Shear rheology senses the electrical room-temperature conductivity optimum in highly Li doped dinitrile electrolytes

Glutaronitrile (GN) doped with lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) at concentrations below and above the room-temperature conductivity optimum near 1M of Li salt is investigated using dielectric spectroscopy and shear rheology. The experiments are carried out from ambient down to the glass transition temperature Tg, which increases considerably as LiTFSI is admixed to GN. As the temperature is lowered, the conductivity optimum shifts to lower salt concentrations, while the power-law exponents connecting resistivity and molecular reorientation time remain smallest for the 1M composition. By contrast, the rheologically detected time constants, as well as those obtained using dielectric spectroscopy, increase monotonically with increasing Li salt concentration for all temperatures. It is demonstrated that the shear mechanical measurements are, nevertheless, sensitive to the 1M conductivity optimum, thus elucidating the interplay of the dinitrile matrix with the mobile species. The data for the Li doped GN and other nitrile solvents all follow about the same Walden line, in harmony with their highly conductive character. The composition dependent relation between the ionic and the reorientational dynamics is also elucidated.

Translational and reorientational dynamics in carboxylic acid-based deep eutectic solvents

The glass formation and the dipolar reorientational motions in deep eutectic solvents (DESs) are frequently overlooked, despite their crucial role in defining the room-temperature physiochemical properties. To understand the effects of these dynamics on the ionic conductivity and their relation to the mechanical properties of the DES, we conducted broadband dielectric and rheological spectroscopy over a wide temperature range on three well-established carboxylic acid-based natural DESs. These are the eutectic mixtures of choline chloride with oxalic acid (oxaline), malonic acid (maline), and phenylacetic acid (phenylaceline). In all three DESs, we observe signs of a glass transition in the temperature dependence of their dipolar reorientational and structural dynamics, as well as varying degrees of motional decoupling between the different observed dynamics. Maline and oxaline display a breaking of the Walden rule near the glass-transition temperature, while the relation between the dc conductivity and dipolar relaxation time in both maline and phenylaceline is best described by a power law. The glass-forming properties of the investigated systems not only govern the orientational dipolar motions and rheological properties, which are of interest from a fundamental point of view, but they also affect the dc conductivity, even at room temperature, which is of high technical relevance.

Rotational dynamics, ionic conductivity, and glass formation in a ZnCl2-based deep eutectic solvent

Glass formation and reorientational motions are widespread but often-neglected features of deep eutectic solvents although both can be relevant for the technically important ionic conductivity at room temperature. Here, we investigate these properties for two mixtures of ethylene glycol and ZnCl2, which were recently considered superior electrolyte materials for application in zinc-ion batteries. For this purpose, we employed dielectric spectroscopy performed in a broad temperature range, extending from the supercooled state at low temperatures up to the liquid phase around room temperature and beyond. We find evidence for a relaxation process arising from dipolar reorientation dynamics, which reveals the clear signatures of glassy freezing. This freezing also governs the temperature dependence of the ionic dc conductivity. We compare the obtained results with those for deep eutectic solvents that are formed by the same hydrogen-bond donor, ethylene glycol, but by two different salts, choline chloride and lithium triflate. The four materials reveal significantly different ionic and reorientational dynamics. Moreover, we find varying degrees of decoupling of rotational dipolar and translational ionic motions, which can partly be described by a fractional Debye-Stokes-Einstein relation. The typical glass-forming properties of these solvents strongly affect their room-temperature conductivity.

Nature of a Tetrabutylammonium Chloride-Levulinic Acid Deep Eutectic Solvent

A deep eutectic solvent was formed by considering the mixtures of tetrabutylammonium chloride and levulinic acid, and it is studied via a combined theoretical and experimental approach. Physicochemical properties were measured as a function of temperature, providing a macroscopic characterization of the fluid. Quantum chemistry and classical molecular dynamics simulations were carried out for the nanoscopic characterization, providing attention to the nature, extension, and dynamics of the hydrogen bonding network, which is at the root of the properties of the fluid. The reported study allows multiscale characterization of this fluid as an archetypical example of a natural, low-cost, and sustainable fluid.

Deep eutectic solvent-infused two-dimensional metal-organic framework membranes as quasi-solid-state electrolytes for wearable micro-supercapacitors

The burgeoning field of miniaturized and portable electronic devices calls for novel advances in micro-energy storage technology. Micro-supercapacitors (MSC) stand at the forefront of this endeavour, yet unlocking their full potential necessitates the exploration of high-performance electrolytes. Herein, we introduce a strategy that leverages flexible metal-organic framework (MOF, CuTCPP) nanosheet-based membranes to construct quasi-solid-state electrolytes (QSSEs) and enhance the ionic conductivity and electrochemical performance of deep eutectic solvent (DES)-based MSCs. Owing to the multiple nanochannel pathways provided by the porous MOF nanosheets, the ionic conductivity of DES within the nanochannels exhibits a 13-fold increment compared with its bulk counterpart. Furthermore, we engineered MSC harnessing the CuTCPP-DES system, whose performance surpasses that reported for most of the ionic liquid and 2D material-based MSCs. The areal-specific capacitance was 81.3 mF cm-2 at a current density of 0.1 mA cm-2, and the energy density was 45.17 μW h cm-2 at a power density of 8.559 mW cm-2. Notably, the performance of MSCs remains consistent and unaffected, even when subjected to bending. These findings contribute to the exploration and potential optimization of the inherent benefits of MOFs, thereby presenting a paradigm shift in nanoconfined systems for microscale energy storage applications.

Therapeutic Deep Eutectic Solvents: A Comprehensive Review of Their Thermodynamics, Microstructure and Drug Delivery Applications

Deep eutectic solvents (DES) are multicomponent liquids that are usually formed by coupling a hydrogen bond donor and acceptor leading to strong non-covalent (NC) intermolecular networking and profound depression in the melting point of the system. Pharmaceutically, this phenomenon has been exploited to improve drugs' physicochemical properties, with an established DES therapeutic subcategory, therapeutic deep eutectic solvents (THEDES). THEDES preparation is usually via straightforward synthetic processes with little involvement of sophisticated techniques, which, in addition to its thermodynamic stability, make these multi-component molecular adducts a very attractive alternative for drug enabling purposes. Other NC bonded binary systems (e.g., co-crystals and ionic liquids) are utilized in the pharmaceutical field for enhancing drug's behaviours. However, a clear distinction between these systems and THEDES is scarcely discussed in the current literature. Accordingly, this review provides a structure-based categorization for DES formers, a discussion of its thermodynamic properties and phase behaviour, and it clarifies the physicochemical and microstructure boundaries between DES and other NC systems. Additionally, a summary of its preparation techniques and their experimental conditions preparation is supplied. Instrumental analysis techniques can be used to characterize and differentiate DES from other NC mixtures, hence this review draws a road map to for this purpose. Since this work mainly focuses on pharmaceutical applications of DES, all types of THEDES including the highly discussed types (conventional, drugs dissolved in DES and polymer based) in addition to the less discussed categories are covered. Finally, the regulatory status of THEDES was investigated despite the current unclear situation.

Structure-Dynamics Interrelation Governing Charge Transport in Cosolvated Acetonitrile/LiTFSI Solutions

Concentrated ionic solutions present a potential improvement for liquid electrolytes. However, their conductivity is limited by high viscosities, which can be attenuated via cosolvation. This study employs a series of experiments and molecular dynamics simulations to investigate how different cosolvents influence the local structure and charge transport in concentrated lithium bis(trifluoromethane-sulfonyl)imide (LiTFSI)/acetonitrile solutions. Regardless of whether the cosolvent's dielectric constant is low (for toluene and dichloromethane), moderate (acetone), or high (methanol and water), they preserve the structural and dynamical features of the cosolvent-free precursor. However, the dissimilar effects of each case must be individually interpreted. Toluene and dichloromethane reduce the conductivity by narrowing the distribution of Li⁺-TFSI^- interactions and increasing the activation energies for ionic motions. Methanol and water broaden the distributions of Li⁺-TFSI^- interactions, replace acetonitrile in the Li⁺ solvation, and favor short-range Li⁺-Li⁺ interactions. Still, these cosolvents strongly interact with TFSI^-, leading to conductivities lower than that predicted by the Nernst-Einstein relation. Finally, acetone preserves the ion-ion interactions from the cosolvent-free solution but forms large solvation complexes by joining acetonitrile in the Li⁺ solvation. We demonstrate that cosolvation affects conductivity beyond simply changing viscosity and provide fairly unexplored molecular-scale perspectives regarding structure/transport phenomena relation in concentrated ionic solutions.

A Zinc-Bromine Battery with Deep Eutectic Electrolytes

A deep eutectic solvent (DES) is an ionic liquid-analog electrolyte, newly emerging due to its low cost, easy preparation, and tunable properties. Herein, a zinc-bromine battery (ZBB) with a Zn-halide-based DES electrolyte prepared by mixing ZnBr₂ , ZnCl₂ , and a bromine-capturing agent is reported. The water-free DES electrolyte allows a closed-cell configuration for the ZBB owing to the prevention of Br₂ evaporation and H₂ evolution. It is found that the Cl^- anion changes the structure of the zinc-halide complex anions and demonstrated that it improves the ion mobility and electrode reaction kinetics. The DES electrolyte with the optimized ZnCl₂ composition shows much higher rate capability and a cycle life 90 times longer than that of a ZnCl₂ -free DES electrolyte. A pouch-type flexible ZBB battery based on the DES electrolyte exhibits swelling-free operation for more than 120 cycles and stable operation under a folding test, suggesting its potential in consumer applications such as wearable electronics.

A Weak Acidic and Strong Coordinated Deep Eutectic Solvent for Recycling of Cathode from Spent Lithium-Ion Batteries

The leaching and recycling of valuable metals via environmentally benign solvents is important because of the ever-increasing waste lithium-ion batteries, but it remains a challenge. Herein, a multi-functional deep eutectic solvent (DES) based on lactic acid (LA) and guanidine hydrochloride (GHC) was used to extract cobalt and lithium ions from LiCoO₂ . Due to the strong acidity (protons) and abundant chlorine coordinating ions of LA/GHC, the solubility of LiCoO₂ in LA/GHC could reach as high as 19.9 mg g^-1 (stirred at 80 °C for 24 h), and a little LiCoO₂ powder even could be dissolved at room temperature without stirring. Oxalic acid was used to strip and separate the oxalates of cobalt and lithium. Furthermore, LA/GHC could be recycled with a similar dissolving performance. This work avoided using corrosive acids and could be realized at low temperature (80 °C), making it energy-saving and cost-effective. It shows DESs have great potential in extracting strategically important metals from LiCoO₂ cathodes and provides an efficient and green alternative for sustainable recycling of spent lithium-ion batteries.

Deuteron magnetic resonance study of glyceline deep eutectic solvents: Selective detection of choline and glycerol dynamics

Glyceline, a green solvent considered for various electrochemical applications, represents a multi-component glass former. Viewed from this perspective, the choline cation and the hydrogen bond donor glycerol, the two major constituents forming this deep eutectic solvent, were studied using nuclear magnetic resonance in a selective manner by means of suitably deuteron-labeled isotopologues. Carried out from far above to far below the glass transition temperature, measurements and analyses of the spin-lattice and spin-spin relaxation times reveal that the reorientational dynamics of the components, i.e., of glycerol as well as of chain deuterated choline chloride are slightly different. Possible implications of this finding regarding the hydrogen-bonding pattern in glyceline are discussed. Furthermore, the deuterated methyl groups in choline chloride are exploited as sensitive probes of glyceline's supercooled and glassy states. Apart from spin relaxometry, a detailed line shape analysis of the CD₃ spectra yields valuable insights into the broad intermolecular and intramolecular energy barrier distributions present in this binary mixture.

Polymer Electrolytes for Lithium-Ion Batteries Studied by NMR Techniques

This review is devoted to different types of novel polymer electrolytes for lithium power sources developed during the last decade. In the first part, the compositions and conductivity of various polymer electrolytes are considered. The second part contains NMR applications to the ion transport mechanism. Polymer electrolytes prevail over liquid electrolytes because of their exploitation safety and wider working temperature ranges. The gel electrolytes are mainly attractive. The systems based on polyethylene oxide, poly(vinylidene fluoride-co-hexafluoropropylene), poly(ethylene glycol) diacrylate, etc., modified by nanoparticle (TiO₂, SiO₂, etc.) additives and ionic liquids are considered in detail. NMR techniques such as high-resolution NMR, solid-state NMR, magic angle spinning (MAS) NMR, NMR relaxation, and pulsed-field gradient NMR applications are discussed. ¹H, ⁷Li, and ¹⁹F NMR methods applied to polymer electrolytes are considered. Primary attention is given to the revelation of the ion transport mechanism. A nanochannel structure, compositions of ion complexes, and mobilities of cations and anions studied by NMR, quantum-chemical, and ionic conductivity methods are discussed.

Recent advances in NMR spectroscopy of ionic liquids

This review presents recent developments in the application of NMR spectroscopic techniques in the study of ionic liquids. NMR has been the primary tool not only for the structural characterization of ionic liquids, but also for the study of dynamics. The presence of a host of NMR active nuclei in ionic liquids permits widespread use of multinuclear NMR experiments. Chemical shifts and multinuclear coupling constants are used routinely for the structure elucidation of ionic liquids and of products formed by their covalent interactions with other materials. Also, the availability of a multitude of NMR techniques has facilitated the study of dynamical processes in them. These include the use of NOESY to study inter-ionic interactions, pulsed-field gradient techniques for probing transport properties, and relaxation measurements to elucidate rotational dynamics. This review will focus on the application of each of these techniques to investigate ionic liquids.

Synthesis and evolution of physicochemical properties of new pharmaceutically active ionic liquids – tetracainium salicylate and tetracainium ibuprofenate

Tetracainium salicylate and tetracainium ibuprofenate were synthesized as active pharmaceutical ingredient ionic liquids (API-ILs). These ILs represent a combination of a drug for local anaesthesia (tetracaine) and nonsteroidal anti-inflammatory drugs (salicylic acid and ibuprofen). After IL synthesis, spectroscopic investigations were performed using infrared and nuclear magnetic resonance spectroscopy to confirm their structures. Differential scanning calorimetry and thermogravimetric analysis determined the obtained thermal behaviour of the ionic liquids. Experimental density, viscosity, and electrical conductivity measurements were performed in a wide temperature range to understand the interactions occurring in the obtained pharmaceutically active ionic liquids. All experimental values were well-fitted by the empirical equations. According to the theoretical calculations, weaker interactions of tetracaine with ibuprofenate than with salicylate are found, ascribed to the decreasing molecular symmetry, weakened hydrogen bonding, and increasing steric hindrance of ibuprofenate's alkyl chain.

Evolution of physicochemical parameters such as thermal stability, density, viscosity and electrical conductivity at various temperatures for two pharmaceutically active ionic liquids – tetracainium salicylate and tetracainium ibuprofenate.

Can the microscopic and macroscopic transport phenomena in deep eutectic solvents be reconciled?

Deep eutectic solvents (DESs) have become ubiquitous in a variety of industrial and pharmaceutical applications since their discovery. However, the fundamental understanding of their physicochemical properties and their emergence from the microscopic features is still being explored fervently. Particularly, the knowledge of transport mechanisms in DESs is essential to tune their properties, which shall aid in expanding the territory of their applications. This perspective presents the current state of understanding of the bulk/macroscopic transport properties and microscopic relaxation processes in DESs. The dependence of these properties on the components and composition of the DES is explored, highlighting the role of hydrogen bonding (H-bonding) interactions. Modulation of these interactions by water and other additives, and their subsequent effect on the transport mechanisms, is also discussed. Various models (e.g. hole theory, free volume theory, etc.) have been proposed to explain the macroscopic transport phenomena from a microscopic origin. But the formation of H-bond networks and clusters in the DES reveals the insufficiency of these models, and establishes an antecedent for dynamic heterogeneity. Even significantly above the glass transition, the microscopic relaxation processes in DESs are rife with temporal and spatial heterogeneity, which causes a substantial decoupling between the viscosity and microscopic diffusion processes. However, we propose that a thorough understanding of the structural relaxation associated to the H-bond dynamics in DESs will provide the necessary framework to interpret the emergence of bulk transport properties from their microscopic counterparts.

Breakdown of the Stokes-Einstein relation in supercooled water: the jump-diffusion perspective

Although water is the most ubiquitous liquid it shows many thermodynamic and dynamic anomalies. Some of the anomalies further intensify in the supercooled regime. While many experimental and theoretical studies have focused on the thermodynamic anomalies of supercooled water, fewer studies explored the dynamical anomalies very extensively. This is due to the intricacy of the experimental measurement of the dynamical properties of supercooled water. Violation of the Stokes-Einstein relation (SER), an important relation connecting the diffusion of particles with the viscosity of the medium, is one of the major dynamical anomalies. In absence of experimentally measured viscosity, researchers used to check the validity of SER indirectly using average translational relaxation time or α-relaxation time. Very recently, the viscosity of supercooled water was accurately measured at a wide range of temperatures and pressures. This allowed direct verification of the SER at different temperature-pressure thermodynamic state points. An increasing breakdown of the SER was observed with decreasing temperature. Increasing pressure reduces the extent of breakdown. Although some well-known theories explained the above breakdown, a detailed molecular mechanism was still elusive. Recently, a translational jump-diffusion (TJD) approach has been able to quantitatively explain the breakdown of the SER in pure supercooled water and an aqueous solution of methanol. The objective of this article is to present a detailed and state-of-the-art analysis of the past and present works on the breakdown of SER in supercooled water with a specific focus on the new TJD approach for explaining the breakdown of the SER.

Viscosity Investigations on the Binary Systems of (1 ChCl:2 Ethylene Glycol) DES and Methanol or Ethanol

In this study, the viscosity behavior of two mixtures of Ethaline (1 ChCl:2 ethylene glycol) with either methanol or ethanol were investigated over the temperature range of 283.15–333.15 K at atmospheric pressure. The measured viscosities of neat Ethaline, methanol, and ethanol showed reliable agreement with the corresponding reported literature values. The mixture viscosities were modeled by an Arrhenius-like model to determine the behavior of viscosity with respect to temperature. The data were also modeled by the four well-known mixture viscosity models of Grunberg–Nissan, Jouyban–Acree, McAllister, and Preferential Solvation. All of the model results were reliable, with the Jouyban–Acree and Preferential Solvation models showing the most accurate agreement with the experimental measurements. The Jones–Dole viscosity model was also investigated for the measured viscosities, and by analyzing the results of this model, strong interactions among Ethaline and the alcohol molecules were proposed for both systems. As a final analysis, viscosity deviations of the investigated systems were calculated to study the deviations of the viscosity behaviors with respect to ideal behavior. Both systems showed negative viscosity deviations at all of the investigated temperatures, with the negative values tending towards zero, and hence more ideal behavior, with increasing temperatures. Moreover, in order to correlate the calculated viscosity deviations, the Redlich–Kister model was successfully used for both systems and at each investigated temperature.

Design and analysis of interactions in ionic liquids based on procaine and pharmaceutically active anions

The present work focuses on modifying a local anaesthetic drug procaine into an ionic liquid and evaluating the resulting thermal behaviour and structural changes. Counter ions, salicylate, ibuprofenate, and docusate, were chosen due to different hydrogen-bonding abilities, molecular size, charge distribution, and functional groups. After synthesis of procaine salicylate, procaine ibuprofenate, and procaine docusate, spectroscopic investigations were performed using infrared (IR) and nuclear magnetic resonance (NMR) spectroscopy to confirm proton transfer. Differential scanning calorimetry (DSC) and thermogravimetric (TG) analysis were used to determine the obtained ionic liquids' thermal behaviour. Experimental measurements of density, viscosity, and electrical conductivity were performed to get insight into the interactions occurring in the obtained ionic liquids. The viscosity and electrical conductivity data were analysed using the Vogel-Fulcher-Tammann (VFT) equation, while thermal expansion coefficients were calculated from measured density data. The obtained results found that the synthesised procaine salicylate and procaine docusate an ionic liquid's behaviours, including weak intermolecular forces, while procaine ibuprofenate showed more liquid co-crystal behaviour due to the absence of proton transfer for ibuprofen. In a theoretical phase of the investigation, the density functional theory (DFT) and molecular dynamics (MD) calculations were conducted. The obtained descriptors and radial distribution functions were used to analyse the interactions between ions of synthesised ionic liquids. In addition, solubility determination results proved that procaine transformation into procaine salicylate and procaine ibuprofenate ionic liquids enhanced its solubility in water, while procaine docusate reduces procaine solubility.

Modulation of Gastrointestinal Mucus Properties with Ionic Liquids for Drug Delivery

The mucus barrier lining the gastrointestinal tract poses a significant barrier to the oral delivery of macromolecular drugs. Successful approaches to overcoming this barrier have primarily focused on reducing drug and carrier interactions with mucus or disrupting the mucus layer directly. Choline-based ionic liquids (ILs) such as choline geranate and choline glycolate (CGLY) have recently been shown to be effective in enhancing the intestinal absorption of macromolecules such as insulin and immunoglobulin (IgG), respectively. Herein, the use of choline-based ILs as mucus-modulating agents for safely improving drug penetration through mucus is described. Choline-based ILs significantly increase the diffusion rates of cationic dextrans through mucin solution. Choline-maleic acid (CMLC 2:1) enhances the diffusion of 4 kDa cationic dextran in mucin solution by more than fourfold when compared to phosphate-buffered saline control. Choline-based ILs also reduce mucus viscosity without significantly impacting the native mucus gel structure. In vitro studies in a mucus-secreting coculture model with Caco-2 and HT29MTX-E12 cells further demonstrate the effectiveness of ILs in improving transport of cationic molecules in the presence of secreted mucus. This work demonstrates the potential for choline-based ionic liquids to be used as nondestructive mucus-modulating agents for enabling enhanced oral delivery of macromolecular drugs.