Emerging Evidences of Mesoscopic-Scale Complexity in Neat Ionic Liquids and Their Mixtures

Evaluating the contributions to conductivity in room temperature ionic liquids

The conductivity of room temperature ionic liquids is not described adequately by the Nernst-Einstein equation, which accounts only for Brownian motion of the ions. We report on the conductivity of the ionic liquid 1-butyl-3-methylimidazolum bis(trifluoromethylsulfonyl) imide (BMIM TFSI), comparing the known conductivity of this RTIL to the diffusion constants of the cationic and anionic species over a range of length scales, using time-resolved fluorescence depolarization and fluorescence recovery after photobleaching (FRAP) measurements of chromophores in the RTIL. Our data demonstrate that the diffusional contribution to molar conductivity is ca. 50%. Another mechanism for the transmission of charged species in RTILs is responsible for the "excess" molar conductivity, and we consider possible contributions.

A Novel Strategy for the Characterization of Self-Assembled Structures Using the Static Solid-State Phosphorus Nuclear Magnetic Resonance Technique

Structural characterization of assemblies in solutions is essential for understanding the relationship between the structure and material properties. In this study, we introduce a novel approach to investigate amphiphilic self-assemblies in solutions using the phospholipid molecule 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (Lyso PC) as a 31P NMR probe. The high natural abundance and gyromagnetic ratio of 31P make it one of the most sensitive nuclei in the low-frequency region, enabling efficient detection even in dilute solutions. Lyso PC can readily co-assemble with amphiphilic molecules and ions in aqueous solutions, forming various structures, such as hexagonal, lamellar, and micellar assemblies. The characteristic line shapes of these assemblies reflect the chemical environment around the probe and provide insights into the different phase states of the assemblies. This strategy offers a simple, cost-effective, and static method for obtaining structural information about various assemblies. Our work not only introduces a sensitive probe for characterizing assemblies in a solvent environment but also inspires new ideas for the development of similar spectroscopic probes.

Phase coexistence in [C22/C1MIm]+[NO3]- ionic-liquid mixtures and first-order phase transitions from homogeneous liquid to smectic B by varying the cation ratio

We perform molecular dynamics simulations to investigate the transition processes of [C22/C1MIm]+[NO3]- binary mixtures by varying the cation ratio of C22 to C1 at a fixed temperature of 400 K. The cation ratio is tuned by ranging C22 percentage from 0% to 100% with a fixed number of 4096 total simulated ion pairs. Our simulated-annealing results indicate that, at 400 K, pure C1 is a homogeneous liquid whilst pure C22 is an ionic liquid crystal (ILC) of smectic-B (SmB) type. With increasing C22 percentage, the system goes through a first-order phase transition from homogeneous liquid to nano-fragment liquid in the range from 15% to 17.5%, during which some of the individual cationic alkyl side chains locally aggregate to form small bundles "floating" in the polar "solvent" composed of anions and cationic head groups. Although the side chains in each bundle are parallelly aligned, the bundles distribute randomly without a global orientation. As the C22 percentage further increases, another first-order phase transition occurs to bring the system into the SmB ILC phase. Particularly, when the C22 percentage is in the range from 45% to 50%, the SmB phase coexists with the liquid phase containing both individual and bundled alkyl side chains.

Designing protein nano-construct in ionic liquid: a boost in efficacy of cytochrome C under stresses

Herein, we present a simple approach to fabricate protein nanoconstructs by complexing cytochrome C (Cyt C) with silk nanofibrils (SNF) and choline dihydrogen phosphate ionic liquid (IL). The peroxidase activity of the IL modified Cyt C nanoconstruct (Cyt C + SNF + IL) increased significantly (2.5 to 10-fold) over unmodified Cyt C and showed enhanced catalytic activity and stability under harsh conditions, proving its potential as a suitable protein packaging strategy.

Solvation structure and dynamics of coumarin 153 in an imidazolium-based ionic liquid with chloroform, benzene, and propylene carbonate

In order to determine the self-diffusion coefficients D of all the species in the solutions at 298.2 K, ¹H and ¹⁹F NMR diffusion ordered spectroscopy (DOSY) has been conducted on coumarin 153 (C153) in binary mixed solvents of an imidazolium-based ionic liquid (IL), 1-dodecyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide (C₁₂mimTFSA), with three molecular liquids (MLs) of chloroform (CL), benzene (BZ), and propylene carbonate (PC) as a function of ML mole fraction x_ML. Below x_ML ≈ 0.8, the D values of each species do not significantly depend on the MLs. However, above this mole fraction, the diffusion of C153 becomes smoother in the order of BZ ≈ CL > PC systems. The interactions among C153, C₁₂mim⁺, TFSA^-, and ML molecules have been investigated using infrared (IR) and ¹H and ¹³C NMR spectroscopic techniques. The relations of the diffusion of the species with the interactions among them have been discussed on the molecular scale. In the IL solution, the C153 carbonyl oxygen atom is hydrogen-bonded with the imidazolium ring C²-H atom of C₁₂mim⁺. C₁₂mim⁺ also forms an ion pair with TFSA^-. Thus, C153, C₁₂mim⁺, and TFSA^- cooperatively move in the CL and BZ solutions at a lower ML content, x_ML < ∼0.8. On the other hand, at a higher ML content, x_ML > ∼0.8, the C153 molecule diffuses with CL and BZ molecules because of the hydrogen bonding between the C153 carbonyl O atom and the CL H atom and the π-π interaction between the C153 and BZ ring planes, respectively. For the PC system, the change in the relative self-diffusion coefficients of each species with increasing x_ML differs from those for the CL and BZ systems because of both hydrogen bonding donor H and acceptor O atoms of PC for C153, the IL cation and anion, and PC themselves.

The Nanostructure of Alkyl-Sulfonate Ionic Liquids: Two 1-Alkyl-3-methylimidazolium Alkyl-Sulfonate Homologous Series

The functionalization of polymers with sulfonate groups has many important uses, ranging from biomedical applications to detergency properties used in oil-recovery processes. In this work, several ionic liquids (ILs) combining 1-alkyl-3-methylimidazolium cations [C_nC₁im]⁺ (4 ≤ n ≤ 8) with alkyl-sulfonate anions [C_mSO₃]^- (4 ≤ m ≤ 8) have been studied using molecular dynamics simulations, totalizing nine ionic liquids belonging to two homologous series. The radial distribution functions, structure factors, aggregation analyses, and spatial distribution functions reveal that the increase in aliphatic chain length induces no significant change in the structure of the polar network of the ILs. However, for imidazolium cations and sulfonate anions with shorter alkyl chains, the nonpolar organization is conditioned by the forces acting on the polar domains, namely, electrostatic interactions and hydrogen bonding.

Recent Advances in Poly(Ionic Liquid)-Based Membranes for CO2 Separation

Poly(ionic liquid)-based membranes have been the subject of intensive research in the last 15 years due to their potential for the separation of CO₂ from other gases. In this short review, different types of PIL-based membranes for CO₂ separation are described (neat PIL membranes; PIL-IL composite membranes; PIL-polymer blend membranes; PIL-based block copolymer membranes, and PIL-based mixed matrix membranes), and their state-of-the-art separation results for different gas pairs (CO₂/N₂, CO₂/H₂, and CO₂/CH₄) are presented and discussed. This review article is focused on the most relevant research works performed over the last 5 years, that is, since the year 2017 onwards, in the field of poly(ionic liquid)-based membranes for CO₂ separation. The micro- and nano-morphological characterization of the membranes is highlighted as a research topic that requires deeper study and understanding. Nowadays there is an array of advanced structural characterization techniques, such as neutron scattering techniques with contrast variation (using selective deuteration), that can be used to probe the micro- and nanostructure of membranes, in length scales ranging from ~1 nm to ~15 μm. Although some of these techniques have been used to study the morphology of PIL-based membranes for electrochemical applications, their use in the study of PIL-based membranes for CO₂ separation is still unknown.

The structuring effect of the alkyl domains on the polar network of ionic liquid mixtures: a molecular dynamics study

By using molecular dynamics simulations, we investigate the structural and dynamic properties of mixtures of 1,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide, [C₁C₁im][Tf₂N] and 1-dodecyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C₁₂C₁im][Tf₂N] (also C₁ and C₁₂ in short). Such mixtures feature an imidazolium bistriflimide salt with a very short alkyl chain, not giving rise to any nano-segregation as a pure component, with another one with a longer alkyl chain that exhibits a substantial nano-segregation as a pure liquid. As the mole fraction of the long-chain component C₁₂ is increased, the so-called pre-peak of the structure factor S(q), occurring in the region 1-3 nm^-1, shows a shift to higher values of the wavevector q, mirroring a decrease of the corresponding correlation length. Moreover, the intensity of the pre-peak strongly increases with the C₁₂ concentration. These results are in very good agreement with experimental X-ray scattering data in the literature. On the other hand, the diffusion of the ions is found to exhibit a simple behaviour consistent with the increased viscosity of the mixture, and these results are also in good agreement with NMR experimental data from the literature. The simulation results are rationalized as caused by a structuring effect, similar to the hydrophobic effect, of the alkyl domains of the C₁₂ component dissolved in the "solvent" represented by the C₁ cation, the Tf₂N^- anion and the C₁₂ cation head. In short, the exclusion of the alkyl chains from the polar network, a process mostly governed by electrostatic interactions, favours the formation of hydrophobic domains, which in turn exert a structuring effect on the ions of the polar domains, favouring a stronger ionic interaction. This is finally reflected in a shorter correlation length and a higher intensity of the pre-peak of the structure factor S(q) as the C₁₂ mole fraction is increased. At variance with the microscopic structure, the diffusion of all three types of ions is not strongly influenced by the nano-segregation and is essentially dependent on the viscosity of the mixture.

Long-Range Organization Study of Piperidinium-Based Ionic Liquids by Polarizable Molecular Dynamics Simulations

The nanoscale organization of some classes of ionic liquids is responsible for their singular properties. In this paper, we use polarizable molecular dynamics simulations and small-angle X-ray scattering to probe the structure of two piperidinium- and (trifluoromethylsulfonyl)imide-based ionic liquids ([EBPip⁺][NTf₂^-] and [EOPip⁺][NTf₂^-]) that differ in the alkyl chain length of their cation. The X-ray scattering intensities calculated numerically, from the radial distribution functions, are in excellent agreement with the experimental data. The analysis of the different contributions of the X-ray scattering data allowed us to highlight the correlations responsible for the low q peak observed for the long-chain alkyl cations. New angular analyses showed that anions were more likely to align with alkyl chains as their size increased, inducing angular correlation between anions at larger distances. They also showed that the long alkyl chains of the cations aligned more with each other than the short ones. These more aligned alkyl chains induce a smaller volume of the apolar microdomains compared to the well-studied imidazolium-based ionic liquids, leading to the smaller correlation distance for piperidinium-based ionic liquids.

Hydrogen Bond Redistribution Effects in Mixtures of Protic Ionic Liquids Sharing the Same Cation: Non-ideal Mixing with Large Negative Mixing Enthalpies

We report a joint experimental and theoretical study characterising the hydrogen bond (HB) redistribution in mixtures of two different protic ionic liquids (PILs) sharing the same cation: triethylammonium-methanesulfonate ([TEA][OMs]) and...

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.

A Brief Guide to the Structure of High-Temperature Molten Salts and Key Aspects Making Them Different from Their Low-Temperature Relatives, the Ionic Liquids

High-temperature molten salt research is undergoing somewhat of a renaissance these days due to the apparent advantage of these systems in areas related to clean and sustainable energy harvesting and transfer. In many ways, this is a mature field with decades if not already a century of outstanding work devoted to it. Yet, much of this work was done with pioneering experimental and computational setups that lack the current day capabilities of synchrotrons and high-performance-computing systems resulting in deeply entrenched results in the literature that when carefully inspected may require revision. Yet, in other cases, access to isotopically substituted ions make those pioneering studies very unique and prohibitively expensive to carry out nowadays. There are many review articles on molten salts, some of them cited in this perspective, that are simply outstanding and we dare not try to outdo those. Instead, having worked for almost a couple of decades already on their low-temperature relatives, the ionic liquids, this is the perspective article that some of the authors would have wanted to read when embarking on their research journey on high-temperature molten salts. We hope that this will serve as a simple guide to those expanding from research on ionic liquids to molten salts and vice versa, particularly, when looking into their bulk structural features. The article does not aim at being comprehensive but instead focuses on selected topics such as short- and intermediate-range order, the constraints on force field requirements, and other details that make the high- and low-temperature ionic melts in some ways similar but in others diametrically opposite.

Structure factor lineshape model gives approximate nanoscale size of polar aggregates in the ionic liquid N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide

A Lorentzian lineshape model is developed and tested for the charge alternation peak in X-ray structure factors calculated from MD simulations for N-methyl-N-propylpyrrolidinium bis(trifluoromethylsulfonyl)imide. Applying the model to published, experimental X-ray scattering data reproduces calculated cation-cation and anion-anion distances within 6% and implies that half of ionic aggregates are larger than 12.7 Å.

Comment on "Bi-layering at ionic liquid surfaces: a sum - frequency generation vibrational spectroscopy - and molecular dynamics simulation-based study" by T. Iwahashi, T. Ishiyama, Y. Sakai, A. Morita, D. Kim and Y. Ouchi, Phys. Chem. Chem. Phys., 2020, 22, 12565

This Comment raises several questions concerning the surface structure concluded in the paper referenced in the title. Specifically, that paper ignores previous experiments and simulations which demonstrate for the same ionic liquids depth-decaying, multilayered surface-normal density profiles rather than the claimed molecular mono- or bi-layers. We demonstrate that the claimed structure does not reproduce the measured X-ray reflectivity, which probes directly the surface-normal density profile. The measured reflectivities are found, however, to be well-reproduced by a multilayered density model. These results, and previous experimental and simulation results, cast severe doubt on the validity of the surface structure claimed in the paper referenced in the title.

Nanoconfinement effects on structural anomalies in imidazolium ionic liquids

Imidazolium Ionic Liquids (ILs) have been found to exhibit unusual nanostructuring behavior below their glass transition temperatures (Tg), which is ascribed to rearrangements in nonpolar domains formed by segregated alkyl chains. However, the dimensions required for such highly cooperative bulk phenomena are still unknown. In this work, we for the first time, investigate the effect of nanoconfinement on structural anomalies in imidazolium ILs. For this purpose, a series of ILs were embedded into the cavities of metal-organic framework (MOF) ZIF-8 and investigated using spin probes and Electron Paramagnetic Resonance (EPR) spectroscopy. The unusual nanostructuring near Tg, previously known for bulk ILs, was also observed for such nanoconfined ILs, and the amplitude of the anomaly was found to be dependent on the structure of the IL, thus showing the effects of molecular packing inside the MOF cavity. The first observation of structural anomalies in nanoconfined ILs opens perspectives for designing smart materials exhibiting these phenomena, and engaging MOFs as platforms creates the basis for potential applications of such functionalities.

Heterogeneous Structures of Ionic Liquids as Probed by CO Rotation with Nuclear Magnetic Resonance Relaxation Analysis and Molecular Dynamics Simulations

The rotational dynamics of carbon monoxide (CO) in ionic liquids (ILs) was investigated by nuclear magnetic resonance (NMR) relaxation measurements and molecular dynamics (MD) simulations. NMR spin-lattice relaxation time measurements were performed for ¹⁷O-enriched CO in 10 ILs (four imidazolium-cation-based, four phosphonium-cation-based, and two ammonium-cation-based ILs, all paired with the bis(trifluorosulfonylmethane)imide anion). In combination with previously reported data for five ILs and viscosity data, our results indicated that the obtained rotational relaxation times (τ_2R) were much smaller than those predicted using the Stokes-Einstein-Debye (SED) theory. For the same viscosity/temperature values, the τ_2R^-1 value increased linearly with increasing carbon number of the alkyl group in the cation. The deviation from the SED equation was due to the insensitivity of τ_2R to the carbon number, even though a higher carbon number generally leads to higher viscosity values for ILs. To investigate the unique rotational properties of CO in the ILs, MD simulations were performed on five representative ILs (two imidazolium, two phosphonium, and one ammonium) containing CO solutes. From rotational correlation function analyses, the CO rotation mainly occurred in a free rotation-like manner within 1 ps, which explained the relative insensitivity of CO rotation to viscosity. In the subsequent time scale (>1 ps), the minor component of the CO rotation was discriminated among different ILs. It was strongly suggested that, because CO preferably locates in the outer part of the alkyl groups in the cation, the slow CO rotation is correlated with the outer alkyl dynamics, which are decoupled from the whole cation rotation.

Surface Propensity of Anions in a Binary Ionic-Liquid Mixture Assessed by Full-Range Angle-Resolved X-ray Photoelectron Spectroscopy and Surface-Tension Measurements

Angle-resolved X-ray photoelectron spectroscopy and contact-angle measurements guided by a signal attenuation model are utilized to extract molar composition and anion enrichment in the vacuum interface of a binary ionic liquid mixture, having a common quaternary ammonium cation and two different anions. By using the intensity ratio of the F1s peaks belonging to the two different anions recorded at the full electron take-off angle range, from 0° to 80°, we have determined that only a fractionally covered and anion enriched surface layer can predict the AR-XPS data, which is also consistent with surface tension measurements. Moreover, the more bulky and non-spherical anion enrichment is evident even at the conventional and the so assumed bulk sensitive take-off angle of 0°. This methodology provides a surface enrichment factor of the molecular ions and clearly serves as an experimental evidence for recently debated surface layering and/or island structure in ionic liquid systems.

Molecular Assembling in Mixtures of Hydrophilic 1-Butyl-1-Methylpyrrolidinium Dicyanamide Ionic Liquid and Water

The infrared absorbance spectrum of the ionic liquid 1-butyl-1-methylpyrrolidinium dicyanamide, mixed with water at two different concentrations, was measured between 160 and 300 K in the mid infrared range. Both mixtures do not crystallize on cooling; however, remarkably, the one with an ionic liquid (IL):water composition of 1:3 displays a cold crystallization process on heating in a restricted temperature range between 240 and 250 K. A portion of the water participates to the cold crystallization. On the contrary, with an IL:water composition of 1:6.6 no crystallization takes place. Upon water addition the vibration frequencies of the anion and of some lines of the cation are blue shifted, while the absorption lines of water are red shifted. These facts are interpreted as the evidence of the occurrence of the hydrogen bonding of water, as the hydrogen bonding acceptor with respect to the anion (anion∙∙∙O-H bonds develop) and as hydrogen donor for the cation (C-H∙∙∙O bonds can form). Microscopic inhomogeneities in the samples and their evolution with temperature are discussed.

Anions as Dynamic Probes for Ionic Liquid Mixtures

Ionic liquid (IL) mixtures have been proposed as a viable alternative to rationally fine-tune the physicochemical properties of ILs for a variety of applications. The understanding of the effects of mixing ILs on the properties of the mixtures is however only in the very early stages. Two series of ionic liquid mixtures, based on the 1-ethyl-3-methylimidazolium and 1-dodecyl-3-methylimidazolium cations, and having a common anion (tetrafluoroborate or bis(trifluoromethylsulfonyl)imide), have been prepared and deeply characterized via multiple NMR techniques. Diffusion and relaxation methods combined with 2D ion-ion correlation (nuclear Overhauser enhancement) experiments have been used for a better understanding of the interplay between dynamics and structure of IL mixtures. A crucial role of the anion in driving the mixture's behavior emerged, making them important "dynamic probes" for gaining information of the polar and nonpolar regions of ionic liquids and their mixtures.

Synchrotron Scattering Methods for Nanomaterials and Soft Matter Research

This article aims to provide an overview of broad range of applications of synchrotron scattering methods in the investigation of nanoscale materials. These scattering techniques allow the elucidation of the structure and dynamics of nanomaterials from sub-nm to micron size scales and down to sub-millisecond time ranges both in bulk and at interfaces. A major advantage of scattering methods is that they provide the ensemble averaged information under in situ and operando conditions. As a result, they are complementary to various imaging techniques which reveal more local information. Scattering methods are particularly suitable for probing buried structures that are difficult to image. Although, many qualitative features can be directly extracted from scattering data, derivation of detailed structural and dynamical information requires quantitative modeling. The fourth-generation synchrotron sources open new possibilities for investigating these complex systems by exploiting the enhanced brightness and coherence properties of X-rays.

Tuning Water Networks via Ionic Liquid/Water Mixtures

Water in nanoconfinement is ubiquitous in biological systems and membrane materials, with altered properties that significantly influence the surrounding system. In this work, we show how ionic liquid (IL)/water mixtures can be tuned to create water environments that resemble nanoconfined systems. We utilize molecular dynamics simulations employing ab initio force fields to extensively characterize the water structure within five different IL/water mixtures: [BMIM+][BF4−], [BMIM+][PF6−], [BMIM+][OTf−], [BMIM+][NO3−] and [BMIM+][TFSI−] ILs at varying water fraction. We characterize water clustering, hydrogen bonding, water orientation, pairwise correlation functions and percolation networks as a function of water content and IL type. The nature of the water nanostructure is significantly tuned by changing the hydrophobicity of the IL and sensitively depends on water content. In hydrophobic ILs such as [BMIM+][PF6−], significant water clustering leads to dynamic formation of water pockets that can appear similar to those formed within reverse micelles. Furthermore, rotational relaxation times of water molecules in supersaturated hydrophobic IL/water mixtures indicate the close-connection with nanoconfined systems, as they are quantitatively similar to water relaxation in previously characterized lyotropic liquid crystals. We expect that this physical insight will lead to better design principles for incorporation of ILs into membrane materials to tune water nanostructure.

Translational Diffusion in a Set of Imidazolium-Based Ionic Liquids [bmim]+A- and Their Mixtures with Water

As the development of the work (J. Phys. Chem. B 2019, 123 (10), 2362-2372), we have investigated the translational mobility in the same set of dried imidazolium-based ionic liquids (ILs) [bmim]A (A = BF₄^-, NO₃^-, TfO^-, I^-, Br^-, and Cl^-) in a wide temperature range using the NMR technique. It is shown that for the [bmim]⁺ cation, the temperature dependencies of product Dη do not follow the Stokes-Einstein relation for most systems studied, that is, the so-called "diffusion-viscosity decoupling" was realized. The correlation between local and translational mobility in pure IL of the [bmim][A] type was investigated using the data on NMR relaxation rates and diffusion coefficients. The most recent hypothesis of "water pockets" in mixtures of IL with water is critically discussed. Considering the totality of data in the literature and obtained here, we propose a specific model of the microstructure which may be applied up to water concentrations of 80-90 mol % (the structure of water-rich solutions is out of our current consideration). To confirm the model, molecular dynamics simulations of "IL-water" mixtures were also carried out.

Mesoscale Organization and Dynamics in Binary Ionic Liquid Mixtures

The impact of mesoscale organization on dynamics and ion transport in binary ionic liquid mixtures is investigated by broad-band dielectric spectroscopy, dynamic-mechanical spectroscopy, X-ray scattering, and molecular dynamics simulations. The mixtures are found to form distinct liquids with macroscopic properties that significantly deviate from weighted contributions of the neat components. For instance, it is shown that the mesoscale morphologies in ionic liquids can be tuned by mixing to enhance the static dielectric permittivity of the resulting liquid by as high as 100% relative to the neat ionic liquid components. This enhancement is attributed to the intricate role of interfacial dynamics associated with the changes in the mesoscopic aggregate morphologies in these systems. These results demonstrate the potential to design the physicochemical properties of ionic liquids through control of solvophobic aggregation.

Microscopic Structural and Dynamic Features in Triphilic Room Temperature Ionic Liquids

Here we report a thorough investigation of the microscopic and mesoscopic structural organization in a series of triphilic fluorinated room temperature ionic liquids, namely [1-alkyl,3-methylimidazolium][(trifluoromethanesulfonyl)(nonafluorobutylsulfonyl)imide], with alkyl = ethyl, butyl, octyl ([C_nmim][IM₁₄], n = 2, 4, 8), based on the synergic exploitation of X-ray and Neutron Scattering and Molecular Dynamics simulations. This study reveals the strong complementarity between X-ray/neutron scattering in detecting the complex segregated morphology in these systems at mesoscopic spatial scales. The use of MD simulations delivering a very good agreement with experimental data allows us to gain a robust understanding of the segregated morphology. The structural scenario is completed with determination of dynamic properties accessing the diffusive behavior and a relaxation map is provided for [C₂mim][IM₁₄] and [C₈mim][IM₁₄], highlighting their natures as fragile glass formers.

On the structural origin of free volume in 1-alkyl-3-methylimidazolium ionic liquid mixtures: a SAXS and 129Xe NMR study

Ionic liquid (IL) mixtures enable the design of fluids with finely tuned structural and physicochemical properties for myriad applications. In order to rationally develop and design IL mixtures with the desired properties, a thorough understanding of the structural origins of their physicochemical properties and the thermodynamics of mixing needs to be developed. To elucidate the structural origins of the excess molar volume within IL mixtures containing ions with different alkyl chain lengths, 3 IL mixtures containing 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ILs have been explored in a joint small angle X-ray scattering (SAXS) and 129Xe NMR study. The apolar domains of the IL mixtures were shown to possess similar dimensions to the largest alkyl chain of the mixture with the size evolution determined by whether the shorter alkyl chain was able to interact with the apolar domain. 129Xe NMR results illustrated that the origin of excess molar volume in these mixtures was due to fluctuations within these apolar domains arising from alkyl chain mismatch, with the formation of a greater number of smaller voids within the IL structure. These results indicate that free volume effects for these types of mixtures can be predicted from simple considerations of IL structure and that the structural basis for the formation of excess molar volume in these mixtures is substantially different to IL mixtures formed of different types of ions.

Substituted Azolium Disposition: Examining the Effects of Alkyl Placement on Thermal Properties

We describe the thermal phase characteristics of a series of 4,5-bis(n-alkyl)azolium salts that were studied using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), polarized-light optical microscopy (POM), and synchrotron-based small- to wide-angle X-ray scattering (SWAXS) measurements. Key results were obtained for 1,3-dimethyl-4,5-bis(n-undecyl)imidazolium iodide (1-11), 1,3-dimethyl-4,5-bis(n-pentadecyl)- imidazolium iodide (1-15), and 1,2,3-trimethyl-4,5-bis(n-pentadecyl)imidazolium iodide (2), which were found to adopt enantiotropic smectic A mesophases. Liquid-crystalline mesophases were not observed for 1,3-dimethyl-4,5-bis(n-heptyl)imidazolium iodide (1-7), 3-methyl-4,5-bis(n-penta-decyl)thiazolium iodide (3), and 2-amino-4,5-bis(n-pentadecyl)imidazolium chloride (4). Installing substituents in the 4- and 5-positions of the imidazolium salts appears to increase melting points while lowering clearing points when compared to data reported for 1,3-disubstituted analogues.

Crystal Polymorphism of 1-Butyl-3-methylimidazolium Hexafluorophosphate: Phase Diagram, Structure, and Dynamics

The ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate ([C4mim]PF6), is one of the most representative ILs. Despite its relatively simple ion structure and popularity, [C4mim]PF6 shows complex and confusing thermal phase behaviours, which stem from crystal polymorphism associated with cation conformational change and large thermal hysteresis. To the best of our knowledge, [C4mim]PF6 is the most investigated IL in terms of phase diagram, whereas full understanding has not yet been achieved due to its complexity. Here we review the current status of understanding of the phase diagram and structure/dynamics of each crystalline phase. Presently, depending on temperature and pressure, five structurally different polymorphic crystals have been reported as α, β, γ, δ, and δ’ in addition to some unspecified phases implied by calorimetric studies. Particularly for the α, β and γ phases, the structure and dynamics are well investigated by Raman, NMR, and X-ray scattering techniques.

On the cost of academic methodologies

Synthetic methodologies can be easily compared using the Cost of Academic Methodologies (CAM) parameter, which estimates the cost of making a mole of a product.

Structural analysis of zwitterionic liquids vs. homologous ionic liquids

Zwitterionic liquids (Zw-ILs) have been developed that are homologous to monovalent ionic liquids (ILs) and show great promise for controlled dissolution of cellulosic biomass. Using both high energy X-ray scattering and atomistic molecular simulations, this article compares the bulk liquid structural properties for novel Zw-ILs with their homologous ILs. It is shown that the significant localization of the charges on Zw-ILs leads to charge ordering similar to that observed for conventional ionic liquids with monovalent anions and cations. A low-intensity first sharp diffraction peak in the liquid structure factor S(q) is observed for both the Zw-IL and the IL. This is unexpected since both the Zw-IL and IL have a 2-(2-methoxyethoxy)ethyl (diether) functional group on the cationic imidazolium ring and ether functional groups are known to suppress this peak. Detailed analyses show that this intermediate range order in the liquid structure arises for slightly different reasons in the Zw-IL vs. the IL. For the Zw-IL, the ether tails in the liquid are shown to aggregate into nanoscale domains.

Mesoscopic Structure Facilitates Rapid CO2 Transport and Reactivity in CO2 Capture Solvents

Mass transfer coefficients of CO₂ are anomalously high in water-lean solvents as compared to aqueous amines. Such phenomena are intrinsic to the molecular and nanoscale structure of concentrated organic CO₂ capture solvents. To decipher the connections, we performed in situ liquid time-of-flight secondary ionization mass spectroscopy on a representative water-lean solvent, 1-((1,3-Dimethylimidazolidin-2-ylidene)amino)propan-2-ol (IPADM-2-BOL). Two-dimensional (2D) and three-dimensional (3D) chemical mapping of the solvent revealed that IPADM-2-BOL exhibited a heterogeneous molecular structure with regions of CO₂-free solvent coexisting with clusters of zwitterionic carbonate ions. Chemical mapping were consistent with molecular dynamic simulation results, indicating CO₂ diffusing through pockets and channels of unreacted solvent. The observed mesoscopic structure promotes and enhances the diffusion and reactivity of CO₂, likely prevalent in other water-lean solvents. This finding suggests that if the size, shape and orientation of the domains can be controlled, more efficient CO₂ capture solvents could be developed to enhance mass-transfer and uptake kinetics.

Structural Anomalies in Ionic Liquids near the Glass Transition Revealed by Pulse EPR

Unusual physical and chemical properties of ionic liquids (ILs) open up prospects for various applications. We report the first observation of density/rigidity heterogeneities in a series of ILs near the glass transition temperature ( T_g) by means of pulse electron paramagnetic resonance (EPR). Unprecedented suppression of molecular mobility is evidenced near the glass transition, which is assigned to unusual structural rearrangements of ILs on the nanometer scale. Indeed, pulse and continuous wave EPR clearly indicate the occurrence of heterogeneities near T_g, which exist in a rather broad temperature range of ∼50 K. The two types of local environments are evidenced, being drastically different by their stiffness. The more rigid one suppresses molecular mobility, whereas the softer one instead promotes diffusive molecular rotation. Such properties of ILs near T_g are of general importance; moreover, the observed density/rigidity heterogeneities controlled by temperature might be considered as a new type of tunable reaction nanoenvironment.

Neutron scattering studies on short- and long-range layer structures and related dynamics in imidazolium-based ionic liquids

Alkyl-methyl-imidazolium ionic liquids CnmimX (n: alkyl-carbon number, X: anion) have short-range layer structures consisting of ionic and neutral (alkylchain) domains. To investigate the temperature dependences of the interlayer, interionic group, and inter-alkylchain correlations, we have measured the neutron diffraction (ND) of C16mimPF₆, C9.5mimPF₆, and C8mimPF₆ in the temperature region from 4 K to 470 K. The quasielastic neutron scattering (QENS) of C16mimPF₆ was also measured to study the dynamics of each correlation. C16mimPF₆ shows a first-order transition between the liquid (L) and liquid crystalline (LC) phases at T_c = 394 K. C8mimPF₆ exhibits a glass transition at T_g = 200 K. C9.5mimPF₆, which is a 1:3 mixture between C8mimPF₆ and C10mimPF₆, has both transitions at T_c = 225 K and T_g = 203 K. In the ND experiments, all samples exhibit three peaks corresponding to the correlations mentioned above. The widths of the interlayer peak at ca. 0.2 Å^-1 changed drastically at the L-LC transitions, while the interionic peaks at ca. 1 Å^-1 exhibited a small jump at T_c. The peak position and area of the three peaks did not change much at the transition. The structural changes were minimal at T_g. The QENS experiments demonstrated that the relaxation time of the interlayer motion increased tenfold at T_c, while those of other motions were monotonous in the whole temperature region. The structural and dynamical changes mentioned above are characteristic of the L-LC transition in imidazolium-based ionic liquids.