Mimicking Polymer Surfaces Using Cyclohexyl- and Perfluorocyclohexyl-Terminated Self-Assembled Monolayers

Abstract

This article describes the synthesis of two cyclohexyl-terminated alkanethiols (HCyHnSH; n = 10 and 11) and their fluorinated analogs C6F11(CH2)nSH (n = 10 and 11; FCyHnSH). These ring-terminated adsorbates were used to generate self-assembled monolayers (SAMs) on gold surfaces to serve as model polymeric interfaces on metal substrates. The corresponding SAMs were characterized using ellipsometry, X-ray photoelectron spectroscopy (XPS), polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS), and contact angle goniometry. While the ellipsometric thicknesses were consistent with monolayer formation, analysis by XPS indicated that the HCyHnSH SAMs exhibited greater packing densities than their fluorocarbon analogs. However, PM-IRRAS spectra revealed that the underlying methylene units in the HCyHnSH SAMs were less conformationally ordered than their fluorocarbon analogs. In all the characterizations, no discernible “odd–even” effect was observed in either the HCyHnSH or the FCyHnSH SAMs, plausibly due to the bulkiness and conformational flexibility of the chain termini. Interestingly, comparison of the contact angles of a wide range of contacting liquids on these SAMs and their polymeric analogs, polyethylene (PE) and polytetrafluoroethylene (PTFE), found that these liquids exhibited similar wettability on the HCyHnSH SAMs and PE surfaces, and separately on the FCyHnSH SAMs and PTFE surfaces. These results suggest that these SAMs can successfully act as mimics of nanoscale polymer films and can be used in future studies (e.g., ion-surface collisions and plasma modification) without concerns of surface reconstruction.