The impact of fluorination on the structure and properties of self-assembled monolayer films

Abstract

The adsorption of partially fluorinated amphiphiles on metal/metal oxide surfaces allows for the generation of specifically fluorinated thin-film interfaces. Such surfaces are often compared to polytetrafluoroethylene (PTFE), which exhibits a low surface energy, accompanied by biological and chemical inertness, making perfluorinated interfaces applicable to a wide range of technologies. In thinfilm research, self-assembled monolayers derived from fluorinated alkanethiols (FSAMs) serve as welldefined systems that can be used to evaluate the physical and chemical properties of interfaces produced with varying degrees of fluorination. The characteristics of these surfaces have been attributed to both the chemical composition of the individual molecular adsorbates and the consequent structural features associated with monolayers formed from these unique partially fluorinated adsorbates. Specifically, this review seeks to correlate the structural and interfacial properties of FSAMs on gold with the structure/composition of the fluorinated moiety present in the adsorbed molecules and to highlight how the degree of fluorination influences the interfacial ordering of the individual alkanethiolate chains and the vacuum energy levels of the modified metal substrate. Additionally, the thermal stability of these organic thin films is analyzed as a function of adsorbate structure. Included are highlights of some of the studies in which FSAMs, formed from a variety of new types of surfactants, were used to modify colloidal systems, to generate anti-adhesive materials, and to enhance the stability of fluorinated thin films toward low-energy electron degradation.

Unsymmetrical Spiroalkanedithiols Having Mixed Fluorinated and Alkyl Tailgroups of Varying Length: Film Structure and Interfacial Properties

“…For the SAMs derived from the F8H10SH, the contact angle values are higher than those obtained from the C18SH film, which is consistent with previously observed values on SAMs derived from partially fluorinated adsorbates with the same degree of terminal fluorination [81,84]. The low surface energy enjoyed by fluorinated interfaces compared to hydrocarbon interfaces is consistent with the higher contact angle values observed on the F8H10SH SAM [85][86][87]. …”

Per- and polyfluorinated substances (PFASs): Environmental challenges

“…carboxylic acid-and phospholipidbased F-surfactants or even FnHm diblocks (Table 1)) readily form sturdy insoluble monolayers on water with high collapse pressures, their 4-or 6-carbon homologues generally fail to form stable molecular films [382]. The self-assembling capacity, packing organization, formation of ordered liquid condensed phases, and viscoelastic properties of interfacial surfactant films are also extremely sensitive to F-chain length [383][384][385][386][387]. 6:2 F-telomer sulfonates were deemed less effective than PFOS for hard-metal plating by the United Nations Environment Programme (UNEP), which encouraged research for better performing substitutes [388].…”

Selected physicochemical aspects of poly- and perfluoroalkylated substances relevant to performance, environment and sustainability—Part one

“…F-alkyl chains are thus more space demanding than alkyl chains. Structurally, the larger size of fluorine as compared to hydrogen forces long F-chains to adopt an all-trans helical conformation rather than the trans-planar zig-zag conformation of alkyl chains (Bunn and Howells, 1954;Clark, 1999;Ellis et al, 2004b;Krafft and Riess, 2009;Zenasni et al, 2013).…”

“…Partially fluorinated thiols (C n F 2n+1 (CH 2 ) m SH, typically n = 6-10; m = 2-17) are important components for self-assembled monolayers (SAMs) adsorbed at the surface of coinage metals or oxide surfaces for nanomaterials fabrication, including nanoscale surface coatings that inhibit corrosion of microelectromechanical systems (Zenasni et al, 2013). Wettability can be modulated by varying Fchain length.…”