Self-assembled monolayers (SAMs) derived from 1,2-bis(mercaptomethyl)-4,5-dialkylbenzene (1), 1-mercaptomethyl-3,4-dialkylbenzene (2), 1-mercaptomethyl-4-alkylbenzene (3), 1-mercapto-4-alkylbenzene (4), and 4-mercaptomethyl-4‘-alkoxybiphenyl (5) were prepared by adsorption from solution onto evaporated gold. The SAMs were characterized by contact angle goniometry, optical ellipsometry, and polarization modulation infrared reflection absorption spectroscopy. Comparison of SAMs generated from 1 and 2 to well-known SAMs generated from normal alkanethiols (6) revealed that the former SAMs are densely packed and highly oriented. The alkyl chains of the SAMs derived from 1 and 2 are, however, less crystalline (i.e., less conformationally ordered) than those derived from 6. The adsorption of compounds 3, 4, and 5 onto gold yielded highly crystalline SAMs. Solution-phase thermal desorption of SAMs 1 − 6 at temperatures ranging from 60 to 110 °C was monitored using ex situ ellipsometry. The desorption profiles of SAMs derived from 1 − 6 revealed two distinct kinetic regimes:  a fast initial desorption followed by a substantially slower desorption. The rates of film desorption in both regimes were observed to increase with increasing temperature. Comparison of the desorption profiles of SAMs 1 − 6 showed that the structure of the adsorbate strongly influenced the rate of desorption; the differences were more apparent during the advanced stages of desorption. These studies provided a framework for evaluating the structural features and the mechanistic pathways that dictate the thermal stability of SAMs on gold.

Self-assembled monolayers of terminally fluorinated alkanethiols, CF3(CH2) n SH with n = 9−15, and their nonfluorinated analogues, CH3(CH2) n SH with n = 9−15, were prepared by adsorption from solution onto evaporated gold. The monolayers were characterized by contact angle goniometry, ellipsometry, and X-ray photoelectron spectroscopy. The analyses indicate that the CF3-terminated alkanethiols generate terminally fluorinated monolayers that are well-ordered, particularly when the chain lengths consist of 12 or more carbon atoms. Comparison of CF3-terminated films to CH3-terminated films of similar length reveals that terminal fluorination of the surface leads to an overall decrease in the surface tension of the films. This decrease arises from a relatively large decrease in the dispersive component of the surface tension upon the introduction of fluorine. Surprisingly, terminal fluorination also leads to a small but significant increase in the nondispersive component(s) of the surface tension. The origin of these opposing effects is discussed.

This study provides a comparison of the rates of adsorption of the following thiols onto the surface of gold: 1,2-bis(mercaptomethyl)-4,5-dihexadecylbenzene (1), 1-mercaptomethyl-3,4-dihexadecyl-benzene (2), hexadecanethiol (4a), and and eicosanethiol (4b). This study also compares the rates of adsorption of these adsorbates to those obtained for the aromatic disulfide analogue of 1 (2,3-dithia-6,7-dihexadecyltetralin, 3) and the normal dialkyl disulfide analogues of 4a and 4b (hexadecyl disulfide, 5a, and eicosyl disulfide, 5b, respectively). The adsorption behavior was monitored using ex situ ellipsometry and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). The adsorption profiles suggest that monolayer formation proceeds via two distinct kinetic regimes: (1) a fast initial adsorption, where ca. 80-90% of the monolayer forms during the first few minutes of immersion, followed by (2) a slower orientational ordering lasting several hours. Comparison of the rates of adsorption of the aromatic dithiols 1 to those of aromatic monothiol 2 and normal alkanethiols 4a and 4b reveals that the structure of the adsorbate plays a substantial role during the initial stages of thiol adsorption. The impact of structural and/or chemical variations is further illustrated by comparing the poor quality of the monolayer generated from the aromatic disulfide 3 to those of monolayers generated from 1, 2, 4, and 5.

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