Wettabilities of self-assembled monolayers on gold generated from progressively fluorinated alkanethiols

The conformational order and wettability of self-assembled monolayers (SAMs) on gold generated from a series of partially fluorinated alkanethiols (F(CF2)(n)(CH2)(11)SH, n = 1- 10; FnH11SH, H11 series), possessing methylene spacers of equivalent length, and the corresponding n-alkanethiol (F0H11SH) were characterized by polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) and contact angle goniometry. The PM-IRRAS analyses revealed that the conformational order of the underlying methylene spacers remains constant with increasing n. Probing the wettabilities of the SAMs by contact angle goniometry using hydrocarbon and fluorocarbon contacting liquids revealed that the dispersive surface energies of the monolayers decreased as n increased from 0 to 6 and then remained constant for n = 6-10. Using cis-perfluorodecalin as the contacting liquid revealed that the decrease in the dispersive surface energies is due to a decrease in the surface density of the CF3 groups that occurs as the length of the perfluorocarbon segment increases with increasing n. The contact angles of the hydrocarbon liquids revealed that the presence of underlying CF2 groups can further reduce the strength of the dispersive interactions at the surface. The contact angles of polar contacting liquids on the SAMs were consistent with an increase in the distance of the oriented fluorocarbon-hydrocarbon (FC-HC) dipoles from the monolayer surface with increasing n. Calculation of the works of adhesion supported this model. Comparison of the data measured for SAMs derived from the H11 series to those measured for SAMs derived from a series of partially fluorinated alkanethiols whose total chain lengths were held constant (FnHmSH; n = 1-10, m = 15-6, respectively; n + m = 16; C16 series) and the corresponding n-alkanethiol (F0H15SH) revealed that differences in underlying monolayer structure fail to significantly influence the interfacial wettabilities of all of the contacting liquids. Calculation of the dispersive surface energies of the SAMs using the methods of both Zisman and Good, Girifalco, and Fowkes (GGF) revealed that both methods underestimate the energies of fluorocarbon surfaces when hydrocarbon contacting liquids are used, with Zisman's method providing lower estimates, compared to the energies estimated using a fluorocarbon contacting liquid in the GGF method. Overall, the results demonstrate that substituting fluorocarbon segments (n greater than or equal to 6) for the terminal hydrocarbon segments in alkanethiol SAMs produces lower energy surfaces having wettabilities that are less sensitive to the underlying film structure.