Adsorption of docosanethiol (CH3(CH2)21SH) onto polycrystalline gold from ethanol solution was investigated
by nonlinear vibrational spectroscopy. The formation of the self-assembled monolayer (SAM) was monitored
by recording of the temporal evolution of the C−H stretching modes. Three different steps with significantly
different time scales were identified. The fastest step in the adsorption process is related to the Au−S bond
formation. Progressing with a rate constant of 2500−3000 L mol-1 s-1, it is characterized by hydrocarbon
chains with a high fraction of gauche kinks. The second step proceeds 3−4 times more slowly and comprises
the straightening of the alkane chains. During the first two steps a pronounced band around 2813 cm-1 appears,
which is interpreted as mode softening of the C−H vibrations due to the interaction with the substrate. The
time scale of the final step is a factor of 35−70 slower than the rate of chain stretching and encompasses the
reorientation of the terminal methyl end groups. The methylene group adjacent to the methyl group exhibits
the same temporal behavior and thus behaves oppositely to the other methylene groups. The SFG data imply
that the last few percent of the thiol molecules adsorbing produce the complete monolayer by inducing the
transition from a high number of gauche defects to an all-trans conformation. The sequence of changes in the
vibrational spectra suggests an ordering and annealing of the film from the interface to the film surface.
Self-assembled monolayers of methyl(1-mercaptoundec-11-yl)tri(ethylene glycol) (CH3O(C2H4O)3C11H22SH,
EG3-OMe) adsorbed on gold were investigated by IR−vis sum frequency generation in the range of the
C−H stretching vibrations. Comparison of the monolayers in ambient atmosphere, in contact with water,
and in contact with carbon tetrachloride revealed that the film structure is strongly disturbed by the
interaction of the liquid with the monolayer. The ordered structure found in air undergoes an amorphization
upon exposure to the solvents. The experiments demonstrate that in situ analysis of the film structure
is mandatory.
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