Self-Assembled Monolayers Derived from Alkoxyphenylethanethiols Having One, Two, and Three Pendant Chains

This article describes the design, synthesis, and study of alkoxyphenylethanethiol-based adsorbates with one (<b>R1ArMT</b>), two (<b>R2ArMT</b>), and three (<b>R3ArMT</b>) pendant octadecyloxy chains substituted at the 4-, 3,5-, and 3,4,5-positions, respectively, of the phenylethanethiol group. These adsorbates are being developed for use in the preparation of compositionally versatile “mixed” self-assembled monolayer (SAM) coatings. The resultant SAMs were characterized by ellipsometry, contact angle goniometry, polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS), and X-ray photoelectron spectroscopy (XPS). The studies revealed that <b>R1ArMT</b> generates a well-ordered monolayer film, while <b>R2ArMT</b> and <b>R3ArMT</b> generate monolayer films with diminished conformational order in which the degree of crystallinity decreases as follows: <b>C18</b> ∼ <b>R1ArMT</b> > <b>R3ArMT</b> > <b>R2ArMT</b>. In addition, comparison of the molecular and chain packing densities of SAMs derived from these new adsorbates reveals that the <b>R2ArMT</b> and <b>R3ArMT</b> adsorbates give rise to SAMs with reduced chain tilt and smaller surface area per chain when compared to the SAMs derived from <b>C18</b> and <b>R1ArMT</b>.