solution to the aqueous–air interface. Liquid–liquid interfaces, such as the
boundary between an oil and water phase, also represent regions at
which adsorption may occur. Surface adsorption plays a central role in
the formation of nanomaterials. Adsorbents present themselves as platforms for the self-assembly of molecules into nanostructures. Specific
examples in which nanomaterials are synthesized this way are presented
in Chapter 10. Solid substrates can be chemically modified so that
adsorption can be selective. This modification is known as surface
functionalization and may be as simple as oxidizing a metal surface to
render it hydrophilic and change the surface energy so that polar molecules are spontaneously adsorbed to the surface.
Many solids have the property of adsorbing large quantities of gases and
solutes from liquid solutions. This process is generally very specific both
with respect to the adsorbent and the material adsorbed and driven
largely by thermodynamic considerations. Adsorption is usually an exothermic process and can be divided into two kinds: chemisorption
(chemical adsorption) and physisorption (physical adsorption). In general, if adsorption is specific and if large amounts of heat are liberated
(greater than about 50 kJmol
−1
), the adsorption process is referred to as
chemisorption (first proposed by Irving Langmuir in 1916). In this
process, bonds are broken in the adsorbate molecules and new covalent
bonds are formed between the adsorbent and the adsorbate until a
complete monolayer has been established. The resulting substrateadsorbate bond strengths range from 200 to 500 kJmol
−1 . The chemisorbed monolayer is irreversibly bound to the solid surface and changes
the surface properties of the solid substrate to resemble those of the
exposed portion of the adsorbate. Thus, chemisorption is an excellent
method of chemically functionalizing a solid surface. As an example of
chemisorption, consider the molecule octadecanethiol (ODT), shown in
Figure 7.7. The molecule contains 18 carbon atoms (17 methylene groups
and one methyl group) terminated by a thiol group (SH). The thiol group
is extremely reactive toward gold, resulting in a strong Au–S covalent
bond. Thus, by placing a gold-coated substrate into a solution of ODT in a
solvent such as chloroform, molecules of ODT spontaneously chemisorb
to the Au surface and form a tightly packed monolayer within hours. A few
features regarding this process are worth noting. First, adsorption is
rapid, irreversible, and stops after a complete monolayer is formed.
Second, in addition to the strong Au–S bonds that are formed, the strong
van der Waals interactions between the neighboring alkyl chains in the
ODT monolayer allow the molecules to pack very tightly and force the
ADSORPTION PHENOMENA: SELF-ASSEMBLED MONOLAYERS 231
boundary between an oil and water phase, also represent regions at
which adsorption may occur. Surface adsorption plays a central role in
the formation of nanomaterials. Adsorbents present themselves as platforms for the self-assembly of molecules into nanostructures. Specific
examples in which nanomaterials are synthesized this way are presented
in Chapter 10. Solid substrates can be chemically modified so that
adsorption can be selective. This modification is known as surface
functionalization and may be as simple as oxidizing a metal surface to
render it hydrophilic and change the surface energy so that polar molecules are spontaneously adsorbed to the surface.
Many solids have the property of adsorbing large quantities of gases and
solutes from liquid solutions. This process is generally very specific both
with respect to the adsorbent and the material adsorbed and driven
largely by thermodynamic considerations. Adsorption is usually an exothermic process and can be divided into two kinds: chemisorption
(chemical adsorption) and physisorption (physical adsorption). In general, if adsorption is specific and if large amounts of heat are liberated
(greater than about 50 kJmol
−1
), the adsorption process is referred to as
chemisorption (first proposed by Irving Langmuir in 1916). In this
process, bonds are broken in the adsorbate molecules and new covalent
bonds are formed between the adsorbent and the adsorbate until a
complete monolayer has been established. The resulting substrateadsorbate bond strengths range from 200 to 500 kJmol
−1 . The chemisorbed monolayer is irreversibly bound to the solid surface and changes
the surface properties of the solid substrate to resemble those of the
exposed portion of the adsorbate. Thus, chemisorption is an excellent
method of chemically functionalizing a solid surface. As an example of
chemisorption, consider the molecule octadecanethiol (ODT), shown in
Figure 7.7. The molecule contains 18 carbon atoms (17 methylene groups
and one methyl group) terminated by a thiol group (SH). The thiol group
is extremely reactive toward gold, resulting in a strong Au–S covalent
bond. Thus, by placing a gold-coated substrate into a solution of ODT in a
solvent such as chloroform, molecules of ODT spontaneously chemisorb
to the Au surface and form a tightly packed monolayer within hours. A few
features regarding this process are worth noting. First, adsorption is
rapid, irreversible, and stops after a complete monolayer is formed.
Second, in addition to the strong Au–S bonds that are formed, the strong
van der Waals interactions between the neighboring alkyl chains in the
ODT monolayer allow the molecules to pack very tightly and force the
ADSORPTION PHENOMENA: SELF-ASSEMBLED MONOLAYERS 231