Microstructure, wettability, and thermal stability of semifluorinated self-assembled monolayers (SAMs) on gold

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 absorption 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 nomlal. The longer methylene spacers (HIE, n = 11, 17, and 33) exhibited well-ordered trans-extended conformations as indicated by the position of the antisymmetric methylene band (nu(a)(CH2) = 2919 cm(-1)). Shortening the length of the methylene spacer to It = 6, however, led to a decrease in conformational order (nu(a)(CH2) = 2925 cm(-1)). Dynamic contact angle measurements using the Wilhelmy plate method showed that the semifluorinated SAMs were poorly wet by both water (average theta(a) = 120 degrees) and hexadecane (average theta(a) = 81 degrees). 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 degrees C for 1 h in air).