Abstract
Surface induced smectic order was found for the ionic liquid 1-methyl-3-docosylimidazolium bis(trifluoromethlysulfonyl)imide by X-ray reflectivity and grazing incidence scattering experiments. Near the free liquid surface, an ordered structure of alternating layers composed of polar and non-polar moieties is observed. This leads to an oscillatory interfacial profile perpendicular to the liquid surface with a periodicity of 3.7 nm. Small angle X-ray scattering and polarized light microscopy measurements suggest that the observed surface structure is related to fluctuations into a metastable liquid crystalline SmA2 phase that was found by supercooling the bulk liquid. The observed surface ordering persists up to 157 °C, i.e. more than 88 K above the bulk melting temperature of 68.1 °C. Close to the bulk melting point, we find a thickness of the ordered layer of L = 30 nm. The dependency of L(τ) = Λ ln(τ/τ1) vs. reduced temperature τ follows a logarithmic growth law. In agreement with theory, the pre-factor Λ is governed by the correlation length of the isotropic bulk phase.
Highlights
24098 Kiel, Germany e Ruprecht Haensel Laboratory, Kiel University, Leibnizstr. 19, 24098 Kiel, Germany f DESY Photon Science, Notkestr. 85, 22607 Hamburg, Germany g Max Planck Institute for Intelligent Systems, Heisenbergstr. 3, 70569 Stuttgart, Germany h Institute for Theoretical Physics IV, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany † Electronic supplementary information (ESI) available
The Ionic Liquids (ILs) was purified by recrystallization from a cold ethanol/water (75 : 25) mixture and zone melted in glass tubes under vacuum.[64,65,66]
Details on material synthesis and purification are described in ref. 67
Summary
3, 70569 Stuttgart, Germany h Institute for Theoretical Physics IV, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany † Electronic supplementary information (ESI) available. That are stable under ambient conditions opened up new fields for applications.[3] ionic liquids hold immense promise as environmentally friendly replacements for conventional solvents and reaction media in chemical, energy and nano applications.[2,4] Most of these applications involve processes at surfaces and interfaces in contact with other media. In the SILP (Supported Ionic Liquids Heterogeneous Phase Catalysis) process the chemical reaction takes place in an IL thin film, wetting a solid support material with high surface area.[5,6,7] Here, one rate limiting step is the diffusion of reactants and products across the IL/gas interface. To better understand the interfacial transport mechanisms and optimize the performance of ILs in SILP catalysis a detailed knowledge of the IL surface structure is highly desirable.[8]
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