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Langmuir–Blodgett Monolayers of Partially Fluorinated Ionic Liquids as Two-Dimensional, More Sustainable Functional Materials and Coatings

Sieling, Thorben, Christoffers, Jens, Brand, Izabella
ACS sustainable chemistry & engineering 2019 v.7 no.13 pp. 11593-11602
X-ray photoelectron spectroscopy, air, anions, carbon, catalytic activity, cations, chemical structure, coatings, electrochemistry, environmental impact, gold, hydrophobicity, imidazoles, ionic liquids, liquid-air interface, perfluorocarbons
The structure of cations and anions in ionic liquids is precisely selected to tune their properties for applications in catalysis, electrochemistry, or development of sensors. Substitution of a hydrocarbon by a fluorocarbon chain in either the cation or the anion yields fluorinated ionic liquids (FILs). The ionic character combined with hydrophobic and lipophobic properties of fluorocarbon chains ensure extraordinary surface properties of FILs. The use of fluorocarbons with chains longer than six carbon atoms, due to their impact on the environment and bioaccumulative properties, is banned. In this work we demonstrate that more sustainable imidazolium- and triazolium-based FILs containing two short, partially fluorinated chains [(CH₂)₂(CF₂)₅CF₃] have amphiphilic properties and form stable monolayers at the air–liquid interface which can be transferred onto a gold substrate by the Langmuir–Blodgett (LB) method. X-ray photoelectron and polarization modulation infrared reflection absorption spectroscopies are used to characterize the composition, conformation, and orientation of FILs in LB monolayers on the gold surface. The positively charged heteroaromatic ring is oriented parallel to the gold surface while the fluorocarbon chains are directed toward air. The fluorocarbon chains adopt a helical conformation. In LB monolayers, the tilt of the helix depends on the chemical structure of the cation and the monolayer transfer conditions. Uniform orientation of the amphiphilic cations in the monolayer assembly yields a hydrophobic surface. Two-dimensional LB films of FILs are proposed here as possible sustainable, functional, ultrathin films and coatings.