Self-assembled monolayers on gold generated from terminally perfluorinated alkanethiols bearing propyl vs. ethyl hydrocarbon spacers

• A new series of terminally perfluorinated propanethiols are reported.
• These new adsorbates were used to form self-assembled monolayers (SAMs) on gold.
• Ellipsometry and X-ray photoelectron spectroscopy confirm monolayer formation.
• Wettability measurements indicate an oleophobic and hydrophobic monolayer.
• Comparison to SAMs of ethanethiol derivatives shows no apparent “odd-even” effect.

This paper examines the structural and interfacial properties of terminally perfluorinated self-assembled monolayers (FSAMs) on gold generated from the adsorption of a new series of terminally perfluorinated propanethiols F(CF2)n(CH2)3SH, where n = 8, 10, and 12. Analysis of these FSAMs by ellipsometry and X-ray photoelectron spectroscopy (XPS) confirmed the formation of the monolayer films. The contact angles of water and n-hexadecane on these FSAMs indicated a high degree of hydrophobicity and oleophobicity. Polarization modulation infrared reflection-adsorption spectroscopy (PM-IRRAS) analysis of the films revealed that the fluorinated chains are oriented largely perpendicular to the gold surface. In addition, the FSAMs formed from the new adsorbates were compared to known FSAMs derived from F(CF2)n(CH2)2SH, where n = 8, 10, and 12, to examine the influence of the number of CH2 groups in the short alkyl spacer upon the conformational order and packing structure of the films. Analysis of the XPS spectra for the normalized peak intensity of the F 1s and S 2p binding energies for both types of films suggest a slight increase in packing density for the chains having the propyl vs. the ethyl hydrocarbon spacer. This conclusion is consistent with the observed decrease in the wetting behavior of hexadecane on the FSAMs formed from the new adsorbates. However, the preponderance of the data indicates that these two series of partially fluorinated alkanethiols form films with highly similar structure/packing characteristics with no discernible “odd-even” effect between the two series.

Synopsis: Analysis of the structural and interfacial properties of terminally perfluorinated self-assembled monolayers (FSAMs) on gold generated from a new series of propanethiol adsorbates of the form F(CF2)n(CH2)3SH, where n = 8, 10, and 12, confirms the formation of an oleophobic and hydrophobic monolayer with well packed perfluorinated chains oriented largely perpendicular to the underlying surface.Figure optionsDownload as PowerPoint slide