Liquid structure and dynamics in the choline acetate:urea 1:2 deep eutectic solvent.

We report on the thermodynamic, structural, and dynamic properties of a recently proposed deep eutectic solvent, formed by choline acetate (ChAc) and urea (U) at the stoichiometric ratio 1:2, hereinafter indicated as ChAc:U. Although the crystalline phase melts at 36-38 °C depending on the heating rate, ChAc:U can be easily supercooled at sub-ambient conditions, thus maintaining at the liquid state, with a glass-liquid transition at about -50 °C. Synchrotron high energy x-ray scattering experiments provide the experimental data for supporting a reverse Monte Carlo analysis to extract structural information at the atomistic level. This exploration of the liquid structure of ChAc:U reveals the major role played by hydrogen bonding in determining interspecies correlations: both acetate and urea are strong hydrogen bond acceptor sites, while both choline hydroxyl and urea act as HB donors. All ChAc:U moieties are involved in mutual interactions, with acetate and urea strongly interacting through hydrogen bonding, while choline being mostly involved in van der Waals mediated interactions. Such a structural situation is mirrored by the dynamic evidences obtained by means of 1H nuclear magnetic resonance techniques, which show how urea and acetate species experience higher translational activation energy than choline, fingerprinting their stronger commitments into the extended hydrogen bonding network established in ChAc:U.

Structural features of selected protic ionic liquids based on a super-strong base.

Protic ionic liquids (PIL) were prepared from a super-strong base 1,7-diazabicyclo[5.4.0]undec-7-ene (DBU) and super-strong acids, trifluoromethane sulfonic acid (TfOH), and (trifluoromethanesulfonyl)-(nonafluorobutylsulfonyl)imide, (IM14H), ([DBUH][TfO] and [DBUH][IM14], respectively; the latter for the first time) and their chemical and physical properties and structural features have been explored using a synergy of experimental and computational tools. The short range order in neat DBU, as well as the long range structural correlations induced by charge correlation and hydrogen bonding interactions in the ionic liquids, have been explored under ambient conditions, where these compounds are proposed for a variety of applications. Similar to other [DBUH]-based PILs, the probed ones behave as good ionic liquids. Molecular dynamics-rationalised X-ray diffraction patterns show the major role played by hydrogen bonding in affecting morphology in these systems. Additionally, we find further evidence for the existence of fluorous domains in [DBUH][IM14], thus potentially extending the range of applications for these PILs.

Mesostructure and physical properties of aqueous mixtures of the ionic liquid 1-ethyl-3-methyl imidazolium octyl sulfate doped with divalent sulfate salts in the liquid and the mesomorphic states.

This paper extends the study of the induced temperature change in the mesostructure and in the physical properties occurring in aqueous mixtures of the ionic liquid 1-ethyl-3-methyl imidazolium octyl-sulfate [EMIm][OSO4]. For some compositions, these mixtures undergo a phase transition between the liquid (isotropic in the mesoscale) and the mesomorphic state (lyotropic liquid crystalline) at about room temperature. The behavior of mixtures doped with a divalent metal sulfate was investigated in order to observe their applicability as electrolytes. Calcium sulfate salt is almost insoluble even in the 20 wt% water mixture. The magnesium salt, in contrast, can be dissolved up to concentrations of 730 ppm in the same mixture and it has a profound impact on its properties. Six aqueous mixtures (with water content from 10 wt% to 33 wt%) of [EMIm][OSO4] were saturated with magnesium sulfate salt, producing the ternary mixture [EMIm][OSO4] + H2O + MgSO4. Viscosity, density and ionic conductivity for these samples were measured from 10 °C to 90 °C. In addition, SAXS, FTIR, diffussion NMR and Raman spectroscopy of the most interesting samples have been performed, and structural data indicate a transition between a hexagonal lyotropic liquid crystalline phase below and an isotropic solution phase above room temperature. The octyl sulfate anions of the cylindrical micelles in the hexagonal phase are coordinated with water molecules through H-bonds (about four per sulfate anion), while the [EMIm] cations seem to be poorly coordinated and so free to move. Inorganic salt addition reinforces that network, increasing the phase transition temperature.

Communication: Anion-specific response of mesoscopic organization in ionic liquids upon pressurization.

One of the outstanding features of ionic liquids is their inherently hierarchical structural organization at mesoscopic spatial scales. Recently experimental and computational studies showed the fading of this feature when pressurising. Here we use simulations to show that this effect is not general: appropriate anion choice leads to an obstinate resistance against pressurization.

Nanostructured solvation in mixtures of protic ionic liquids and long-chained alcohols.

The structural and dynamical properties of bulk mixtures of long-chained primary and secondary alcohols (propanol, butanol, and 2-pentanol) with protic ionic liquids (ethylammonium and butylammonium nitrate) were studied by means of molecular dynamics simulations and small angle X-ray scattering (SAXS). Changes in the structure with the alcohol concentration and with the alkyl chain length of the alcohol moieties were found, showing variations in the radial distribution function and in the number of hydrogen bonds in the bulk liquids. Moreover, the structural behaviour of the studied mixtures is further clarified with the spatial distribution functions. The global picture in the local scale is in good agreement with the nanostructured solvation paradigm [T. Méndez-Morales et al. Phys. Chem. B 118, 761 (2014)], according to which alcohols are accommodated into the hydrogen bonds' network of the ionic liquid instead of forming clusters in the bulk. Indeed, our study reveals that the alcohol molecules are placed with their polar heads at the interfaces between polar and nonpolar nanodomains in the ionic liquid, with their alkyl chains inside the nonpolar organic nanodomains. The influence of alcohol chain length in the single-particle dynamics of the mixtures is also reported calculating the velocity autocorrelation function and vibrational densities of states of the different species in the ionic liquid-alcohol mixtures, and a weak caging effect for the ethylammonium cations independent of the chain size of the alcohols was found. However, the SAXS data collected for the studied mixtures show an excess of the scattering intensities which indicates that there are also some structural heterogeneities at the nanoscale.

Liquid structure of dibutyl sulfoxide.

We present experimental (X-ray diffraction) data on the structure of liquid dibutyl sulfoxide at 320 K and rationalise the data by means of molecular dynamics simulations. Not unexpectedly, DBSO bearing a strong dipolar moiety and two medium length, apolar butyl chains, this compound was characterised by a distinct degree of polar vs. apolar structural differentiation at the nm spatial scale, which was fingerprinted by a low Q peak in its X-ray diffraction pattern. Similar to, but to a larger extent than its shorter chain family members (such as DMSO), DBSO was also characterised by an enhanced dipole-dipole correlation, which was responsible for a moderate Kirkwood correlation factor as well as for the self-association detected in this compound. We show, however, that the supposedly relevant hydrogen bonding correlations between oxygen and the butyl chain hydrogens are of a limited extent only, and only in the case of α-hydrogens is an appreciable indication of the existence of such an interaction found, albeit this turned out to be a mere consequence of the strong dipole-dipole correlation.

Triphilic Ionic-Liquid Mixtures: Fluorinated and Non-fluorinated Aprotic Ionic-Liquid Mixtures.

We present here the possibility of forming triphilic mixtures from alkyl- and fluoroalkylimidazolium ionic liquids, thus, macroscopically homogeneous mixtures for which instead of the often observed two domains-polar and nonpolar-three stable microphases are present: polar, lipophilic, and fluorous ones. The fluorinated side chains of the cations indeed self-associate and form domains that are segregated from those of the polar and alkyl domains. To enable miscibility, despite the generally preferred macroscopic separation between fluorous and alkyl moieties, the importance of strong hydrogen bonding is shown. As the long-range structure in the alkyl and fluoroalkyl domains is dependent on the composition of the liquid, we propose that the heterogeneous, triphilic structure can be easily tuned by the molar ratio of the components. We believe that further development may allow the design of switchable, smart liquids that change their properties in a predictable way according to their composition or even their environment.

Pressure-responsive mesoscopic structures in room temperature ionic liquids.

Among the most spectacular peculiarities of room temperature ionic liquids, their mesoscopically segregated structural organization keeps on attracting attention, due to its major consequences for the bulk macroscopic properties. Herein we use molecular dynamics simulations to explore the nm-scale architecture in 1-octyl-3-methylimidazolium tetrafluoroborate, as a function of pressure. This study reveals an intriguing new feature: the mesoscopic segregation in ionic liquids is characterized by a high level of pressure-responsiveness, which progressively vanishes upon application of high enough pressure. These results are in agreement with recent X-ray scattering data and are interpreted in terms of the microscopic organization. This new feature might lead to new methods of developing designer solvents for enhanced solvation capabilities and selectivity.

Nanostructure of mixtures of protic ionic liquids and lithium salts: effect of alkyl chain length.

The bulk structure of mixtures of two protic ionic liquids, propylammonium nitrate and butylammonium nitrate, with a salt with a common anion, is analyzed at room temperature by means of small angle X-ray scattering and classical molecular dynamics simulations. The study of several structural properties, such as density, radial distribution functions, spatial distribution functions, hydrogen bonds, coordination numbers and velocity autocorrelation functions, demonstrates that increasing the alkyl chain length of the alkylammonium cation results in more segregated, better defined polar and apolar domains, the latter having a larger size. This increase, ascribed to the erosion of the H-bond network in the ionic liquid polar regions as salt is added, is confirmed by means of small angle X-ray scattering measurements, which show a clear linear increase of the characteristic spatial sizes of the studied protic ionic liquids with salt concentration, similar to that previously reported for ethylammonium nitrate (J. Phys. Chem. B, 2014, 118, 761-770). In addition, larger ionic liquid cations lead to a lower degree of hydrogen bonding and to more sparsely packed three-dimensional structures, which are more easily perturbed by the addition of lithium salts.

Liquid Structure Scenario of the Archetypal Supramolecular Deep Eutectic Solvent: Heptakis(2,6-di-O-methyl)-ß-cyclodextrin/levulinic Acid.

The concept of supramolecular solvents has been recently introduced, and the extended liquid-state window accessible for mixtures of functionalized cyclodextrins (CDs) with hydrogen bond (HB) donor species, e.g., levulinic acid, led to the debut of supramolecular deep eutectic solvents (SUPRA-DES). These solvents retain CD's inclusion ability and complement it with enhanced solvation effectiveness due to an extended HB network. However, so far, these promising features were not rationalized in terms of a microscopic description, thus hindering a more complete capitalization. This is the first joint experimental and computational study on the archetypal SUPRA-DES: heptakis(2,6-di-O-methyl)-ß-CD/levulinic acid (1:27). We used X-ray scattering to probe CD's aggregation level and molecular dynamics simulation to determine the nature of interactions between SUPRA-DES components. We discover that CDs are homogeneously distributed in bulk and that HB interactions, together with the electrostatic ones, play a major role in determining mutual interaction between components. However, dispersive forces act in synergy with HB to accomplish a fundamental task in hindering hydrophobic interactions between neighbor CDs and maintaining the system homogeneity. The mechanism of mutual solvation of CD and levulinic acid is fully described, providing fundamental indications on how to extend the spectrum of SUPRA-DES combinations. Overall, this study provides the key to interpreting structural organization and solvation tunability in SUPRA-DES to extend the range of sustainable applications for these new, unique solvents.

NMR investigation of imidazolium-based ionic liquids and their aqueous mixtures.

(1)H and (13)C NMR spectroscopy is employed to investigate the interaction of water with two imidazolium-based ionic liquids (ILs), 1-hexyl-3-methylimidazolium bromide ([C(6)mim]Br) and 1-octyl-3-methylimidazolium bromide ([C(8)mim]Br), at IL concentrations well above the critical aggregation concentration (CAC). The results are compared with those of the neat samples. To this aim, a detailed analysis of the changes in the (1)H chemical shifts, (13)C relaxation parameters, and 2D ROESY data due to the presence of water is performed. The results for both neat ILs are consistent with a packed structure where head-to-head, head-to-tail, and tail-to-tail contacts occur and where the site of maximal mobility restriction is at the polar head. At the lowest investigated water content, the presence of water influences mainly the environment around the IL polar head, slowing down the motional dynamics of the aromatic ring with respect to the alkyl chain. At higher water contents this difference diminishes, the motional freedom of the whole molecule increasing. The presence of ROESY cross-peaks between protons in the polar and apolar IL regions, as well as between protons in non-neighboring alkyl groups, at all investigated water contents suggests that the alkyl tails are not fully segregated in hydrophobic domains, as expected for micelle-like structures.

Solubility and solvation features of native cyclodextrins in 1-ethyl-3-methylimidazolium acetate.

The comprehension of the mechanism entailing efficient solvation of cyclodextrins (CD) by green solvents is of great relevance to boost environmentally sustainable usages of smart supramolecular systems. Here, 1-ethyl-3-methylimidazolium acetate, an ecofriendly ionic liquid (IL), is considered as an excellent solvent for native CDs. This IL efficiently dissolves up to 40 wt.% ß- and γ-CD already at ambient temperature and X-ray scattering indicates that CDs do not tend to detrimental flocculation under these drastic concentration conditions. Simulation techniques reveal the intimate mechanism of CD solvation by the ionic species: while the strong hydrogen bonding acceptor acetate anion interacts with CD's hydroxyl groups, the imidazolium cation efficiently solvates the hydrophobic CD walls via dispersive interactions, thus hampering CD's hydrophobic driven flocking. Overall the amphiphilic nature of the proposed IL provides an excellent solvation environment for CDs, through the synergic action of its components.

Structural organization and phase behaviour of 1-butyl-3-methylimidazolium hexafluorophosphate: an high pressure Raman spectroscopy study.

The complexity of the phase diagram of a representative room temperature ionic liquid (1-butyl-3-methylimidazolium hexafluorophosphate, [bmim][PF(6)]) is explored by means of Raman spectroscopy at high pressure (up to 1000 MPa) and high temperature (from room temperature to 100 °C) conditions. The first experimental evidence of the existence of a second crystalline phase for this salt at high pressure conditions is provided. By comparing the low frequency vibrational bands for the liquid state and the two observed crystalline phases, we confirm the scenario that considers the crystal polymorphism in this class of materials as a consequence of the rotational isomerism of the butyl chain. Furthermore the pressure dependence of other vibrational bands indicates the existence of a structural rearrangement across p≈ 50 MPa at ambient temperature.

Liquid Structure of a Water-in-Salt Electrolyte with a Remarkably Asymmetric Anion.

Water-in-salt systems, i.e., super-concentrated aqueous electrolytes, such as lithium bis(trifluoromethanesulfonyl)imide (21 mol/kgwater), have been recently discovered to exhibit unexpectedly large electrochemical windows and high lithium transference numbers, thus paving the way to safe and sustainable charge storage devices. The peculiar transport features in these electrolytes are influenced by their intrinsically nanoseparated morphology, stemming from the anion hydrophobic nature and manifesting as nanosegregation between anions and water domains. The underlying mechanism behind this structure-dynamics correlation is, however, still a matter of strong debate. Here, we enhance the apolar nature of the anions, exploring the properties of the aqueous electrolytes of lithium salts with a strongly asymmetric anion, namely, (trifluoromethylsulfonyl)(nonafluorobutylsulfonyl) imide. Using a synergy of experimental and computational tools, we detect a remarkable level of structural heterogeneity at a mesoscopic level between anion-rich and water-rich domains. Such a ubiquitous sponge-like, bicontinuous morphology develops across the whole concentration range, evolving from large fluorinated globules at high dilution to a percolating fluorous matrix intercalated by water nanowires at super-concentrated regimes. Even at extremely concentrated conditions, a large population of fully hydrated lithium ions, with no anion coordination, is detected. One can then derive that the concomitant coexistence of (i) a mesoscopically segregated structure and (ii) fully hydrated lithium clusters disentangled from anion coordination enables the peculiar lithium diffusion features that characterize water-in-salt systems.

Structural Features of ß-Cyclodextrin Solvation in the Deep Eutectic Solvent, Reline.

The inherently amphiphilic nature of native cyclodextrins (CDs) determines their peculiar molecular encapsulation features, enabling applications such as targeted drug nanodelivery, aroma protection, etc. On the contrary, it may also lead to poor solubility in water and other organic solvents and to potentially detrimental flocking in these media, thus posing limitations to more extensive usage. Here we use small angle X-ray scattering to show that deep eutectic solvent reline (1:2 choline chloride:urea) succeeds in dissolving large amounts of ß-CD (at least 800 mg/mL, compared with the solubility in water of 18 mg/mL), without aggregation phenomena occurring. At the microscopic level, molecular dynamics simulations highlight the complex interplay of hydrogen bonding-mediated hydrophilic interactions and hydrophobic force mitigation occurring between ß-CD and reline components, leading to energetically favorable ß-CD solvation. The possibility of achieving very high concentration conditions for unaggregated ß-CD in an environmentally responsible media, such as reline, can open the way to new, so far unpredictable applications, addressing multiple societal challenges.

Liquid structure of trihexyltetradecylphosphonium chloride at ambient temperature: an X-ray scattering and simulation study.

We report on an experimental and simulation study done on a representative room temperature ionic liquid, namely tetradecyltrihexylphosphonium chloride, at ambient conditions. The study was conducted using small and wide angle X-ray scattering and molecular dynamics simulations. Both approaches converge in indicating that this material is characterized by the existence of strong P-Cl interactions (with characteristic distances between 3.5 and 5.0 A) and by the occurrence of nanoscale segregation, despite the symmetric nature of the cation and similarly to other room temperature ionic liquids. A good agreement is found between the structure factor and pair correlation functions obtained from MD simulations and the corresponding experimental observables, thus strongly validating the interaction potential used in the simulations.

Effect of cation symmetry and alkyl chain length on the structure and intermolecular dynamics of 1,3-dialkylimidazolium bis(trifluoromethanesulfonyl)amide ionic liquids.

In this article, the structure and intermolecular dynamics of 1,3-alkylmethylimidazolium bis(trifluoromethanesulfonyl)amides [C(n)mim][NTf(2)] with n = 2-5 are compared to those of 1,3-dialkylimidazolium bis(trifluoromethanesulfonyl)amides [(C(n))(2)im][NTf(2)] with n = 2-5. The structures of these room-temperature ionic liquids (RTILs) were studied by small-wide-angle X-ray scattering (SWAXS), and their intermolecular dynamics were studied by optical Kerr effect (OKE) spectroscopy. The SWAXS measurements indicate that, on a microscopic scale, the liquid structure of RTILs with symmetric cations is similar to that of RTILs with asymmetric cations. The OKE measurements indicate that the intermolecular dynamics of RTILs with symmetric cations are higher in frequency than those of RTILs with asymmetric cations. These results suggest that the local structure of RTILs with symmetric cations is more solid-like than that of RTILs with asymmetric cations. Further evidence for this difference in local structure on a mesoscopic spatial scale is that the width of the low-Q peak in the SWAXS data is narrower for [(C(5))(2)im][NTf(2)] than for [C(5)mim][NTf(2)]. Moreover, the structure and intermolecular dynamics of the RTILs with ethyl-substituted cations appear to be quite different from those of other RTILs within a given series. This difference is evidenced by a clear change in the dependence of the spectral parameters of the intermolecular part of the OKE spectrum on the alkyl chain length in going from n = 2 to n = 3. The dependence of the SWAXS and OKE data on alkyl chain length is discussed within the context of the nanoscale heterogeneities of RTILs.

Temperature dependence of the primary relaxation in 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide.

We present results from complementary characterizations of the primary relaxation rate of a room temperature ionic liquid (RTIL), 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl} imide, [C6mim][Tf2N], over a wide temperature range. This extensive data set is successfully merged with existing literature data for conductivity, viscosity, and NMR diffusion coefficients thus providing, for the case of RTILs, a unique description of the primary process relaxation map over more than 12 decades in relaxation rate and between 185 and 430 K. This unique data set allows a detailed characterization of the VTF parameters for the primary process, that are: B=890 K, T0=155.2 K, leading to a fragility index m=71, corresponding to an intermediate fragility. For the first time neutron spin echo data from a fully deuteriated sample of RTIL at the two main interference peaks, Q=0.76 and 1.4 A(-1) are presented. At high temperature (T>250 K), the collective structural relaxation rate follows the viscosity behavior; however at lower temperatures it deviates from the viscosity behavior, indicating the existence of a faster process.

An energy dispersive x-ray scattering and molecular dynamics study of liquid dimethyl carbonate.

In this work, we report on the first x-ray diffraction study on liquid dimethyl carbonate. Diffraction spectra were collected with an energy-dispersive instrument, whose wide Q-range allows the structure determination of weakly ordered systems (such as liquids). The structural correlation in this liquid ranges up to about 20 A. The observed patterns are interpreted with a structural model derived from classical molecular dynamics simulations. The simulations were run using OPLS force field, only slightly modified to restrain bond distances to the experimental values. The model structure function and radial distribution functions, averaged among the productive trajectory frames, are in very good agreement with the corresponding experimental ones. Molecular dynamics results show that the deviations from C(2v) cis-cis structure, predicted by ab initio calculations and observed by electron diffraction in the gas phase, are small. By analyzing the intra- and intermolecular pair distribution functions, it was possible to assign the peaks of the experimental radial distribution function to specific structural correlations, and to compute the different average intermolecular coordination numbers. The intermolecular methyl-carbonyl oxygen distance is thoroughly discussed to assess the presence of weak C-H...O hydrogen bonds.

Nanoscale organization in piperidinium-based room temperature ionic liquids.

Here we report on the complex nature of the phase diagram of N-alkyl-N-methylpiperidinium bis(trifluoromethanesulfonyl)imide ionic liquids using several complementary techniques and on their structural order in the molten state using small-wide angle x-ray scattering. The latter study indicates that the piperidinium aliphatic alkyl chains tend to aggregate, forming alkyl domains embedded into polar regions, similar to what we recently highlighted in the case of other ionic liquids.