Microstructure, Wettability, and Thermal Stability of Semifluorinated Self-Assembled Monolayers (SAMs) on Gold

Abstract

The microstructure, wettability, and thermal stability of self-assembled monolayers (SAMs) on gold generated from semifluorinated alkanethiols F(CF2)10(CH2) n SH, where n = 2, 6, 11, 17, and 33 (F10HnSH), were examined by polarization modulation infrared reflection absoprtion spectroscopy (PM-IRRAS) and dynamic contact angle measurements. Analysis by PM-IRRAS revealed that the length of the methylene spacer (Hn) influenced the tilt angle of the fluorocarbon segments in the semifluorinated SAMs. As the length of the methylene spacer was increased, the tilt angle of the perfluorocarbon moiety increased with respect to the surface normal. The longer methylene spacers (Hn, n = 11, 17, and 33) exhibited well-ordered trans-extended conformations as indicated by the position of the antisymmetric methylene band (νa CH 2 = 2919 cm-1). Shortening the length of the methylene spacer to n = 6, however, led to a decrease in conformational order (νa CH 2 = 2925 cm-1). Dynamic contact angle measurements using the Wilhelmy plate method showed that the semifluorinated SAMs were poorly wet by both water (average θa = 120°) and hexadecane (average θa = 81°). The wettability varied with the length of the methylene spacer; in particular, both the thinnest and the thickest semifluorinated SAMs (derived from F10H2SH and F10H33SH, respectively) exhibited relatively low dynamic contact angle values. In addition, the thermal stability of the semifluorinated SAMs was found to increase as the length of the methylene spacer was increased. Overall, these films exhibited remarkable resistance to thermal degradation (e.g., SAMs derived from F10H33SH sustained a relatively high contact angles after incubation at 150 °C for 1 h in air).

Tunable water-based lubrication behavior of alkyl- and fluoroalkyl-silanes

“…This result is the same as that in our previous work [23,24], and could be explained as follows. In aqueous solutions, perfluorocarbon chains adopt a helical conformation and consequently possess larger cross-sections than those of simple hydrocarbon chains, resulting in a strong ability to endure high sliding-velocities, even though the perfluorocarbon chains are more rigid than hydrocarbon chains, and high resistance to oxidation and corrosion [27]. Another important phenomenon to note is that the differences between the friction coefficients of C8/Si and C10/Si and those of C8F/Si and C10F/Si, i.e., between hydrocarbon chains and perfluorocarbon chains, are much smaller than those we observed in the dry-state experiments.…”

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…For the fluorinated adsorbates, the as CH 2 of the F8H10SH SAMs appears at 2919 cm The bands arising from the C-F vibrations in the region from 1000 to 1400 cm -1 (shown in Figure 6 and discussed below), were assigned based on literature examples using similar fluorocarbon materials [60,63]. Additionally, an estimate of the orientation of the fluorocarbon segments can be obtained by comparing the relative intensities of bands associated with transitions perpendicular and parallel to the fluorinated helix, pd CF 2 and ax CF 2 respectively (vide infra) [63,78,79].…”

“…Additionally, an estimate of the orientation of the fluorocarbon segments can be obtained by comparing the relative intensities of bands associated with transitions perpendicular and parallel to the fluorinated helix, pd CF 2 and ax CF 2 respectively (vide infra) [63,78,79]. The band at 1150 cm -1 has been assigned as the symmetric C-F stretch of the CF2, while those at 1205 and 1250 cm -1 to the antisymmetric C-F stretches [60,80]. Complicating our analysis of the C-F spectra is the overlap of other vibrational modes, including C-C bond stretching and C-C-C bending at ~1220 cm -1 [60,81].…”

“…The band at 1150 cm -1 has been assigned as the symmetric C-F stretch of the CF2, while those at 1205 and 1250 cm -1 to the antisymmetric C-F stretches [60,80]. Complicating our analysis of the C-F spectra is the overlap of other vibrational modes, including C-C bond stretching and C-C-C bending at ~1220 cm -1 [60,81]. The bands in the 1150-1270 cm -1 range can be attributed to the stretches whose transitions dipole moments are perpendicular to the fluorocarbon chain, pd CF 2 [61]; correspondingly, the bands at 1336 and 1373 cm -1 have been identified as stretches with a transition dipole moment parallel to the fluorocarbon helical axis [61].…”

“…The discovery of poly(tetrafluoroethylene) (PTFE) by Plunkett in 1938 [58] has led to the extensive use of fluoropolymers in various applications due to their chemical inertness, thermal and mechanical resistance, and low adhesion [59]. Therefore, fluorocarbon-based films can offer certain advantages over hydrocarbon-based films in terms of chemical resistance, chain rigidity, thermal stability, hydrophobicity, and oleophobicity [60][61][62]. For this report, two new fluorinated bidentate dithiols were synthesized having the molecular structures shown in Figure 1.…”