The influence of hydrogen bonding on the physical properties of ionic liquids

Deep eutectic solvent (TMAH·5H2O/Urea) with low viscosity for cellulose dissolution at room temperature

A deep eutectic solvent (DES) formulated with tetramethylammonium hydroxide pentahydrate /urea (TMAH·5H2O/Urea) was designed for the first time to dissolve cellulose at room temperature. The optimized system, characterized by a 1:3 M ratio, demonstrates the capability to dissolve approximately 7.5 wt% cellulose, boasting a high degree of polymerization (DP = 526). Notably, both the pure DES and 4.0 wt% cellulose/TMAH·5H2O/Urea mixtures manifests low viscosity, establishing its potential as an effective spinning aid in fiber manufacturing. The structural analyses shows that the cellulose crystal type shifts from type I to type II form, accompanied by a reduction in both crystallinity and DP. A pivotal aspect of this research involves determining Kamlet-Taft parameters for TMAH·5H2O/Urea-DES with different molar ratios. The results reveal these solvate DESs exhibit the high hydrogen bond basicity, which enables them to easily form hydrogen bonds with hydroxyl groups of cellulose and demonstrate good cellulose solubility. In conclusion, this solvent system presents notable advantages, including straightforward synthesis procedures, low viscosity, and well cellulose solubility, paving the way for new approaches and techniques in cellulose utilization.

Environmental impact assessment of dicationic ionic liquids with ammonium-phosphonium cations and amino acid anions

Progress in the development of biodegradable or biobased ionic liquids (ILs) has led to the design of green compounds for several applications. Herein, four biocompatible dicationic ionic liquids (DILs) with ammonium-phosphonium cations and amino acid anions were synthesized and investigated their environmental impact. The structures of the DILs were confirmed by spectral analyses (1H, 13C and 31P NMR). Furthermore, physicochemical properties such as density, viscosity and refractive index were determined. Water content, bromide content and solubility were thereafter determined as the parameters needed for further studies. Subsequently, their antifeedant activity towards economically important pests of grain in storage warehouses: the granary weevil, the confused flour beetle, and the khapra beetle was examined, showing the dependence on structure. Moreover, selected DILs were investigated for toxicity towards white mustard, Daphnia magna, and Artemia franciscana to specify the environmental impact. These studies were complemented by understand the biodegradation of DILs by bacterial communities derived from soil at the agricultural land. The result was DILs with limited environmental footprints that have great potential for further application studies.

Crystal Phase Ionic Liquids for Energy Applications: Heat Capacity Prediction via a Hybrid Group Contribution Approach

In the selection and design of ionic liquids (ILs) for various applications, including heat transfer fluids, thermal energy storage materials, fuel cells, and solvents for chemical processes, heat capacity is a key thermodynamic property. While several attempts have been made to develop predictive models for the estimation of the heat capacity of ILs in their liquid phase, none so far have been reported for the ILs' solid crystal phase. This is particularly important for applications where ILs will be used for thermal energy storage in the solid phase. For the first time, a model has been developed and used for the prediction of crystal phase heat capacity based on extending and modifying a previously developed hybrid group contribution model (GCM) for liquid phase heat capacity. A comprehensive database of over 5000 data points with 71 unique crystal phase ILs, comprising 42 different cations and 23 different anions, was used for parameterization and testing. This hybrid model takes into account the effect of the anion core, cation core, and subgroups within cations and anions, in addition to the derived indirect parameters that reflect the effects of branching and distribution around the core of the IL. According to the results, the developed GCM can reliably predict the crystal phase heat capacity with a mean absolute percentage error of 6.78%. This study aims to fill this current gap in the literature and to enable the design of ILs for thermal energy storage and other solid phase applications.

Unraveling the Morphology of [CnC1Im]Cl Ionic Liquids Combining Cluster and Aggregation Analyses

A characteristic feature of ionic liquids is their nanosegregation, resulting in the formation of polar and nonpolar domains. The influence of increasing the alkyl side chain on the morphology of ionic liquids has been the subject of many studies. Typically, the polar network (charged part of the cation and anion) constitutes a continuous subphase that partially breaks to allow the formation of a nonpolar domain with the increase of the alkyl chain. As the nonpolar network expands, the number of tails per aggregate increases until the ionic liquid percolates. In this work, we demonstrate how the complementary software packages TRAVIS and AGGREGATES can be employed in conjunction to gain insights into the size and morphology of the [CnC1Im]Cl family, with n ∈ {2, 4, 6, 8, 10, 12}. The combination of the two approaches rounds off the picture of the intricate arrangement and structural features of the alkyl chains.

The Coiling Effect in Ether Ionic Liquids: Exploiting Acetate as a Probe for Transport Properties and Microenvironment Analysis

The inherently high viscosity of ionic liquids (ILs) can limit their potential applications. One approach to address this drawback is to modify the cation side chain with ether groups. Herein, we assessed the structure-property relationship by focusing on acetate (OAc), a strongly coordinating anion, with 1,3-dialkylimidazolium cations with different side chains, including alkyl, ether, and hydroxyl functionalized, as well as their combinations. We evaluated their viscosity, thermal stabilities, and microstructure using Raman and infrared (IR) spectroscopies, allied to density functional theory (DFT) and ab initio molecular dynamics (AIMD) simulations. The viscosity data showed that the ether insertion significantly enhances the fluidity of the ILs, consistent with the coiling effect of the cation chain. Through a combined experimental and theoretical approach, we analyzed how the OAc anion interacts with ether ILs, revealing a characteristic bidentate coordination, particularly in hydroxyl functionalized ILs due to specific hydrogen bonding with the OH group. IR spectroscopy showed subtle shifts in the acidic hydrogens of imidazolium ring C(2)-H and C(4,5)-H, suggesting weaker interactions between OAc and the imidazolium ring in ether-functionalized ILs. Additionally, spatial distribution functions (SDF) and dihedral angle distribution obtained via AIMD confirmed the intramolecular hydrogen bonding due to the coiling effect of the ether side chain.

Combined Spectroscopic, Thermodynamic, and Theoretical Approach for Detecting and Quantifying Hydrogen Bonding and Dispersion Interaction in Ionic Liquids

ConspectusIonic liquids (ILs) are attracting increasing interest in science and engineering due to their unique properties that can be tailored for specific applications. Clearly, a better understanding of their behavior on the microscopic scale will help to elucidate macroscopic fluid phenomena and thereby promote potential applications. The advantageous properties of these innovative fluids arise from the delicate balance of Coulomb interactions, hydrogen bonding, and dispersion forces. The development of these properties requires a fundamental understanding of the strength, location, and direction of the different types of interactions and their contribution to the overall phase behavior. Contrary to expectations, hydrogen bonding and dispersion interactions have a significant influence on the structure, dynamics, and phase behavior of ILs.The synergy between experimental and theoretical methods has now advanced to a stage where hydrogen bonds and dispersion effects as well as the competition between the two can be studied in detail. In this account, we demonstrate that a suitable combination of spectroscopic, thermodynamic, and theoretical methods enables the detection, dissection, and quantification of noncovalent interactions, even in complex systems such as ionic liquids. This approach encompasses far-infrared vibrational spectroscopy (FIR), various thermodynamic methods for determining enthalpies of vaporization, and quantum chemical techniques that allow us to switch dispersion contributions on or off when calculating the energies and spectroscopic properties of clusters.We briefly discuss these experimental and theoretical methods, before providing various examples illustrating how the mélange of Coulomb interaction, hydrogen bonds, and dispersion forces can be analyzed, and their individual contributions quantified. First, we demonstrated that both hydrogen bonding and dispersion interactions are manifested in the FIR spectra and can be quantified by observed shifts of characteristic spectral signatures. Through the selection of suitable protic ionic liquids (PILs) featuring anions with varying interaction strengths and alkyl chain lengths, we were able to demonstrate that dispersion interactions can compete with hydrogen bonding. The resultant transition enthalpy serves as a measure of the dispersion interaction. Contrary to expectations, PILs possess lower enthalpies of vaporization compared with aprotic ILs (AILs). The reason for this is simple: In protic ILs, ion pairs carry both the hydrogen bond and attractive dispersion between the cation and anion into the gas phase. By utilizing a well-curated set of protic ILs and molecular analogues, we successfully disentangled Coulomb interaction, hydrogen bonding, and dispersion interaction through purely thermodynamic methods.

Uncovering the Properties of Dicationic Ionic Liquid Nanodroplets through Ab Initio Molecular Dynamics Simulations

The behavior of nanodroplets of an imidazolium-based dicationic ionic liquid, i.e., [C1(mim)2][PF6]2, was investigated in this study using ab initio molecular dynamics simulations. The vibrational features as well as the structural, interfacial, and dynamical properties of different sized droplets were analyzed and compared to the bulk phase system. Structural properties of the droplets, such as π-π stacking, radial distribution functions, structure factors, combined distribution functions, and angular distribution functions were analyzed to understand the interactions and orientations of their ions. The vibrational features and hydrogen bonding strength of droplets were studied by calculating their infrared (IR) and power spectra, determining the contribution of different types of hydrogen bonding to each vibrational mode. The calculated spectra showed good overall agreement with the experimental results. The interfacial properties of the droplets and the orientation of their ions were analyzed using density profiles and an exposed surface. The results showed that, in all systems studied, cations and anions were equally likely to exist in both inner and outer layers, and the cations tended to be oriented toward the center of droplets with obtuse angles. Additionally, the droplet densities were extrapolated to predict the bulk phase density with less than 2% deviation. The dynamical properties of hydrogen bonds, mean square displacement, and van Hove correlations of cations and anions were also analyzed. The results indicated that there was no regular trend in the dynamic properties of droplets with an increasing system size.

Recent Advances in Cellulose-Based Hydrogels Prepared by Ionic Liquid-Based Processes

This review summarizes the recent advances in preparing cellulose hydrogels via ionic liquid-based processes and the applications of regenerated cellulose hydrogels/iongels in electrochemical materials, separation membranes, and 3D printing bioinks. Cellulose is the most abundant natural polymer, which has attracted great attention due to the demand for eco-friendly and sustainable materials. The sustainability of cellulose products also depends on the selection of the dissolution solvent. The current state of knowledge in cellulose preparation, performed by directly dissolving in ionic liquids and then regenerating in antisolvents, as described in this review, provides innovative ideas from the new findings presented in recent research papers and with the perspective of the current challenges.

Ionic liquid mixtures as energy storage materials: a preliminary and comparative study based on thermal storage density

Fifteen equimolar binary mixtures are synthesized and thermophysically evaluated in this study. These mixtures are derived from six ionic liquids (ILs) based on methylimidazolium and 2,3-dimethylimidazolium cations with butyl chains. The objective is to compare and elucidate the impact of small structural changes on the thermal properties. The preliminary results are compared to previously obtained results with mixtures containing longer eight-carbon chains. The study demonstrates that certain mixtures exhibit an increase in their heat capacity. Additionally, due to their higher densities, these mixtures achieve a thermal storage density equivalent to that of mixtures with longer chains. Moreover, their thermal storage density surpasses that of some conventional materials commonly used for energy storage.

Quantification and Distribution of Three Types of Hydrogen Bonds in Mixtures of an Ionic Liquid with the Hydrogen-Bond-Accepting Molecular Solvent DMSO Explored by Neutron Diffraction and Molecular Dynamics Simulations

The concept of hydrogen bonding is celebrating its 100th birthday. Hydrogen bonds (H-bonds) play a key role in the structure and function of biological molecules, the strength of materials, and molecular binding. Herein, we study H-bonding in mixtures of a hydroxyl-functionalized ionic liquid with the neutral, H-bond-accepting molecular liquid dimethylsulfoxide (DMSO) using neutron diffraction experiments and molecular dynamics simulations. We report the geometry, strength, and distribution of three different types of H-bond OH···O, formed between the hydroxyl group of the cation and either the oxygen atom of another cation, the counteranion, or the neutral molecule. Such a variety of different strengths and distributions of H-bonds in one single mixture could hold the promise of providing solvents with potential applications in H-bond-related chemistry, for example, to alter the natural selectivity patterns of catalytic reactions or the conformation of catalysts.

Hydrogen Bonding and Infrared Spectra of Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide/Water Mixtures: A View from Molecular Dynamics Simulations

Simulations of ab initio molecular dynamics have been performed for mixtures of ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (EMIM-TFSI) ionic liquid and water. Statistics of donors and acceptors of hydrogen bonds has revealed that with increasing water content, hydrogen bonds between EMIM cations and TFSI anions are replaced by bonds to water molecules. In the mixture of liquids, the total number of bonds (from EMIM cations or water molecules) formed by TFSI acceptors increases. IR spectra obtained from ab initio molecular dynamics trajectories are in good agreement with literature data for ionic liquid/water systems. Analysis of oscillations of individual C-H and O-H bonds has shown correlations between vibrational frequencies and hydrogen bonds formed by an EMIM cation or water molecule and has indicated that the changes in the IR spectrum result from the decreased number of water-water hydrogen bonds in the mixture. The tests of DFTB methodology with tailored parameterizations have yielded reasonably good description of the IR spectrum of bulk water, whereas available parameterizations have failed in satisfactory reproduction of the IR spectrum of EMIM-TFSI/water mixtures in the region above 3000 cm^-1.

Insights into cation-anion hydrogen bonding in mesogenic ionic liquids: an NMR study

The hydrogen-bonding interaction is studied in imidazolium-based mesogenic ionic liquids in their isotropic, smectic, and solid phases and in a nanoconfined state by proton solid-state nuclear magnetic resonance (NMR). In the smectic phase, the more basic anions form stronger hydrogen bonds. A small decrease of H-bonding in the mesophase with respect to that in the isotropic phase is associated with the presence of a layered assembly with high orientational order and limited conformational freedom. Hydrogen bond strength is not sensitive to the cation structural modification as long as the aprotic nature of the material is preserved. The strong cation-anion hydrogen bonding observed in the smectic phases provides direct support for the presence of ionic sublayers which form in ionic liquid crystals regardless of the location and alignment of the charged group in the cation, particularly irrespective of whether the charged group occupies a terminal or central position in the cation structure. A comparison of the results obtained in isotropic, liquid-crystalline, and solid states shows that in the bulk materials the dynamic state of ions ranging from high reorientational and translational freedom to partial orientation and positional order to full immobilization, respectively, has no strong impact on the cation-anion hydrogen bond strength. On the other hand, nanoconfinement of ionic liquid crystals led to hydrogen bond disruption due to competing interactions of anions with a solid interface.

Infrared and Terahertz Spectroscopic Investigation of Imidazolium, Pyridinium, and Tetraalkylammonium Tetrafluoroborate Ionic Liquids

We performed terahertz time-domain spectroscopy and infrared spectroscopy of imidazolium-based, pyridinium-based, and tetraalkylammonium-based tetrafluoroborate ionic liquids to study their characteristic intermolecular and intramolecular vibrational modes to clarify interactions between various cations and the tetrafluoroborate anion. It was found that the central frequency of the intermolecular vibrational band for these ionic liquids has a relatively high frequency, ranging from 90 to 100 cm^-1. In the 900-1150 cm^-1 range, the intramolecular vibrational absorption band of the 3-fold degenerate mode of tetrafluoroborate anions in the ionic liquids was observed. Although the tetrafluoroborate anion is attributable to one of the weakly coordinated anions, the spectroscopic splitting behavior of the 3-fold degenerate mode differs depending on the cation species. It was revealed that the degenerate mode is very sensitive to local interactions between the tetrafluoroborate anion and each cation.

Alkylimidazolium Protic Ionic Liquids: Structural Features and Physicochemical Properties

We focus on a series of protic ionic liquids (PILs) with imidazolium and alkylimidazolium (1R3HIm, R = methyl, ethyl, propyl, and butyl) cations. Using the literature data and our experimental results on the thermal and transport properties, we analyze the effects of the anion nature and the alkyl radical length in the cation structure on the above properties. DFT calculations in gas and solvent phase have resulted in microscopic insights into the structure and cation-anion binding in these PILs. We show that the higher thermodynamic stability of an ion pair raises the PIL decomposition temperature. The melting points of the salts with the same cation decrease as the hydrocarbon radical in the cation becomes longer, which correlates with the weaker ion-ion interaction in the ion pairs. A comparative analysis of the protic ILs and corresponding ILs (1R3MeIm) with the same radical (R) in the cation structure and the same anion has been performed. The lower melting points of the ILs with 1R3MeIm cations are assumed to result from the weakening both of the ion-ion interaction and hydrogen bond.

Spectroscopic Signatures of Hydrogen-Bonding Motifs in Protonic Ionic Liquid Systems: Insights from Diethylammonium Nitrate in the Solid State

Diethylammonium nitrate, [N0 0 2 2][NO3], and its perdeuterated analogue, [N D D 2 2] [NO3], were structurally characterized and studied by infrared, Raman, and inelastic neutron scattering (INS) spectroscopy. Using these experimental data along with state-of-the-art computational materials modeling, we report unambiguous spectroscopic signatures of hydrogen-bonding interactions between the two counterions. An exhaustive assignment of the spectral features observed with each technique has been provided, and a number of distinct modes related to NH···O dynamics have been identified. We put a particular emphasis on a detailed interpretation of the high-resolution, broadband INS experiments. In particular, the INS data highlight the importance of conformational degrees of freedom within the alkyl chains, a ubiquitous feature of ionic liquid (IL) systems. These findings also enable an in-depth physicochemical understanding of protonic IL systems, a first and necessary step to the tailoring of hydrogen-bonding networks in this important class of materials.

Dissolution of Cellulose in Ionic Liquid-DMSO Mixtures: Roles of DMSO/IL Ratio and the Cation Alkyl Chain Length

The dissolution behavior of cellulose in the mixtures of dimethyl sulfoxide (DMSO) and different ionic liquids (ILs) at 25 °C was studied. High solubility of cellulose was reached in the mixtures of ILs and DMSO at mole fractions of 1:2, 1:2, and 1:1 for 1-butyl-3-methylimidazolium acetate, 1-propyl-3-methylimidazolium acetate, and 1-ethyl-3-methylimidazolium acetate, respectively. At high DMSO/IL molar ratios (10:1-2:1), a longer alkyl chain of the IL cation led to higher cellulose solubility. However, shorter cation alkyl chains favored cellulose dissolution at 1:1. Rheological, Fourier transform infrared spectroscopy (FTIR), and nuclear magnetic resonance (NMR) measurements were used to understand cellulose dissolution. It was found out that the increase of the DMSO ratio in binary mixtures caused higher cellulose solubility by decreasing the viscosity of systems. For cations with longer alkyl chains, stronger interaction between the IL and cellulose and higher viscosity of DMSO/IL mixtures were observed. The new knowledge obtained here could be useful to the development of cost-effective solvent systems for biopolymers.

Evidence of the CH···O HydrogenBonding in Imidazolium-Based Ionic Liquids from Far-Infrared Spectroscopy Measurements and DFT Calculations

Knowledge of all the intermolecular forces occurring in ionic liquids (ILs) is essential to master their properties. Aiming at investigating the weaker hydrogen bonding in aprotic liquids, the present work combined computational study and far-infrared spectroscopy on four imidazolium-based ILs with different anions. The DFT calculations of the ionic couples, using the ωB97X-D functional and considering both the empirical dispersion corrections and the presence of a polar solvent, show that, for all samples, the lowest energy configurations of the ion pair present H atoms, directly bound to C atoms of the cation and close to O atoms of the anion, capable of creating moderate to weak hydrogen bonding with anions. For the liquids containing anions of higher bonding ability, the absorption curves generated from the calculated vibrational frequencies and intensities show absorption bands between 100 and 125 cm⁻¹ corresponding to the stretching of the hydrogen bond. These indications are in complete agreement with the presently reported temperature dependence of the far-infrared spectrum, where the stretching modes of the hydrogen bonding are detected only for samples presenting a moderate interaction and become particularly prominent at low temperatures. Moreover, from the analysis of the infrared spectra, the occurrence of various phase transitions as a function of temperature was detected, and the difference in the average energy between the H-bonded and the dispersion-governed molecular configurations was evaluated.

Cation-Anion Interactions, Stability, and IR Spectra of Dicationic Amino Acid-Based Ionic Liquids Probed Using Density Functional Theory

In this work, we have theoretically studied the dicationic ionic liquids (DILs) constructed from geminal methylimidazolium dication with varying amino acid anions and spacers using density functional theory. Amino acid-based DILs form via strong C-H···O hydrogen bonds. These hydrogen bonds have a significant role in stabilizing the DILs. The higher cation-anion interaction energy in the order of covalent bond energy and liquid density of DILs imply higher thermal stability than their mono analogues. The C-H stretching frequencies are above 3100 cm^-1 in all complexes and form a signature for DILs. Interestingly, aliphatic and aromatic amino acid anions show similar molecular properties. Overall, the DILs formed from amino acids exhibit high stability and large surface tension and are chemically non-toxic; hence, they can replace inorganic DILs.

Balance Between Contact and Solvent-Separated Ion Pairs in Mixtures of the Protic Ionic Liquid [Et3NH][MeSO3] with Water Controlled by Water Content and Temperature

The formation of aggregates of ionic species is a crucial process in liquids and solutions. Ion speciation is particularly interesting for the case of ionic liquids (ILs) since these Coulombic fluids consist solely of ions. Most of their unique properties, such as enthalpies of vaporization and conductivities, are strongly related to ion pair formation. Here, we show that the balance of hydrogen-bonded contact ion pairs (CIP) and solvent-separated (SIP) ion pairs in protic ionic liquids (PILs) and in their mixtures with water can be well understood by a combination of far-infrared (FIR) and mid-infrared (MIR) spectroscopy, density functional theory (DFT) calculations of PIL/water aggregates, and molecular dynamics (MD) simulations of PIL/water mixtures. This combined approach is applied to mixtures of triethylammonium methanesulfonate [Et₃NH][MeSO₃] with water. It is shown that ion speciation in this mixture depends on three parameters: the relative hydrogen bond acceptor strength of the counter ion and the molecular solvent, the solvent concentration, and the temperature. For selected PIL/water mixtures, the equilibrium constants for CIPs and SIPs were determined as a function of the solvent content and temperature. Finally, for the studied PIL/water mixtures, the transition from CIPs to SIPs could be understood on enthalpic and entropic grounds. A detailed picture of this interconversion process could be described at the molecular level by means of MD simulations. In addition, the concentration dependence of ion pair formation can be well understood with help of a simplified "cartoon-like" statistical model describing hydrogen bond redistribution.

Targeted modifications in ionic liquids - from understanding to design

Ionic liquids are extremely versatile and continue to find new applications in academia as well as industry. This versatility is rooted in the manifold of possible ion types, ion combinations, and ion variations. However, to fully exploit this versatility, it is imperative to understand how the properties of ionic liquids arise from their constituents. In this work, we discuss targeted modifications as a powerful tool to provide understanding and to enable design. A 'targeted modification' is a deliberate change in the structure of an ionic liquid. This includes chemical changes in an experiment as well as changes to the parameterisation in a computer simulation. In any case, such a change must be purposeful to isolate what is of interest, studying, as far as is possible, only one concept at a time. The concepts can then be used as design elements. However, it is often found that several design elements interact with each other - sometimes synergistically, and other times antagonistically. Targeted modifications are a systematic way of navigating these overlaps. We hope this paper shows that understanding ionic liquids requires experimentalists and theoreticians to join forces and provides a tool to tackle the difficult transition from understanding to design.

Inter- and Intramolecular Interactions in Ether-Functionalized Ionic Liquids

The intra- and intermolecular interactions in ether-functionalized ionic liquids (ILs) are studied by means of infrared (IR) spectroscopy measurements of N-ethoxyethyl-N-methylpiperidiniumbis(fluorosulfonyl)imide (P_1,2O2-FSI) and N-ethoxyethyl-N-methylmorpholiniumbis(fluorosulfonyl)imide (M_1,2O2-FSI). The temperature dependence of the spectra in the medium IR range allows the study of the anion conformer distribution and its variation during phase transitions. In particular, it is found that for both ILs the trans conformer of FSI is more stable than the cis conformer, and the enthalpy differences between them are calculated and are found to decrease upon the addition of a Li salt. The results obtained in the far IR range, combined with ab initio calculation of the ionic couple performed using the B3LYP-D functional and considering both empirical dispersion corrections and the presence of a polar solvent, provide evidence for the occurrence of a hydrogen bonding between the O atom of the anion and its closest H atoms directly linked to a C atom of the cation. The comparison with samples having the same cations but with bis(trifluoromethanesulfonyl)imide (TFSI) as an anion, that is, M_1,2O2-TFSI and P_1,2O2-TFSI, as well as with samples having cations without the ether-functionalization neither in the ring nor in the side chain, such as N-propyl-N-methylpyrrolidinium-FSI (PYR₁₃-FSI) and 1-butyl-1-methylpyrrolidinium-TFSI (PYR₁₄-TFSI), indicates that the occurrence of such highly directional interaction between anion and cation is better observable in the ether-functionalized samples, in particular in those containing FSI as an anion.

Infrared Spectroscopy in the Middle Frequency Range for Various Imidazolium Ionic Liquids-Common Spectroscopic Characteristics of Vibrational Modes with In-Plane +C(2)-H and +C(4,5)-H Bending Motions and Peak Splitting Behavior Due to Local Symmetry Breaking of Vibrational Modes of the Tetrafluoroborate Anion

Various alkyl-methylimidazolium ionic liquids (ILs) were inspected using infrared spectroscopy in the middle frequency range. In the 1050-1200 cm^-1 range, there is a skeletal vibrational mode accompanied with a large in-plane ⁺C(2)-H bending motion and ⁺C(4)-H and ⁺C(5)-H motions, and in the 1500-1650 cm^-1 range, there are two skeletal vibrational modes with in-plane ⁺C(4,5)-H bending motions. Interestingly, in both ranges, we found that skeletal vibrational modes with a large in-plane ⁺C(2)-H bending motion and in-plane ⁺C(4,5)-H bending motions are insensitive to increases in the basicity of anions or the strengthening of hydrogen bond-type interactions, and the behaviors are completely different from those in the ⁺C-H stretching vibrational modes in the 3000-3200 cm^-1 range and the skeletal vibrational modes with large out-of-plane ⁺C-H motions in the 700-950 cm^-1 range. Furthermore, in alkyl-methylimidazolium tetrafluoroborate [C _n mim⁺][BF₄ ^-] ILs, we found that absorption due to the (threefold) degenerate vibrational mode of [BF₄ ^-] was observed as a broad absorption band with three splitting peaks in the 900-1150 cm^-1 range as a result of local symmetry breaking due to the cation-anion interactions.

A force field for bio-polymers in ionic liquids (BILFF) - part 1: [EMIm][OAc]/water mixtures

We present BILFF, a novel force field for bio-polymers in ionic liquids. In the first part of our study, we introduce optimized force field parameters for mixtures of the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate ([EMIm][OAc]) with water. This imidazolium-based IL is of particular practical importance as it can dissolve significant amounts of cellulose even at room temperature. An understanding of this dissolution process via molecular dynamics simulations requires a quantitative description of the microscopic structure and the strong hydrogen bonds with a method able of simulating at least several dozen nanoseconds, which is the main aim of our novel force field. To reach this goal, we optimize the force field parameters to reproduce radial, spatial, and combined distribution functions, hydrogen bond lifetimes, diffusion coefficients, and several other quantities from reference ab initio molecular dynamics (AIMD) simulations. Non-trivial effects such as dispersion interactions between the side chains and π-π stacking of the cations are reproduced very well. We further validate the force field by comparison to experimental data such as thermal expansion coefficients, bulk modulus, and density at different temperatures, which yields good agreement and correct trends. No other force field with optimized parameters for mixtures of [EMIm][OAc] and water has been presented in the literature yet. Optimized force field parameters for cellulose and other ILs will be published in upcoming articles.

Multi-Interactions in Ionic Liquids for Natural Product Extraction

Natural products with a variety of pharmacological effects are important sources for commercial drugs, and it is very crucial to develop effective techniques to selectively extract and isolate bioactive natural components from the plants against the background of sustainable development. Ionic liquids (ILs) are a kind of designable material with unique physicochemical properties, including good thermal stability, negligible vapor pressure, good solvation ability, etc. ILs have already been used in pharmaceuticals for extraction, purification, drug delivery, etc. It has been reported that multi-interactions, like hydrogen bonding, hydrophobic interactions, play important roles in the extraction of bioactive components from the plants. In this review, recent progress in the understanding of scientific essence of hydrogen bonding, the special interaction, in ILs was summarized. The extraction of various natural products, one important area in pharmaceutical, by conventional and functional ILs as well as the specific roles of multi-interactions in this process were also reviewed. Moreover, problems existing in bioactive compound extraction by ILs and the future developing trends of this area are given, which might be helpful for scientists, especially beginners, in this field.

Probing the Effect of Side Alkyl Chain Length on the Structural and Dynamical Micro-heterogeneities in Dicationic Ionic Liquids

The molecular dynamics simulations and Voronoi tessellation analysis of two dicationic ionic liquids (DILs) including [C₅(mim)₂][NTf₂]₂ and [C₅(mim)₂C₄][NTf₂]₂ have been carried out to investigate the effects of side alkyl chain length on the structural and dynamical micro-heterogeneity of these DILs. Radial distribution functions (RDFs), spatial distribution functions (SDFs), and also neighborhood analysis of ions have been calculated to determine the arrangement of the nearest neighboring ions. To better understand the hydrogen-bonding network, microstructures, inter- and intramolecular orientations of ions in the studied DILs, different kinds of combined distribution functions (CDFs) were computed and analyzed. Also, qualitative and quantitative analyses of the structural heterogeneity were explored through total/partial structure factors, heterogeneity order parameters (HOPs), and domain analysis from Voronoi tessellation. The results showed that the side alkyl chains in DILs have significant effects on their micro-organizations in such a way that [C₅(mim)₂C₄][NTf₂]₂ with longer side chains has more microstructural heterogeneity than [C₅(mim)₂][NTf₂]₂ where the linkage alkyl chain is the same in both of them. Furthermore, to shed light on the dynamical heterogeneity, ion pair, ion cage, and hydrogen-bond stabilities and also the reorientation dynamics of ions have been investigated. Results demonstrated that local dynamics differences originate from local structural heterogeneity.

Aqueous Co-Solvent in Zwitterionic-based Protic Ionic Liquids as Electrolytes in 2.0 V Supercapacitors

High-performance energy-storage devices are receiving great interest in sustainable terms as a required complement to renewable energy sources to level out the imbalances between supply and demand. Besides electrode optimization, a primary objective is also the judicious design of high-performance electrolytes combining novel ionic liquids (ILs) and mixtures of aqueous solvents capable of offering "à la carte" properties. Herein, it is described the stoichiometric addition of a zwitterion such as betaine (BET) to protic ILs (PILs) such as those formed between methane sulfonic acid (MSAH) or p-toluenesulfonic acid (PTSAH) with ethanolamine (EOA). This addition resulted in the formation of zwitterionic-based PILs (ZPILs) containing the original anion and cation as well as the zwitterion. The ZPILs prepared in this work ([EOAH]⁺ [BET][MSA]^- and [EOAH]⁺ [BET][PTSA]^- ) were liquid at room temperature even though the original PILs ([EOAH]⁺ [MSA]^- and [EOAH]⁺ [PTSA]^- ) were not. Moreover, ZPILs exhibited a wide electrochemical stability window, up to 3.7 V vs. Ag wire for [EOAH]⁺ [BET][MSA]^- and 4.0 V vs. Ag wire for [EOAH]⁺ [BET][PTSA]^- at room temperature, and a high miscibility with both water and aqueous co-solvent (WcS) mixtures. In particular, "WcS-in-ZPIL" mixtures of [EOAH]⁺ [BET][MSA]^- in 2 H₂ O/ACN/DMSO provided specific capacitances of approximately 83 F g^-1 at current densities of 1 A g^-1 , and capacity retentions of approximately 90 % after 6000 cycles when operating at a voltage of 2.0 V and a current density of 4 A g^-1 .

Clusters of Hydroxyl-Functionalized Cations Stabilized by Cooperative Hydrogen Bonds: The Role of Polarizability and Alkyl Chain Length

We explore quantum chemical calculations for studying clusters of hydroxyl-functionalized cations kinetically stabilized by hydrogen bonding despite strongly repulsive electrostatic forces. In a comprehensive study, we calculate clusters of ammonium, piperidinium, pyrrolidinium, imidazolium, pyridinium, and imidazolium cations, which are prominent constituents of ionic liquids. All cations are decorated with hydroxy-alkyl chains allowing H-bond formation between ions of like charge. The cluster topologies comprise linear and cyclic clusters up to the size of hexamers. The ring structures exhibit cooperative hydrogen bonds opposing the repulsive Coulomb forces and leading to kinetic stability of the clusters. We discuss the importance of hydrogen bonding and dispersion forces for the stability of the differently sized clusters. We find the largest clusters when hydrogen bonding is maximized in cyclic topologies and dispersion interaction is properly taken into account. The kinetic stability of the clusters with short-chained cations is studied for the different types of cations ranging from hard to polarizable or exhibiting additional functional groups such as the acidic C(2)-H position in the imidazolium-based cation. Increasing the alkyl chain length, the cation effect diminishes and the kinetic stability is exclusively governed by the alkyl chain tether increasing the distance between the positively charged rings of the cations. With adding the counterion tetrafluoroborate (BF₄⁻) to the cationic clusters, the binding energies immediately switch from strongly positive to strongly negative. In the neutral clusters, the OH functional groups of the cations can interact either with other cations or with the anions. The hexamer cluster with the cyclic H-bond motive and “released” anions is almost as stable as the hexamer built by H-bonded ion pairs exclusively, which is in accord with recent IR spectra of similar ionic liquids detecting both types of hydrogen bonding. For the cationic and neutral clusters, we discuss geometric and spectroscopic properties as sensitive probes of opposite- and like-charge interaction. Finally, we show that NMR proton chemical shifts and deuteron quadrupole coupling constants can be related to each other, allowing to predict properties which are not easily accessible by experiment.

Different Hydrogen Bond Changes Driven by Surface Segregation Behavior of Imidazolium-Based Ionic Liquid Mixture at the Liquid-Vacuum Interface

In this work, molecular dynamics (MD) simulations were carried out to study the behaviors of a binary ionic liquid (IL) mixture consisting of equimolar [C₂C₁Im][BF₄] and [C₄C₁Im][BF₄], as well as two corresponding pure ILs, at the liquid-vacuum interface. Our simulation results show that the competition of nonpolar interactions between different alkyl chains of two cations results in an obvious surface segregation behavior of the IL mixture at the interface, indicating an enhanced aggregation of the [C₄C₁Im]⁺ cations but a weakened aggregation of the [C₂C₁Im]⁺ cations at the outermost surface. More interestingly, different hydrogen bond (HB) changes between two imidazolium cations at the interface can be driven by such surface segregation behavior, where the [C₂C₁Im]⁺ cations rather than the [C₄C₁Im]⁺ ones have more and stronger HBs with the [BF₄]^- anions by comparison with the corresponding pure ILs at the interface. Meanwhile, it is interesting to find that such a stronger HB would lower the rotations of the imidazolium rings of interfacial [C₂C₁Im]⁺ cations. By contrast, the [C₄C₁Im]⁺ cations at the outermost surface rotate faster owing to their weaker HB. In addition, the orientation analysis uncovers that there is a major decrease for the orderliness of interfacial [C₂C₁Im]⁺ cations, but a minor decrease for that of interfacial [C₄C₁Im]⁺ cations, from the pure IL to the IL mixture. Such distinct results are closely related to the surface segregation between the [C₂C₁Im]⁺ and [C₄C₁Im]⁺ cations in the IL mixture and their interfacial HB properties. Thus, our simulation results afford a deep insight into the surface segregation effect on the HB behavior of the imidazolium-based IL mixture at liquid-vacuum interface.

Confinement Effects on Glass-Forming Aqueous Dimethyl Sulfoxide Solutions

Combining broadband dielectric spectroscopy and nuclear magnetic resonance studies, we analyze the reorientation dynamics and the translational diffusion associated with the glassy slowdown of the eutectic aqueous dimethyl sulfoxide solution in nano-sized confinements, explicitly, in silica pores with different diameters and in ficoll and lysozyme matrices at different concentrations. We observe that both rotational and diffusive dynamics are slower and more heterogeneous in the confinements than in the bulk but the degree of these effects depends on the properties of the confinement and differs for the components of the solution. For the hard and the soft matrices, the slowdown and the heterogeneity become more prominent when the size of the confinement is reduced. In addition, the dynamics are more retarded for dimethyl sulfoxide than for water, implying specific guest-host interactions. Moreover, we find that the temperature dependence of the reorientation dynamics and of the translational diffusion differs in severe confinements, indicating a breakdown of the Stokes–Einstein–Debye relation. It is discussed to what extent these confinement effects can be rationalized in the framework of core-shell models, which assume bulk-like and slowed-down motions in central and interfacial confinement regions, respectively.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Development of Nanoscale Hybrids from Ionic Liquid-Peptide Amphiphile Assemblies as New Functional Materials

Over the years, ionic liquids (ILs) have gained tremendous importance because of their unique properties and plethora of applications. In this work, we have developed a new nanoscale hybrid gel consisting of 1-ethyl-3-methylimidazolium dimethyl phosphate, [C2mim][dmp], and self-assembled peptide nanoassemblies. The peptide nanoassemblies were formed by self-assembly of a newly synthesized peptide bolaamphiphile bis(N-α-amido-threonine) 1,7 heptane dicarboxylate (ThrC7). Upon mild heating and sonication of the IL and ThrC7 nanoassemblies, ThrC7-IL nanocomposites were formed. We explored the formation of nanohybrids by varying the ratio of IL to ThrC7 assemblies. While at lower IL ratios, a gelatinous matrix was formed, at higher IL ratios, highly ordered multilayered structures were observed by atomic force microscopy (AFM) imaging. The interactions between the ThrC7 nanofibers and [C2mim][dmp] IL were probed by Fourier transform infrared spectroscopy, transmission electron microscopy, and AFM imaging. Differential scanning calorimetry and thermogravimetric analysis showed that the nanohybrids illustrated distinct thermal phase changes due to changes in hydrogen bonding interactions and unfolding of the nanoassemblies. The viscoelastic behavior of the nanohybrids indicated that the materials displayed higher storage modulus upon incorporation of the ThrC7 nanoassemblies when compared to the IL. Furthermore, the nanohybrids were found to adhere to and promote proliferation of human dermal fibroblasts, while cytotoxicity was observed toward MCF-7 breast cancer cells. Thus, for the first time, we have developed peptide-based nanohybrids with an imidazolium-based IL with unique structural properties that may open new avenues for exploring potential biological applications.