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

Self-assembled monolayers (SAMs) were prepared from a novel two-tailed partially fluorinated thiol (F8C11/C16), possessing one hydrocarbon chain and one chain with an extended fluorinated segment, and from mixtures of F8C11/C16 and hexadecanethiol (C16) on gold, with the expectation that the internal chemical dissimilarity and wedge-like shape of F8C11/C16 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 F8C11/C16 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 F8C11 and F8C11/C16 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 C16 matrix, the F8C11/C16 component organized into the two-dimensional analogue of discrete bilayers. Control experiments involving mixed SAMs comprised of F8C11/C16 and a single-tailed partially fluorinated thiol (F8C11) or C16 and F8C11 exhibited no appreciable indication of interesting self-organization beyond an evenly dispersed mixing of the thiolates or phase separation, respectively.