Influence of Different Substituents on the Surface Composition of Ionic Liquids Studied Using ARXPS

Lovelock, Kevin R. J.; Kolbeck, Claudia; Cremer, Till; Paape, Natalia; Schulz, Peter S.; Wasserscheid, Peter; Maier, Florian; Steinrück, Hans‐Peter

doi:10.1021/jp810637d

Cited by 183 publications

(397 citation statements)

References 67 publications

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“…The absence of significant signal intensity differences for the various core levels in 80° and 0° emission indicates a homogeneous distribution of anions and cations at the surface and in the bulk of the IL. This is in line with previous experience that surface enrichment effects are typically found for systems with functionalized and non‐functionalized alkyl chains with a chain length of at least 4 carbon units 52,53. Notably, the small peak at 285.0 eV, is due to a minor contamination of the IL (see Experimental Section).…”

Section: Resultssupporting

confidence: 91%

Surface Enrichment in Equimolar Mixtures of Non‐Functionalized and Functionalized Imidazolium‐Based Ionic Liquids

et al. 2018

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For equimolar mixtures of ionic liquids with imidazolium‐based cations of very different electronic structure, we observe very pronounced surface enrichment effects by angle‐resolved X‐ray photoelectron spectroscopy (XPS). For a mixture with the same anion, that is, 1‐methyl‐3‐octylimidazolium hexafluorophosphate+1,3‐di(methoxy)imidazolium hexafluorophosphate ([C8C1Im][PF6]+[(MeO)2Im][PF6]), we find a strong enrichment of the octyl chain‐containing [C8C1Im]+ cation and a corresponding depletion of the [(MeO)2Im]+ cation in the topmost layer. For a mixture with different cations and anions, that is, [C8C1Im][Tf2N]+[(MeO)2Im][PF6], we find both surface enrichment of the [C8C1Im]+ cation and the [Tf2N]− (bis[(trifluoromethyl)sulfonyl]imide) anion, while [(MeO)2Im]+ and [PF6]− are depleted from the surface. We propose that the observed behavior in these mixtures is due to a lowering of the surface tension by the enriched components. Interestingly, we observe pronounced differences in the chemical shifts of the imidazolium ring signals of the [(MeO)2Im]+ cations as compared to the non‐functionalized cations. Calculations of the electronic structure and the intramolecular partial charge distribution of the cations contribute to interpreting these shifts for the two different cations.

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Section: Resultssupporting

confidence: 91%

Surface Enrichment in Equimolar Mixtures of Non‐Functionalized and Functionalized Imidazolium‐Based Ionic Liquids

et al. 2018

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“…Furthermore, the quantum-chemically calculated pDOS provides invaluable information for analysis and interpretation of MIES spectra. For the increasing surface sensitivity from XPS over UPS to MIES, our quantum-chemically supported technique of reconstruction and analysis provides us information about the chemical composition and steric structure of ionic liquid surfaces, in full agreement with previously suggested surface structures of [EMIm]Tf 2 N and [OMIm]Tf 2 N. 6,25,26,[35][36][37] …”

Section: Discussionsupporting

confidence: 84%

“…Again a shift of 1.1 eV between the cation and the anion is necessary to fit the expected alkylchain contribution to the lower binding energy edge of the difference spectrum. Due to the very high surface sensitivity of MIES this feature of the difference spectrum means that the octyl-chains of the [OMIm] + cations are present at the outermost part of the surface-a result that agrees well with our UPS (He II) reconstruction and the results from angle resolved XPS, 25,26 HRBS, 35,36 and SFG. 6,37 Note that the experimental MIES spectra have been normalized to 12.5 eV, and this may influence the obtained difference spectrum.…”

Section: Ups Spectrasupporting

confidence: 85%

Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids

Reinmöller

Ulbrich

Ikari

et al. 2011

Phys. Chem. Chem. Phys.

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In the present work we analyze these spectra by means of partial density of states (pDOS) as calculated from a single ion pair of the respective ionic liquid using density functional theory (DFT). Subsequently we reconstruct the XPS and UPS spectra by considering photoemission cross sections and analyze the MIES spectra by pDOS, which provides us decisive hints to the ionic liquid surface structure.

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“…This directly observed near-surface ordering of the molecular moieties is consistent with the ordering deduced from indirect studies by surface tension and spectroscopy of ILs comprising [C n≥4 mim] + cations. Those measurements were interpreted as showing that the first layer of imidazolium rings and anions is located at a depth close to that of a fully extended C 18 chain (20,21) and that the alkyl chains protrude into the vapor (6,(22)(23)(24). The molecular model presented in Fig.…”

Section: Resultsmentioning

confidence: 97%

Surface layering and melting in an ionic liquid studied by resonant soft X-ray reflectivity

Mezger

Ocko

Reichert

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

100

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The molecular-scale structure of the ionic liquid [C 18 mim] + [FAP] − near its free surface was studied by complementary methods. X-ray absorption spectroscopy and resonant soft X-ray reflectivity revealed a depth-decaying near-surface layering. Element-specific interfacial profiles were extracted with submolecular resolution from energy-dependent soft X-ray reflectivity data. Temperature-dependent hard X-ray reflectivity, small-and wide-angle Xray scattering, and infrared spectroscopy uncovered an intriguing melting mechanism for the layered region, where alkyl chain melting drove a negative thermal expansion of the surface layer spacing.liquid surface | resonant scattering | surface structure T he defining feature of all interfaces is the break in symmetry, and this may induce interfacial order or disorder not found in the bulk. For liquids, a prominent example of surface-induced order are alkane melts where a surface-frozen crystalline monolayer is formed, coexisting with the molten bulk over a range of a few degrees above the bulk freezing temperature (1). In liquid crystals, a smectic, or nematic, multilayer wets the free surface of the isotropic bulk where the number of layers increases upon cooling toward the bulk's phase transition temperature (2). In liquid metals, a depth-decaying near-surface multilayer of a temperature-independent thickness and number is observed (3). The ionic liquid (IL), studied here, belongs to a unique class of organic salts with melting points below 100°C whose molecular interactions include van der Waals, dipolar, unscreened Coulomb, and hydrogen bonding. ILs also hold immense promise as environmentally friendly replacements for currently used solvents and reaction media (4, 5) in chemical, energy, and nano applications. Because most of these processes occur at interfaces, a detailed knowledge of the interfacial structural motifs caused by the complex nature of the ILs interactions is highly desirable. The very few Angstrom-resolution X-ray reflectivity (XRR) studies of IL free surfaces published to date reveal the presence of a dense surface monolayer, commonly interpreted as being a surface enrichment by the hydrophobic cation (6). Recently, surface multilayers were reported for two ILs with the bis(nonafluorobutanesulfonyl)amide anion (7). However, standard hard XRR, used in all surface studies to date, is not atom-specific for the low-Z atoms found in ILs; it is only sensitive to the surface-normal total electron density profile, but cannot directly resolve the distribution of the IL's various chemical moieties. In contrast, resonant XRR (8, 9) takes advantage of the strong variation of the atomic scattering factor with X-ray energy near an atom's absorption edge to distinguish between moieties of different atomic composition (10), functional groups (11), and molecular orientation (12, 13). Nevertheless, the challenge posed by molecular-resolution resonant soft XRR measurements at liquid surfaces hitherto prohibited such measurement, despite the widely acknowledged need ...

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Influence of Different Substituents on the Surface Composition of Ionic Liquids Studied Using ARXPS

Cited by 183 publications

References 67 publications

Surface Enrichment in Equimolar Mixtures of Non‐Functionalized and Functionalized Imidazolium‐Based Ionic Liquids

Surface Enrichment in Equimolar Mixtures of Non‐Functionalized and Functionalized Imidazolium‐Based Ionic Liquids

Theoretical reconstruction and elementwise analysis of photoelectron spectra for imidazolium-based ionic liquids

Surface layering and melting in an ionic liquid studied by resonant soft X-ray reflectivity

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