5.1.9 Hydrogen bonds
The hydrogen bond is a special type of intermolecular interaction of great
importance in colloidal systems and nanomaterials in general. The
hydrogen bond is essentially electrostatic in origin and so is a subset of
the dipole interactions already discussed. However, it is of particular
importance and strength, and so has acquired a special classification.
Hydrogen bonds occur between molecules that have hydrogen covalently
bonded to a strongly electronegative atom such as N, O, or F. In such
cases, the electron density surrounding the hydrogen atom is mostly
drawn toward the more electronegative atom, leaving the hydrogen atom
“exposed” with a strong partial positive charge through which it may form
a strong dipole–dipole interaction with the electronegative element on an
adjacent molecule. Perhaps more importantly, the reduced electron
density around the hydrogen atom means that the neighboring molecule
with which it is hydrogen bonding can draw much closer than it could
otherwise (within ~1.5 to 2.0 Å). In other words, the overlap repulsion is
minimized because the electron “cloud” surrounding the hydrogen atom
has been reduced in size. Because the neighboring molecule can draw
much closer, the magnitude of the attractive interaction energy is much
greater than it would normally be (remember that in our equations for
electrostatic interaction energies, r 12 was in the denominator and usually
raised to some power; so if r 12 is smaller, then the magnitude of the
interaction potential energy is much greater). Figure 5.11 depicts a
hydrogen bond between two water molecules.
5.1.10 The hydrophobic effect
The van der Waals interactions discussed so far are responsible for many
of the physical properties (e.g., solubility) of organic molecules. For
example, in methanol (CH 3 OH), the hydrocarbon portion of the molecule is relatively small and the polar hydroxyl group is largely responsible
for the weak intermolecular van der Waals interactions. However, as the
length of the hydrocarbon moiety increases [e.g., as in decanol, CH 3
(CH 2 ) 9 OH] the nonpolar hydrocarbon portion of the molecule dominates
intermolecular interactions and defines its solubility. Hydrocarbon
chains are essentially oil and have little or no tendency to interact with
water. If the hydrocarbon chain is long enough the molecules may drop
out of solution (precipitate) and interact with themselves instead of the
water molecules. This brings us to a discussion of the hydrophobic effect,
CHAPTER 5: Intermolecular Interactions and Self-Assembly
154
The hydrogen bond is a special type of intermolecular interaction of great
importance in colloidal systems and nanomaterials in general. The
hydrogen bond is essentially electrostatic in origin and so is a subset of
the dipole interactions already discussed. However, it is of particular
importance and strength, and so has acquired a special classification.
Hydrogen bonds occur between molecules that have hydrogen covalently
bonded to a strongly electronegative atom such as N, O, or F. In such
cases, the electron density surrounding the hydrogen atom is mostly
drawn toward the more electronegative atom, leaving the hydrogen atom
“exposed” with a strong partial positive charge through which it may form
a strong dipole–dipole interaction with the electronegative element on an
adjacent molecule. Perhaps more importantly, the reduced electron
density around the hydrogen atom means that the neighboring molecule
with which it is hydrogen bonding can draw much closer than it could
otherwise (within ~1.5 to 2.0 Å). In other words, the overlap repulsion is
minimized because the electron “cloud” surrounding the hydrogen atom
has been reduced in size. Because the neighboring molecule can draw
much closer, the magnitude of the attractive interaction energy is much
greater than it would normally be (remember that in our equations for
electrostatic interaction energies, r 12 was in the denominator and usually
raised to some power; so if r 12 is smaller, then the magnitude of the
interaction potential energy is much greater). Figure 5.11 depicts a
hydrogen bond between two water molecules.
5.1.10 The hydrophobic effect
The van der Waals interactions discussed so far are responsible for many
of the physical properties (e.g., solubility) of organic molecules. For
example, in methanol (CH 3 OH), the hydrocarbon portion of the molecule is relatively small and the polar hydroxyl group is largely responsible
for the weak intermolecular van der Waals interactions. However, as the
length of the hydrocarbon moiety increases [e.g., as in decanol, CH 3
(CH 2 ) 9 OH] the nonpolar hydrocarbon portion of the molecule dominates
intermolecular interactions and defines its solubility. Hydrocarbon
chains are essentially oil and have little or no tendency to interact with
water. If the hydrocarbon chain is long enough the molecules may drop
out of solution (precipitate) and interact with themselves instead of the
water molecules. This brings us to a discussion of the hydrophobic effect,
CHAPTER 5: Intermolecular Interactions and Self-Assembly
154