compared to the same molecules “disordered” and occupying a larger
volume. Since entropy changes are positive for spontaneous processes, it
is favorable for a water molecule to be “released” from a small space and
enter a larger volume, especially if such a process leads to a loss of order.
Consider an organic molecule with a large hydrocarbon chain [e.g.,
CH 3 (CH 2 ) 9 OH] placed in water. It turns out that water molecules, in
avoiding contact with the hydrocarbon chain, form a cage around the
chain. This cage contains ordered water molecules essentially immobilized around the organic molecule. When two such “hydrated” molecules
approach each other and make contact, this cage is disrupted and the
confined water molecules are liberated into the bulk solution. This
process is accompanied by an increase in entropy of the water molecules.
In a sense the van der Waals attractions between the two hydrocarbon
chains is driven in part by the large increase in entropy due to the disruption of the cage. If this process occurs among many organic molecules
it will lead to aggregation, typically resulting in nanoscale entities dispersed within the aqueous solution.
The hydrophobic effect is paramount in many self-assembly processes,
including some biological processes such as the formation of the cell
membrane. Reference to the effect is made throughout the text, and in
particular, Section 5.3 contains a discussion of the hydrophobic effect in
the context of surfactant chemistry.
5.2 ELECTROSTATIC FORCES BETWEEN
SURFACES: THE ELECTRICAL
DOUBLE LAYER
Surface chemistry plays a vital role in the self-assembly of nanomaterials.
The forces discussed so far (van der Waals interactions, hydrogen bonds,
hydrophobic interactions, etc.) may exist between a planar surface and a
molecule some distance away. The strength and nature of these interactions will determine the extent to which molecules adsorb to the
surface and perhaps initiate the growth of a nanomaterial. Furthermore,
surface forces play an important role in catalysis, where a surface-bound
molecule may be immobilized on the surface in an optimal geometry for a
reaction to ensue. This section focuses on electrostatic interactions at
surfaces. A more thorough treatment of the subject can be found in
Israelachvili’s classic book Intermolecular and Surface Forces.
CHAPTER 5: Intermolecular Interactions and Self-Assembly
156
volume. Since entropy changes are positive for spontaneous processes, it
is favorable for a water molecule to be “released” from a small space and
enter a larger volume, especially if such a process leads to a loss of order.
Consider an organic molecule with a large hydrocarbon chain [e.g.,
CH 3 (CH 2 ) 9 OH] placed in water. It turns out that water molecules, in
avoiding contact with the hydrocarbon chain, form a cage around the
chain. This cage contains ordered water molecules essentially immobilized around the organic molecule. When two such “hydrated” molecules
approach each other and make contact, this cage is disrupted and the
confined water molecules are liberated into the bulk solution. This
process is accompanied by an increase in entropy of the water molecules.
In a sense the van der Waals attractions between the two hydrocarbon
chains is driven in part by the large increase in entropy due to the disruption of the cage. If this process occurs among many organic molecules
it will lead to aggregation, typically resulting in nanoscale entities dispersed within the aqueous solution.
The hydrophobic effect is paramount in many self-assembly processes,
including some biological processes such as the formation of the cell
membrane. Reference to the effect is made throughout the text, and in
particular, Section 5.3 contains a discussion of the hydrophobic effect in
the context of surfactant chemistry.
5.2 ELECTROSTATIC FORCES BETWEEN
SURFACES: THE ELECTRICAL
DOUBLE LAYER
Surface chemistry plays a vital role in the self-assembly of nanomaterials.
The forces discussed so far (van der Waals interactions, hydrogen bonds,
hydrophobic interactions, etc.) may exist between a planar surface and a
molecule some distance away. The strength and nature of these interactions will determine the extent to which molecules adsorb to the
surface and perhaps initiate the growth of a nanomaterial. Furthermore,
surface forces play an important role in catalysis, where a surface-bound
molecule may be immobilized on the surface in an optimal geometry for a
reaction to ensue. This section focuses on electrostatic interactions at
surfaces. A more thorough treatment of the subject can be found in
Israelachvili’s classic book Intermolecular and Surface Forces.
CHAPTER 5: Intermolecular Interactions and Self-Assembly
156