Surface induced smectic order in ionic liquids – an X-ray reflectivity study of [C₂₂C₁im]⁺[NTf₂]⁻

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 SmA 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/τ) 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.

Fluorenone imidazolium salts as novel de Vries materials

“…The DSC curves reveal supercooling for the mesophase-to-crystalline transition upon cooling from the isotropic liquid while clearing temperatures are maintained in heating/cooling scans. It should be noted that supercooling is quite common among ILCs [93][94][95][96][97] and was recently rationalized by cationic clustering. 98 Under the POM Br(On,Om) displayed focal-conic fan textures, which are characteristic of SmA phases (Fig.…”

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

“…XRR study of mercury supported Langmuir films of imidazolium-based ionic liquids showed a gradual penetration of mercury into the ionic liquid film over several hours [182]. Surface-induced smectic order was found for the ionic liquid 1-methyl-3-docosylimidazolium bis(trifluoromethlysulfonyl) imide by XRR and GID experiments [183]. An ordered structure of alternating layers composed of polar and nonpolar moieties was observed near the free liquid surface that persists up to 88 K above the bulk melting temperature.…”

Nanoscale Structure in Short‐Chain Ionic Liquids

“…[36][37][38][39] This lineshape is derived employing the two-point correlations in liquids where the intermolecular potential has a finite range. [46,47] Previous studies [38,39,45] have shown this lineshape to be accurately approximated by an asymmetric Lorentzian, which is mathematically much simpler and easier to use. [21,22,38,39,45] Thus, our model consists of a sum of asymmetric Measured intensity I(q) of scattered x-rays vs. scattering vector q for C 1 -C 6 (a) at a fixed 25°C, and (b) at the temperatures listed.…”

“…The strong overlap of peaks II and III precludes confident extraction by eye of T-and n-dependent trends in the raw curves in Figure 1, and inferring structural conclusions therefrom. Resolution of the scattering curves into individual lines by model fits has been shown to support a much more detailed elucidation of the structure [22,29,39,45] and is consequently used here as discussed in the next subsections.…”

“…[46,47] Previous studies [38,39,45] have shown this lineshape to be accurately approximated by an asymmetric Lorentzian, which is mathematically much simpler and easier to use. [21,22,38,39,45] Thus, our model consists of a sum of asymmetric Measured intensity I(q) of scattered x-rays vs. scattering vector q for C 1 -C 6 (a) at a fixed 25°C, and (b) at the temperatures listed. (c) A cartoon of the layered structure of C n for n � 4, provided for illustrating the concepts used in this paper.…”