always repulsive. By comparison, this repulsive interaction begins to
occur when the two surfaces are at a greater distance than if one of the
surfaces was neutral.
Finally, let’s consider the case of the interaction between two surfaces of
opposite charge. As we would expect, electrostatic attraction between the
surfaces dominates at long range. One surface can, in effect, operate as
the “counterion” for the apposing surface. As the surfaces approach,
counterions are released into solution and expelled from the gap between
the two surfaces, resulting in an increase in entropy of the system. If the
charge densities of each surface are equivalent, the electrostatic attraction
continues up until contact between the surfaces. If the charge densities
are not equal, some counterions must remain in the gap. Gradually they
become more concentrated as the surfaces approach each other, and at
some point the repulsive force from these counterions balances the
electrostatic attraction between the surfaces.
5.3 INTERMOLECULAR FORCES
AND AGGREGATION
Supramolecular chemistry is dominated by the host of noncovalent
interactions present in molecular subunits. A simple illustration can help
us understand the interplay between the various interactions discussed
so far, and how this interplay leads to molecular self-assembly into
nanomaterials with a specific structure. Consider the set of generic
molecules shown in Figure 5.15. For simplicity, only characteristics
emphasizing interactions are shown, such as ionic moieties, hydrophobic
regions, dipoles, and hydrogen bonding groups. The molecules represent
molecular building blocks and the aggregation of these individual blocks
will be affected by intermolecular interactions.
The organization of the building blocks into more complex structures will
largely be driven by thermodynamics, whether energetics or entropy.
The latter factor, for example, can be an entropic gain due to the
hydrophobic effect. Minimization of energy will be achieved by minimizing unfavorable interactions such as bringing two like-charged moieties to the same vicinity. Possible ways the molecular building blocks
may assemble are shown in Figure 5.15b. Self-assembly will lead to a
three-dimensional aggregate, but if the assembly is occurring on a
surface, then a two-dimensional aggregate will form. A planar support
will also impose some restrictions on the exact structure. For example, in
CHAPTER 5: Intermolecular Interactions and Self-Assembly
162
occur when the two surfaces are at a greater distance than if one of the
surfaces was neutral.
Finally, let’s consider the case of the interaction between two surfaces of
opposite charge. As we would expect, electrostatic attraction between the
surfaces dominates at long range. One surface can, in effect, operate as
the “counterion” for the apposing surface. As the surfaces approach,
counterions are released into solution and expelled from the gap between
the two surfaces, resulting in an increase in entropy of the system. If the
charge densities of each surface are equivalent, the electrostatic attraction
continues up until contact between the surfaces. If the charge densities
are not equal, some counterions must remain in the gap. Gradually they
become more concentrated as the surfaces approach each other, and at
some point the repulsive force from these counterions balances the
electrostatic attraction between the surfaces.
5.3 INTERMOLECULAR FORCES
AND AGGREGATION
Supramolecular chemistry is dominated by the host of noncovalent
interactions present in molecular subunits. A simple illustration can help
us understand the interplay between the various interactions discussed
so far, and how this interplay leads to molecular self-assembly into
nanomaterials with a specific structure. Consider the set of generic
molecules shown in Figure 5.15. For simplicity, only characteristics
emphasizing interactions are shown, such as ionic moieties, hydrophobic
regions, dipoles, and hydrogen bonding groups. The molecules represent
molecular building blocks and the aggregation of these individual blocks
will be affected by intermolecular interactions.
The organization of the building blocks into more complex structures will
largely be driven by thermodynamics, whether energetics or entropy.
The latter factor, for example, can be an entropic gain due to the
hydrophobic effect. Minimization of energy will be achieved by minimizing unfavorable interactions such as bringing two like-charged moieties to the same vicinity. Possible ways the molecular building blocks
may assemble are shown in Figure 5.15b. Self-assembly will lead to a
three-dimensional aggregate, but if the assembly is occurring on a
surface, then a two-dimensional aggregate will form. A planar support
will also impose some restrictions on the exact structure. For example, in
CHAPTER 5: Intermolecular Interactions and Self-Assembly
162