Siloxane-Terminated Self-Assembled Monolayers for Mimicking Nanoscale Hydrophobic Polydimethylsiloxane Surfaces

Hydrophobic surfaces are important in daily life and in industries, including antifouling coatings, self-cleaning surfaces, microfluidic devices, and medicine. Here, we report the preparation and study of a nanoscale hydrophobic coating based on self-assembled monolayers (SAMs) of a thiol-functionalized methacryloxypropyl T-structure siloxane adsorbate, HS(CH₂)CH(CH₃)COO(CH₂)₄Si[O(Si(CH₃)₂O)_<i>n</i>Si(CH₃)₃]₃ (MAPTSiSH). The siloxane adsorbate was characterized by ¹H, ¹³C, and COSY nuclear magnetic resonance spectroscopy. The resulting MAPTSiSH SAMs were characterized by ellipsometry, X-ray photoelectron spectroscopy (XPS), and polarization modulation infrared reflection-absorption spectroscopy (PM-IRRAS). Importantly, comparison of the hydrophobic properties of the SAMs derived from MAPTSiSH with its polymeric analogue, polydimethylsiloxane (PDMS), using contact-angle measurements over a broad set of probe liquids showed comparable surface wettability. Thus, the new siloxane-terminated SAMs serve as nanoscale mimics of hydrophobic PDMS surfaces and can be used to investigate the interfacial properties of industrially important PDMS without interference from surface reconstruction.