Fibrillar Self-Organization of a Line-Active Partially Fluorinated Thiol within Binary Self-Assembled Monolayers

Abstract

Self-assembled monolayers (SAMs) were prepared from a novel two-tailed partially fluorinated thiol (<b>F8C11/C16</b>), possessing one hydrocarbon chain and one chain with an extended fluorinated segment, and from mixtures of <b>F8C11/C16</b> and hexadecanethiol (<b>C16</b>) on gold, with the expectation that the internal chemical dissimilarity and wedge-like shape of <b>F8C11/C16</b> would lead to unique self-organizational motifs. The SAMs were systematically characterized using ellipsometry, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and polarization modulation infrared reflection–absorption spectroscopy (PM-IRRAS). Based on this characterization, the one-component <b>F8C11/C16</b> SAMs exhibited relatively poor molecular organization compared to traditional alkanethiols, forming low coverage monolayers with significant molecular disorder. However, the series of mixed SAMs formed from <b>F8C11</b> and <b>F8C11/C16</b> were anomalously well ordered as indicated by film thickness, surface coverage, and the frequencies of characteristic vibrational modes. AFM images of these mixed SAMs exhibited nanoscale fibrillar structures in a birds-nest morphology, suggesting that in the presence of a <b>C16</b> matrix, the <b>F8C11/C16</b> component organized into the two-dimensional analogue of discrete bilayers. Control experiments involving mixed SAMs comprised of <b>F8C11/C16</b> and a single-tailed partially fluorinated thiol (<b>F8C11</b>) or <b>C16</b> and <b>F8C11</b> exhibited no appreciable indication of interesting self-organization beyond an evenly dispersed mixing of the thiolates or phase separation, respectively