hydrocarbon backbone into an all-trans conformation (Figure 7.7). The
resulting film is solid in nature, has a thickness almost the length of a fully
extended ODT molecule, and is referred to as a self-assembled monolayer
(SAM). Finally, the SAM renders the solid surface hydrophobic because
the surface now contains a very high density of closely packed methyl
groups. Consequently, a drop of water on this surface has a large contact
angle (>110°).
If adsorption is nonspecific and if only small amounts of heat are liberated, comparable to the heat of vaporization of the adsorbed material, the
process is physisorption. The interaction between the adsorbent and
adsorbate is much weaker (~20 kJmol
−1
) in a physisorbed film compared
to a chemisorbed film. It should be noted that although physisorption is typically nonspecific, thermodynamic considerations remain very
important for physisorption processes. As an example, we consider the
physisorption of a long-chain alcohol, such as dodecanol, from an aqueous solution onto a solid hydrophobic surface. Let the hydrophobic surface
be composed of an ODT monolayer chemisorbed to a gold surface. The
adsorption process is driven by a strong tendency of the alcohol to avoid
water (dodecanol is only sparingly soluble in water) and to associate with
any accessible hydrophobic surface. Thus, the dodecanol adsorbs to the
ODT SAM (Figure 7.8). The resulting dodecanol monolayer is closely
packed with the OH headgroup orientated toward the water phase. Like
the ODT SAM, the alkyl chains of dodecanol are closely associated with
each other due to favorable van der Waals interactions, and form a solid-like
SAM on the surface of the hydrophobic film. Unlike the ODT, the adsorption of dodecanol is reversible. The molecules in the film are in dynamic
equilibrium with the dodecanol molecules in the aqueous bulk phase.
CH 3
CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3
S S
+
S S S S S S S S S
SH
Gold-coated substrate
CH 3 (CH 2 ) 17 SH + Au surface
SAM
Figure 7.7 The formation
of a self-assembled monolayer by the chemisorption of
octadecane thiol on a gold
surface.
CHAPTER 7: Fundamentals of Surface Nanoscience
232
resulting film is solid in nature, has a thickness almost the length of a fully
extended ODT molecule, and is referred to as a self-assembled monolayer
(SAM). Finally, the SAM renders the solid surface hydrophobic because
the surface now contains a very high density of closely packed methyl
groups. Consequently, a drop of water on this surface has a large contact
angle (>110°).
If adsorption is nonspecific and if only small amounts of heat are liberated, comparable to the heat of vaporization of the adsorbed material, the
process is physisorption. The interaction between the adsorbent and
adsorbate is much weaker (~20 kJmol
−1
) in a physisorbed film compared
to a chemisorbed film. It should be noted that although physisorption is typically nonspecific, thermodynamic considerations remain very
important for physisorption processes. As an example, we consider the
physisorption of a long-chain alcohol, such as dodecanol, from an aqueous solution onto a solid hydrophobic surface. Let the hydrophobic surface
be composed of an ODT monolayer chemisorbed to a gold surface. The
adsorption process is driven by a strong tendency of the alcohol to avoid
water (dodecanol is only sparingly soluble in water) and to associate with
any accessible hydrophobic surface. Thus, the dodecanol adsorbs to the
ODT SAM (Figure 7.8). The resulting dodecanol monolayer is closely
packed with the OH headgroup orientated toward the water phase. Like
the ODT SAM, the alkyl chains of dodecanol are closely associated with
each other due to favorable van der Waals interactions, and form a solid-like
SAM on the surface of the hydrophobic film. Unlike the ODT, the adsorption of dodecanol is reversible. The molecules in the film are in dynamic
equilibrium with the dodecanol molecules in the aqueous bulk phase.
CH 3
CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3 CH 3
S S
+
S S S S S S S S S
SH
Gold-coated substrate
CH 3 (CH 2 ) 17 SH + Au surface
SAM
Figure 7.7 The formation
of a self-assembled monolayer by the chemisorption of
octadecane thiol on a gold
surface.
CHAPTER 7: Fundamentals of Surface Nanoscience
232