Depleting the number of dodecanol molecules in the bulk phase reduces
the packing density in the monolayer at the solid–aqueous interface.
Adsorption characteristics such as temperature, bulk concentration
dependence, and reversibility are often used to distinguish chemisorption
and physisorption. The energetic difference between the two adsorption
processes is nicely illustrated in the one-dimensional Lennard-Jones
potential energy curve for the adsorption of a diatomic molecule on a
planar surface. V(x) = 0 at large distances corresponds to zero interaction
between the substrate and adsorbate molecule at infinite separation. As
the molecule approaches the surface (going from point A to B in Figure
7.9), there is a negative attractive potential between the surface and the
adsorbate molecule. The potential reaches a minimum at distance B in
the case of physisorption and at distance C in the case of chemisorption.
OH
OH
OH OH
OH
OH OH
CH 3 (CH 2 ) 17 OH
CH 3
CH 3
CH 3 CH 3 CH 3 CH 3 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
Figure 7.8 The formation
of a physisorbed monolayer of
dodecanol on top of a SAM
composed of octadecane thiol
on a gold surface.
ADSORPTION PHENOMENA: SELF-ASSEMBLED MONOLAYERS 233