The acid-base equilibrium of ω-functionalized alkanethiol monolayers on Au(111) has been studied using the change in double-layer capacitance accompanied by the protonation-deprotonation of ω-terminals. The pK of ω-carboxyl alkanethiols on Au(111) increases by four pH units. The shift becomes greater as the alkyl chain length increases. The same magnitude of the pK shift to the acidic side occurs in the monolayers of aminoethane thiol. The surface pK varies little with the electrode potential, whereas the increase in the supporting electrolyte concentration slightly diminishes the pK shift. The drawn-out shape of the titration curves is consistent with the mean-field model taking account of the repulsive interaction between adsorbed molecules, indicating the significance of the strong electrostatic repulsion between adsorbed thiol molecules in its charged states. The magnitude of the pK shift is, more than that predicted by the mean-field model, however, and suggests the considerable contribution from other factors that stabilize the uncharged state, for example, hydrogen bonding and low dielectric constant in the vicinity of the self-assembled monolayer. The possibility of counterion binding when the degree of deprotonation is large has been suggested.
The reductive desorption process of self-assembled monolayers of
1-hexadecanethiol, 1-propanethiol,
and 3-mercaptopropionic acid on Au(111) has been studied in 0.5 M
KOH solution by in-situ scanning
tunneling microscopy (STM) and cyclic voltammetry. In-situ STM
images of the monolayers at the potentials
between −0.2 V and the reduction potentials of each thiols show the
pits that are commonly seen in STM
images of thiol self-assembled monolayers. A drastic morphological
change takes place in the STM image
around the peak potential in a cyclic voltammogram for the reductive
desorption of adsorbed thiols. The
images indicate that 3-mercaptopropionic acid molecules diffuse away
from the surface after the reduction
because of its higher solubility, while 1-propanethiol and
1-hexadecanethiol molecules stay in the vicinity
of the surface forming aggregates. The partial recovery of the
1-hexadecanethiol monolayer after the
anodic scan, suggested by cyclic voltammograms, is confirmed by STM,
whereas 1-propanethiol aggregates
remain at the surface without being reoxidized. The difference in
the reoxidation behavior reflects the
different amphiphilic properties of the desorbed molecules and the
resultant molecular organizations
formed on the surface.
A new method for preparing phase-separated binary self-assembled
monolayers (SAMs) of alkanethiols
on gold is proposed. Binary SAMs of 3-mercaptopropionic acid (MPA)
and hexadecanethiol (HDT) at
various mixing ratios which exhibit phase separation in nanometer scale
are used as starting substrates.
Because the MPA domains in the binary SAMs have a less negative
desorption potential, they can be
selectively removed by controlling the potential of gold substrates in
0.5 M KOH. Other alkanethiols with
various chain lengths and terminal groups can subsequently be adsorbed
where MPA was originally
adsorbed, resulting in phase-separated binary SAMs with different
combinations of alkanethiols.
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