Molecular Layering of Fluorinated Ionic Liquids at a Charged Sapphire (0001) Surface

Abstract

Room-temperature ionic liquids (RTILs) are promising candidates for a broad range of "green" applications, for which their interaction with solid surfaces plays a crucial role. In this high-energy x-ray reflectivity study, the temperature-dependent structures of three ionic liquids with the tris(pentafluoroethyl)trifluorophosphate anion in contact with a charged sapphire substrate were investigated with submolecular resolution. All three RTILs show strong interfacial layering, starting with a cation layer at the substrate and decaying exponentially into the bulk liquid. The observed decay length and layering period point to an interfacial ordering mechanism, akin to the charge inversion effect, which is suggested to originate from strong correlations between the unscreened ions. The observed layering is expected to be a generic feature of RTILs at charged interfaces.

Checkerboard Self-Patterning of an Ionic Liquid Film on Mercury

“…The surface-parallel molecules self-organize into stripes, which exhibit onedimensional, smectic-like order, implying a checkerboardlike ionic surface tiling. This is consistent with the expected domination of the lateral order by the strong ionic Coulomb interaction, but differs from the ordering found for the bulk of the same RTIL at a charged solid interface, where alternately-charged ion layering was observed [10].…”

“…Surface-parallel ion orientations are also found in simulations of similar RTILs, albeit on graphite [22]. This ionic arrangement has a zero net local and global charge, and a lower free energy than that of the alternating uniformly charged layers found at the same RTIL's bulk interface with a charged sapphire substrate [10]. The average of the fitted exclusion areas, A 0 = 90.5Å 2 /molecule, and the molar volume, 0.370 l/mol, yield a monolayer thickness d 0 = 6.8Å, supported by the x-ray results below.…”

“…Whether a similar order emerges in our RTIL upon increasing its alkyl tail length, in spite of the irregular ionic shapes and the strong Coulomb interaction, remains to be investigated. The dichotomy between the electrically neutral mono-and bi-layers observed here and the alternating, uniformly charged, (multi)layers formed at the charged sapphire interface in contact with a bulk of the same RTIL [10] also deserves further investigation, to elucidate the role of the overlying RTIL bulk and the level of substrate charging in the structure formation and characteristics. Above all, a greater body of experiments on…”

“…Theoretical progress in understanding the interplay of interactions in, and of the strong-correlation aspects of, RTILs is hampered by the scarcity of molecular-resolution structural data, particularly at interfaces, where such measurements are highly challenging. While a few such studies of liquid/vapour (l/v ) RTIL interfaces are available [8,9], only two address liquid/solid (l/s) interfaces [10,11]. No bulk liquid/liquid (l/l ) interface studies were published.…”

The interface between Au(100) and 1-butyl-3-methyl-imidazolium-bis(trifluoromethylsulfonyl)imide

“…Other physical methods also reveal ordering and layering. High-energy X-ray reflectivity studies exhibit the existence of a strong interfacial layering between ILs and charged sapphire electrodes [39]. This intense interaction was also revealed for Au(111) in contact with BMPTf 2 N in form of a 2D-liquid state adlayer by UHV-STM [32].…”