Structure of water-in-salt and water-in-bisalt electrolytes

We report a systematic diffraction study of two "water-in-salt" electrolytes and a "water-in-bisalt" electrolyte combining high-energy X-ray diffraction (HEXRD) with polarized and unpolarized neutron diffraction (ND) on both H₂O and D₂O solutions. The measurements provide three independent combinations of correlations between the different pairs of atom types that reveal the short- and intermediate-range order in considerable detail. The ND interference functions show pronounced peaks around a scattering vector Q ∼ 0.5 Å^-1 that change dramatically with composition, indicating significant rearrangements of the water network on a length scale around 12 Å. The experimental results are compared with two sets of Molecular Dynamics (MD) simulations, one including polarization effects and the other based on a non-polarizable force field. The two simulations reproduce the general shapes of the experimental structure factors and their changes with concentration, but differ in many detailed respects, suggesting ways in which their force fields might be modified to better represent the actual systems.

Solvent extraction of rare earths elements from nitrate media in DMDOHEMA/ionic liquid systems: performance and mechanism studies

Extraction of La(iii), Eu(iii) and Fe(iii) was compared in n-dodecane and two ionic liquids (ILs) (1-ethyl-1-butylpiperidinium bis (trifluoromethylsulfonyl)imide [EBPip⁺] [NTf₂⁻] and 1-ethyl-1-octylpiperidinium bis (trifluoromethylsulfonyl)imide [EOPip⁺] [NTf₂⁻]). Using the extractant N,N′-dimethyl-N,N′-dioctylhexylethoxymalonamide (DMDOHEMA), the effect of pH was investigated in detail to recover extraction mechanisms. The use of ILs as the organic solvent instead of n-dodecane, greatly enhances extraction efficiency, and an ionic liquid with a shorter alkyl chain [EBPip⁺] [NTf₂⁻] provides higher extraction than [EOPip⁺] [NTf₂⁻]. The mechanistic study points out that for low nitric acid concentrations ([HNO₃] ≤ 0.01 M), metal is extracted via a cation of the ionic liquids, while for higher nitric acid concentrations ([HNO₃] ≥ 1.0 M), extraction occurs through pure solvation mechanism of DMDOHEMA as in conventional diluents. This latter case is of high interest for applications, as higher extraction can be obtained without any loss of ILs by ion exchange mechanisms.

Extraction of La(iii), Eu(iii) and Fe(iii) was compared in n-dodecane and in two ionic liquids (ILs) [EBPip⁺] [NTf₂⁻] and [EOPip⁺] [NTf₂⁻]. Extraction mechanisms have been investigated as a function of pH.

Ionic liquids for regulating biocatalytic process: Achievements and perspectives

Biocatalysis has found enormous applications in sorts of fields as an alternative to chemical catalysis. In the pursue of green and sustainable chemistry, ionic liquids (ILs) have been considered as promising reaction media for biocatalysis, owing to their unique characteristics, such as nonvolatility, inflammability and tunable properties as regards polarity and water miscibility behavior, compared to organic solvents. In recent years, great developments have been achieved in respects to biocatalysis in ILs, especially for preparing various chemicals. This review tends to give illustrative examples with a focus on representative chemicals production by biocatalyst in ILs and elucidate the possible mechanism in such systems. It also discusses how to regulate the catalytic efficiency from several aspects and finally provides an outlook on the opportunities to broaden biocatalysis in ILs.

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.

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 + ][BF 4 - ], [BMIM + ][PF 6 - ], [BMIM + ][OTf - ], [BMIM + ][NO 3 - ]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 + ][PF 6 - ], 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.

3D structure of the electric double layer of ionic liquid-alcohol mixtures at the electrochemical interface

Mixtures of the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate with amphiphilic cosolvents, such as methanol and ethanol, nanoconfined between graphene walls are studied by means of molecular dynamics simulations and the results are compared with those of the pure ionic liquid and its mixtures with water confined in the same conditions. We investigate the adsorption of cosolvent molecules at the graphene walls as well as their distribution across the system. The results show that, due to a higher affinity of the polar groups to be close to the anions in combination with the electrostatic and excluded volume interactions, there exists a high tendency of the OH groups to lie close to the anode, inducing small changes in the first cation layer. The orientation of cosolvent molecules is found to be closely related to the alignment of the molecular dipole moment. We also investigate the lateral ionic distribution in the layers close to the electrodes, which shows a structural transition from liquid-like lamellar ordering to solid-like hexagonal patterns as the size of the cosolvent molecules increases leading to smaller position fluctuations of the ions. The dependence of the specific patterns on the nature of the electrodes is also studied. This study strongly suggests that the ionic patterns formed in the first ionic layers next to the charged interfaces are universal since their existence does not crucially depend on the atomic composition of the interfacial material, but only on the net charge density of the considered ionic layer, which significantly changes the ionic mobility in this region.

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.

"Solvent-in-salt" systems for design of new materials in chemistry, biology and energy research

Inorganic and organic "solvent-in-salt" (SIS) systems have been known for decades but have attracted significant attention only recently. Molten salt hydrates/solvates have been successfully employed as non-flammable, benign electrolytes in rechargeable lithium-ion batteries leading to a revolution in battery development and design. SIS with organic components (for example, ionic liquids containing small amounts of water) demonstrate remarkable thermal stability and tunability, and present a class of admittedly safer electrolytes, in comparison with traditional organic solvents. Water molecules tend to form nano- and microstructures (droplets and channel networks) in ionic media impacting their heterogeneity. Such microscale domains can be employed as microreactors for chemical and enzymatic synthesis. In this review, we address known SIS systems and discuss their composition, structure, properties and dynamics. Special attention is paid to the current and potential applications of inorganic and organic SIS systems in energy research, chemistry and biochemistry. A separate section of this review is dedicated to experimental methods of SIS investigation, which is crucial for the development of this field.

Relaxation in a Prototype Ionic Liquid: Influence of Water on the Dynamics

The influence of water on the relaxation of a prototype ionic liquid (IL) C₈mimBF₄ is examined in the IL-rich regime combining quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations. The QENS and MD simulations results for relaxation of IL and the equimolar mixture with water probed by the dynamics of the C₈mim hydrogen atoms in the time range of 2 ps to 1 ns are in excellent agreement. The QENS data show that translational relaxation increases by a factor of 7 on the addition of water, while rotational relaxation involving multiple processes fitted by a KWW function with low β values is speeded up by a factor of 3 on the time scale of QENS measurements. The MD simulations show that the cation diffusion coefficient, inverse viscosity, and ionic conductivity increase on the addition of water, consistent with the very small change in ionicity. The difficulties in obtaining rotational and translational diffusion coefficients from fits to QENS experiments of pure ILs and IL-water mixtures are discussed.

Proof of ion-pair structures in ammonium-based protic ionic liquids using combined NMR and DFT/PCM-based chemical shift calculations

The self-assembly of triethylammonium bis(trifluoromethylsulfonyl)imide, i.e. [(C₂H₅)₃NH][TFSI], in chloroform and aqueous solutions has been investigated using ¹H NMR spectroscopy and computational (DFT/PCM prediction) methods.

Water modulates the ultraslow dynamics of hydrated ionic liquids near CG rich DNA: consequences for DNA stability

The higher affinity of water towards CG-base pairs can alter the dynamics of ionic liquids in solution, causing entropic adversity in binding to the base pairs.