Ionic liquids (ILs) are known as �designer solvents� because their properties can be tuned by appropriate combination of a wide range of anions and cations. The concept of designer solvents can be extended by using mixtures of different ILs to tune the properties more finely. Because the surface properties of ILs are often of prime importance for some applications understanding of the surface structures of ILs is indispensable for these applications. We will present the results of our recent studies on the surface structures of ILs using high-resolution Rutherford backscattering spectroscopy (HRBS) with a particular emphasis placed on the surface structures of IL mixtures. HRBS can provide compositional depth profiles with a depth resolution ~ 0.2 nm at the surface. The orientation of surface molecules can also be deduced. The chosen substances are imidazolium based ILs consisting of 1-alkyl-3-methylimidazolium cations and various anions, Cl, [BF4], [PF6] or [TFSI]. In the surfaces of pure ILs, the alkyl chains of the cations protrude to the vacuum and the anions are located at almost the same depth as the imidazolium rings of the cations. The [TFSI] anion also shows a preferential orientation at the surface with CF3 groups pointing toward the vacuum. In the mixtures, the surface molecules have the same preferential orientations as in the pure ILs and surface segregation of particular cations or anions is observed in some mixtures. The observed surface segregation can be roughly explained in terms of the surface tension but larger anions tend to segregate more efficiently than the expected one.

Surface structures of ionic liquids observed by high-resolution Rutherford backscattering spectroscopy

248th National Meeting (2014)

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