Morphology of 1-alkyl-3-methylimidazolium hexafluorophosphate room temperature ionic liquids

Triolo, Alessandro; Russina, Olga; Fazio, Barbara; Triolo, R.; Cola, Emanuela Di

doi:10.1016/j.cplett.2008.04.027

Cited by 307 publications

(342 citation statements)

References 25 publications

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“…The layering is promoted by the microphase separation of the IL's long alkyl chains from the charged groups. Similar effects were found in the bulk by XRD (14) and in atomistic simulations (15). Temperature-dependent hard XRR, smalland wide-angle X-ray scattering (SAXS/WAXS), and infrared spectroscopy (FTIR), reported below, uncover an intriguing melting mechanism for the layered region, in which alkyl chain melting drives a negative thermal expansion of the surface layer spacing.…”

supporting

confidence: 67%

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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supporting

confidence: 67%

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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“…This points to a breathing mode of mesoscopic aggregates essentially normal to the average dipole orientation of the cluster. Aggregates of π-stacked cations and their surrounding anions are reasonable candidates compatible with MD simulations [40,41,42] and scattering experiments [43]. It should be noted that the lifetime of the caging effects observed in MD simulations of RTIL diffusion, see e.g.…”

Section: Sub-α Relaxationmentioning

confidence: 57%

Dynamics of RTILs: A comparative dielectric and OKE study

Sonnleitner

Turton

Waselikowski

et al. 2014

Journal of Molecular Liquids

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The dynamics of room-temperature ionic liquids (RTILs) were studied by investigating their dielectric relaxation (DR) and timeresolved optical Kerr-effect (OKE) spectra in the frequency range of ∼10 MHz to ∼20 THz. For the studied RTILs the OKE and DR spectra are dominated by a relaxation in the GHz region and extend to a relatively sharp band at around 10 THz. Whilst the first feature is mainly associated with the structural relaxation of the fluid through ion rotation (α relaxation), the second indicates the short-time limit of intermolecular dynamics. The rather featureless intermediate region is mainly associated with intermolecular vibrations that are strongly coupled to hindered rotations. In contrast to other RTILs, imidazolium salts show an additional sub-α relaxation which dominates the OKE signal and is indicative of the breathing motion of rather long-lived cages.Mixed with polar solvents RTILs were found to retain their ionic liquid-like character up to relatively high levels of dilution, but with the overall dynamics considerably speeded up. Below RTIL mole fractions of ∼0.2-0.4 these systems behave like conventional electrolyte solutions with more or less pronounced ion pairing.

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“…[16] As the dielectric spectrum and the OKE spectrum should show the same dynamics albeit with different strengths for the various components, [40] comparison of the two can give rise to greatly improved fitting. In the case of RTILs, it was found that mesoscopic structure [15,18] gives rise to a sub-α relaxation peak that is very strong in the OKE spectrum but nearly undetectable in the dielectric relaxation spectrum. [16] A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT TFISH measures the correlation function of the dipole moment and the hyperpolarizability.…”

Section: Methodsmentioning

confidence: 99%

“…There are hints that such nanoscale aggregation may even occur in molecules as simple as n-alcohols. [18] It is widely thought that solutes -in particular charged solutes -can increase or reduce the extent of the structure of the solvent, and are classified in the Hofmeister series as kosmotropes (structure makers) and chaotropes (structure breakers), through their influence on viscosity, and on protein folding and unfolding. [19][20][21] Mesoscopic structure has been observed in aqueous solutions [22][23][24] as well as in mixtures [25] by a variety of techniques.…”

Section: Introductionmentioning

confidence: 99%

Rattling the cage: Micro- to mesoscopic structure in liquids as simple as argon and as complicated as water

Turton

Hunger

Stoppa

et al. 2011

Journal of Molecular Liquids

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ÔØ Å ÒÙ× Ö ÔØThis is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Abstract. The water molecule has the convenient property that its molecular polarizability tensor is nearly isotropic while its dipole moment is large. As a result, the low-frequency anisotropic Raman spectrum of liquid water is mostly collision induced and therefore reports primarily translational motions while the far-infrared (terahertz) and dielectric spectrum is dominated by rotational modes. A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPTAtomic and globular-molecular liquids have a zero dipole moment as well as an isotropic polarizability tensor. These spectrum-simplifying properties were exploited in a study of a number of liquids and solutions using ultrafast optical Kerr-effect (OKE) spectroscopy combined with dielectric relaxation spectroscopy (DRS), terahertz time-domain spectroscopy (THz-TDS), and terahertz field-induced second-harmonic generation (TFISH) spectroscopy. For room-temperature ionic liquids (RTILs), liquid water, aqueous salt solutions, noble gas liquids, and globular molecular liquids it was found that, in each case, surprising structure and/or inhomogeneity is observed, ranging from mesoscopic clustering in RTILs to stretched exponential dynamics in the noble gas liquids. For aqueous electrolyte solutions it is shown that the viscosity, normally described by the Jones-Dole expression, can be explained in terms of a jamming transition, a concept borrowed from soft condensed matter studies of glass transitions in colloidal suspensions.

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Morphology of 1-alkyl-3-methylimidazolium hexafluorophosphate room temperature ionic liquids

Cited by 307 publications

References 25 publications

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

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

Dynamics of RTILs: A comparative dielectric and OKE study

Rattling the cage: Micro- to mesoscopic structure in liquids as simple as argon and as complicated as water

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