason that water is so well adapted as the
medium for life is that it is a superb solvent. More different substances
dissolve in water, and in greater quantities, than in any other liquid. Water’s
superiority as a solvent arises from the marked polarity of the water molecule
and its resulting readiness to interact with ions and polar molecules. Thanks
to this property, both ionic substances and nonionic polar substances are
soluble in water.
Consider, for example, what happens when a dry crystal of a
salt, such as sodium chloride, is dissolved in an aqueous medium. Within the
dry crystal, the ionic bonds between the positive sodium ions and negative
chloride ions are very strong, and much energy would be required to pull these
ions away from each other. When the crystal is put into water, however, the
attraction of the electronegative oxygen end of the water molecules for the
positively charged sodium ions and the similar attraction of the electropositive
hydrogen ends of the water molecules for the negatively charged chloride ions
are greater than the mutual attraction between the sodium and chloride ions.
In water, then, the ionic bonds are broken with extreme ease, because of the
competitive attraction of the water molecules for the ions. Consequently, the
sodium and chloride ions dissociate, and each becomes surrounded by a hydration
sphere of regularly arranged water molecules that are attracted to it (Fig.
1). Such an ion is said to be hydrated.
Fig. 1. Hydration spheres of Na+ and Cl-. When
dissolved in water, each of the Na+ and Cl- ions is hydrated–that is,
surrounded by water molecules electrostatically attracted to it. Note that
the oxygen of the water molecules is attracted to the positively charged Na+,
while the hydrogen of the water molecules is attracted to the negatively charged
Cl-. Water molecules in a hydration sphere are called bound water. This bonding
between ion and polar molecules (red bands) makes evident the common electrostatic
basis of ionic bonds and polar (hydrogen) bonds.
Water is also an excellent solvent for nonionic, polar molecules. Indeed, such
molecules are said to be hydrophilic (“water-loving”).
The solubility of such molecules—such as ethyl alcohol, for instance,
arises from an electrostatic attraction between the charged portions of the
solute molecules and the oppositely charged parts of the water molecules.
This occurs especially when the molecule has an oxygen with a hydrogen attached
to it (-OH). As in water molecules, the hydrogen in such a group has a slight
positive charge and is therefore attracted by the negatively charged oxygen
end of a nearby water molecule, with the result that a hydrogen bond is formed.
The dissolved (solute) molecules and the water molecules thus become linked
to one another.
In short, substances dissolve in water if their molecules can interact
with the polar water molecules. The old adage “like dissolves
like” is useful in determining a substance’s solubility in
a particular solvent. Water, because it is polar, can interact with other polar
or charged substances, and such substances will readily dissolve in water (Fig.
2). Substances that are electrically neutral and nonpolar, however, dissolve
poorly in water. They show no tendency to interact electrostatically with water
and, indeed, are repulsed by it. When a hydrophobic substance such as oil is
stirred into water, it will soon begin to separate out, because the water molecules
tend slowly to reestablish the hydrogen bonds broken by the physical intrusion
of the insoluble material. In a very real sense, the water “pushes” the
nonpolar molecules together, and the nonpolar molecules tend to coalesce to
form droplets (Fig. 3).
Fig. 2. Polar basis of solubility. When
a polar substance such as glucose, an energy-rich sugar (left), is placed in
contact with water, the water molecules are attracted to the polar atoms of
the sugar. (For clarity, the polar –OH groups are shown for only two
of the sugar molecules.) The water forms hydrogen bonds with the substance,
surrounding it with water molecules, and so dissolves it (right).
Fig. 3. Water-induced clumping of hydrophobic
molecules. Dispersed hydrophobic molecules disrupt the polar bonding
pattern of pure water so that few hydrogen bonds can form in the solution
(A). As hydrophobic molecules (schematically represented as brown ovals)
encounter one another randomly in a solution of water, they tend to become
trapped in clumps by polar bonding of water molecules to one another (B).
Because there is more polar bonding when hydrophobic molecules are clumped,
the solution becomes stabilized in this form.
Detergents. Sodium dodecyl sulfate is
a strong detergent often used to dissolve cell membranes and other
hydrophobic molecules in experiments requiring the separation of these
components for further analysis. It has a long, straight hydrophobic
tail and, because it ionizes in water, a charged head (A). Water molecules
dissolve the heads and drive the tails into tightly packed clumps
that dissolve hydrophobic grease molecules (B). During washing, whether
in a laboratory preparation or a home washing machine, the entire
assembly is rinsed out.
