10.4 SELF-ASSEMBLED MONOLAYERS
A self-assembled monolayer (SAM) is formed when a particular group on
a molecule has a strong affinity for a specific surface. Thiols and silanes are
examples of such molecules, and these are discussed in the next two
sections. Spontaneous chemisorptions of these adsorbate molecules may
occur from either the vapor or liquid phase and often leads to the formation
of an organized monolayer. The adsorbate, being a self-assembling
molecular building block, often has a relatively simple structure with the
reacting group attached to an alkyl chain and terminated by some other
inert group, although similar processes can be used for more complex
molecules. The chemisorption step is usually rapid, lowering the surface
energy of the substrate. This is followed by slow two-dimensional organization of the alkyl chains. This organization step is dominated by interchain hydrophobic interactions.
Surface organization itself typically occurs in stages, where each stage
represents a distinct phase. First, a low-density phase is formed comprised of adsorbate molecules randomly dispersed on the surface of the
substrate. This is followed by a transition to a second intermediate phase
with randomly dispersed molecules lying flat on the substrate. The final
transition to a high-density phase involves the molecules orienting
themselves normal to the substrate surface. Exactly how these phase
transitions proceed depends on temperature. If lateral interactions are
ignored, chemisorption follows the Langmuir adsorption isotherm. The
overall kinetics of the process are first-order and are approximately
described by Equation 10.3:
dq=dt = k 1 − q
ð
Þ
(10.3)
where q is proportional to the amount of area occupied and k is the rate
constant.
10.4.1 Thiols on gold
Thiols are compounds having the structure RSH (where R ≠ H). These
molecules are also referred to as mercaptans. Some commercially available thiols are shown in Figure 10.17. The thiol group is extremely reactive
to gold and other noble metals, resulting in the formation of a strong
metal–sulfur covalent bond having a bond strength on the order of 100 kJ
mol
−1 . The resulting monolayer on the metal surface is thermally stable
and is resistant to various solvents and electrolytes.
SELF-ASSEMBLED MONOLAYERS 407
A self-assembled monolayer (SAM) is formed when a particular group on
a molecule has a strong affinity for a specific surface. Thiols and silanes are
examples of such molecules, and these are discussed in the next two
sections. Spontaneous chemisorptions of these adsorbate molecules may
occur from either the vapor or liquid phase and often leads to the formation
of an organized monolayer. The adsorbate, being a self-assembling
molecular building block, often has a relatively simple structure with the
reacting group attached to an alkyl chain and terminated by some other
inert group, although similar processes can be used for more complex
molecules. The chemisorption step is usually rapid, lowering the surface
energy of the substrate. This is followed by slow two-dimensional organization of the alkyl chains. This organization step is dominated by interchain hydrophobic interactions.
Surface organization itself typically occurs in stages, where each stage
represents a distinct phase. First, a low-density phase is formed comprised of adsorbate molecules randomly dispersed on the surface of the
substrate. This is followed by a transition to a second intermediate phase
with randomly dispersed molecules lying flat on the substrate. The final
transition to a high-density phase involves the molecules orienting
themselves normal to the substrate surface. Exactly how these phase
transitions proceed depends on temperature. If lateral interactions are
ignored, chemisorption follows the Langmuir adsorption isotherm. The
overall kinetics of the process are first-order and are approximately
described by Equation 10.3:
dq=dt = k 1 − q
ð
Þ
(10.3)
where q is proportional to the amount of area occupied and k is the rate
constant.
10.4.1 Thiols on gold
Thiols are compounds having the structure RSH (where R ≠ H). These
molecules are also referred to as mercaptans. Some commercially available thiols are shown in Figure 10.17. The thiol group is extremely reactive
to gold and other noble metals, resulting in the formation of a strong
metal–sulfur covalent bond having a bond strength on the order of 100 kJ
mol
−1 . The resulting monolayer on the metal surface is thermally stable
and is resistant to various solvents and electrolytes.
SELF-ASSEMBLED MONOLAYERS 407