Self-Assembly of n-Alkanethiol Monolayers. A Study by IR−Visible Sum Frequency Spectroscopy (SFG)

Abstract

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.

Formation of tethered bilayer lipid membranes probed by various surface sensitive techniques

“…Displacement of contaminants and expulsion of included solvent may also be involved in this slow step, 12 which can take several hours up to days. 6,13,14 Several reports have confirmed the two-step adsorption process by using different methods such as ellipsometry and contact angle ͑wettability͒ measurements, 12 x-ray photoelectron spectroscopy, 15 IRvisible sum frequency spectroscopy, 13,14 surface plasmon resonance ͑SPR͒ spectroscopy, 16 atomic force microscopy ͑AFM͒, 17 or quartz crystal microbalance ͑QCM͒. 18 SAMs can be used as functional coatings of substrates.…”

Evidence of a molecular boundary lubricant at snakeskin surfaces

“…Moving onto the CH stretching region, we observe two distinct vibrational modes in the spectra collected from the dorsal scale-2850 and 2865 cm 21 . Both of these modes stem from symmetric CH 2 vibrations [49]. The peak at 2865 cm 21 is also a CH 2 symmetric stretch but is shifted by 15 cm 21 , because it represents a symmetric vibrational mode from a CH 2 group adjacent to a CH 3 group [49].…”

Interference effects in the sum frequency generation spectra of thin organic films. II: Applications to different thin-film systems

“…The Y-type LB multilayers of the per-deuterated arachidate ͓Fig. 1͑b͔͒ with different layer thicknesses ͑4, 10,20,30,40,50,60,70, and 80 monolayers͒ except for the top-monolayer of the per-protonated arachidate, were deposited by sequential vertical dipping of the hydrophobic gold substrate. The layer thickness of the perdeuterated arachidate bilayer was estimated to be 5.7Ϯ 0.1 nm/ bilayer based on the results obtained by ellipsometry 5 and grazing incidence x-ray diffraction.…”

“…I, in situ SFG observations have been carried out for electrochemical systems. [39][40][41][42][43][44] In this case, in order to reduce the IR absorption by the aqueous electrolyte solution, a thin-layer geometry has usually been employed in which the electrode is usually placed on an optical window ͑usually, CaF 2 or BaF 2 is used͒ to form a thin solution layer estimated to be several microns thick. Since it is not possible to control the thickness of the thin layer exactly, it is necessary to consider the possibility of interference in the thin layer of electrolyte solution contained between the electrode and the optical window.…”

“…In fact, this prediction concurs with the reported results even though the thickness of the thin solution layer cannot be accurately controlled, and the spectral line shapes obtained from different groups for the same sample were similar to each other. [39][40][41][42][43][44] On the other hand, the intensity of the SFG signal for different film thicknesses exhibits some thickness dependent features. For example, Fig.…”