Targeted modifications in ionic liquids - from understanding to design

Planting the Seeds of a Decision Tree for Ionic Liquids: Steric and Electronic Impacts on Melting Points of Triarylphosponium Ionic Liquids

While machine learning and artificial intelligence offer promising avenues in the computer-aided design of materials, the complexity of these computational techniques remains a barrier for scientists outside of the specific fields of study. Leveraging decision tree models, inspired by empirical methodologies, offers a pragmatic solution to the knowledge barrier presented by artificial intelligence (AI). Herein, we present a model allowing for the qualitative prediction of melting points of ionic liquids derived from the crystallographic analysis of a series of phosphonium-based ionic liquids. By carefully tailoring the steric and electronic properties of the cations within these salts, trends in the melting points are observed, pointing toward the critical importance of π interactions to forming the solid state. Quantification of the percentage of these π interactions using modern quantum crystallographic approaches reveals a linear trend in the relationship of C-Hπ and π-π stacking interactions with melting points. These structure-property relationships are further examined by using computational studies, helping to demonstrate the inverse relationship of dipole moments and melting points for ionic liquids. The results provide valuable insights into the features and relationships that are consistent with achieving low Tm values in phosphonium salts, which were not apparent in earlier studies. The data gathered are presented in a simple decision tree format, allowing for visualization of the data and providing guidance toward developing yet unreported compounds.

Evolving better solvate electrolytes for lithium secondary batteries

The overall performance of lithium batteries remains unmatched to this date. Decades of optimisation have resulted in long-lasting batteries with high energy density suitable for mobile applications. However, the electrolytes used at present suffer from low lithium transference numbers, which induces concentration polarisation and reduces efficiency of charging and discharging. Here we show how targeted modifications can be used to systematically evolve anion structural motifs which can yield electrolytes with high transference numbers. Using a multidisciplinary combination of theoretical and experimental approaches, we screened a large number of anions. Thus, we identified anions which reach lithium transference numbers around 0.9, surpassing conventional electrolytes. Specifically, we find that nitrile groups have a coordination tendency similar to SO2 and are capable of inducing the formation of Li+ rich clusters. In the bigger picture, we identified a balanced anion/solvent coordination tendency as one of the key design parameters.

Experiences with an Inquiry-Based Ionic Liquid Module in an Undergraduate Physical Chemistry Laboratory

The topic of ionic liquids is typically not taught at the undergraduate level. Many properties, such as conductivity, vapor pressure, and viscosity, of these so-called "green solvents" are unique compared to traditional molecular solvents. Using active learning techniques, we introduced an ionic liquid module in the physical chemistry laboratory where their structures and physical properties, namely, viscosity, conductivity, and vapor pressure, were explored in relation to molecular solvents. Summative and formative assessments show that a majority of the participants were able to grasp the key concepts of ionic liquids. We envision that our methods and strategies can be one of the building blocks of introducing ionic liquids into the undergraduate chemistry curriculum.

Thermodynamic Study of Alkylsilane and Alkylsiloxane-Based Ionic Liquids

The thermodynamic properties of ionic liquids (ILs) bearing alkylsilane and alkylsiloxane chains, as well as their carbon-based analogs, were investigated. Effects such as the replacement of carbon atoms by silicon atoms, the introduction of a siloxane linkage, and the length of the alkylsilane chain were explored. Differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) were used to study the thermal and phase behavior (glass transition temperature, melting point, enthalpy and entropy of fusion, and thermal stability). Heat capacity was obtained by high-precision drop calorimetry and differential scanning microcalorimetry. The volatility and cohesive energy of these ILs were investigated via the Knudsen effusion method coupled with a quartz crystal microbalance (KEQCM). Gas phase energetics and structure were also studied to obtain the gas phase heat capacity as well as the energy profile associated with the rotation of the IL side chain. The computational study suggested the existence of an intramolecular interaction in the alkylsiloxane-based IL. The obtained glass transition temperatures seem to follow the trend of chain flexibility. An increase of the alkylsilane chain leads to a seemingly linear increase in molar heat capacity. A regular increment of 30 J·K-1·mol-1 in the molar heat capacity was found for the replacement of carbon by silicon in the IL alkyl chain. The alkylsilane series was revealed to be slightly more volatile than its carbon-based analogs. A further increase in volatility was found for the alkylsiloxane-based IL, which is likely related to the decrease of the cohesive energy due to the existence of an intramolecular interaction between the siloxane linkage and the imidazolium headgroup. The use of Si in the IL structure is a suitable way to significantly reduce the IL's viscosity while preserving its large liquid range (low melting point and high thermal stability) and low volatilities.

BF3 Complexing Phosphate/Phosphonate Ionic Liquids: Synthesis, Characterization and Thermophysical Properties

A new group of BF3 complexing phosphate/phosphonate ionic liquids (ILs) [Emim][X(BF3)2] (X=dimethyl phosphate, diethyl phosphate, methyl phosphonate, and ethyl phosphonate) were synthesized and characterized. Key thermophysical properties of the new complex ionic liquids, including density, viscosity, conductivity, surface tension, solid-liquid phase transition, and thermal stability were determined and compared with those of [Emim][X]. Some other important thermophysical properties such as isobaric thermal expansion coefficient, molecular volume, standard molar entropy, and lattice potential energy were obtained from measured density data, and the free volume was estimated by a linear equation presented in this article, while critical temperature, normal boiling temperature, and enthalpy of vaporization were estimated from measured surface tension and density data. Furthermore, Fragility study shows that [Emim][X(BF3)2] should be considered as fragile liquids, while [Emim][X] could be considered as extremely fragile liquids. The ionicity of [Emim][X(BF3)2] was predicted by Walden rule, and the result shows that these ILs fit well with Walden law. The key features of these complex ILs are their extremely low glass transition (-95.33~-98.46 °C) without melting, considerably low viscosities (33.876~58.117 mPa ⋅ s), and high values of free volume fraction (comparable to [Omim][BF4], [Emim][NTf2], and [Emim][TCB]).

Are nature's strategies the solutions to the rational design of low-melting, lipophilic ionic liquids?

Ionic liquids (ILs) have emerged as a new class of materials, displaying a unique capability to self-assemble into micelles, liposomes, liquid crystals, and microemulsions. Despite evident interest, advancements in the controlled formation of amphiphilic ILs remain in the early stages. Taking inspiration from nature, we introduced the concept of lipid-like (or lipid-inspired) ILs more than a decade ago, aiming to create very low-melting, highly lipophilic ILs that are potentially bio-innocuous - a combination of attributes that is frequently antithetical but highly desirable from several application-specific standpoints. Lipid-like ILs are a subclass of functional organic liquid salts that include a range of lipidic side chains such as saturated, unsaturated, linear, branched, and thioether while retaining melting points below room temperature. It was observed in several homologous series of [Cnmim] ILs that elongation of N-appended alkyl chains to greater than seven carbons leads to a substantial increase in melting point (Tm) - which is the most characteristic feature of ILs. Accordingly, it is challenging to develop ILs with low Tm values while preserving their hydrophobicity and self-organizing properties. We found that two alternative Tm depressive approaches are useful. One of these is the replacement of the double bonds with thioether moieties in the alkyl chains, as detailed in several published papers detailing the chemistry of these ILs. Employing thiol-ene and thiol-yne click reactions is a facile, robust, and orthogonal method to overcome the challenges associated with the synthesis of alkyl thioether-functionalized ILs. The second approach involves replacing the double bonds with the cisoid cyclopropyl motif, mimicking the strategy used by certain organisms to modulate cell membrane fluidity. This discovery has the potential to greatly impact the utilization of lipid-like ILs in various applications, including gene delivery, lubricants, heat transfer fluids, and haloalkane separations, among others. This feature article presents a concise, historical overview, highlighting key findings from our work while offering speculation about the future trajectory of this de novo class of soft organic-ion materials.

Understanding the effects of targeted modifications on the 1 : 2 Choline And GEranate structure

1 : 2 Choline-and-geranate (CAGE) is an ionic liquid (IL) widely studied for its biomedical applications. However, both its industrial-scale preparation and its long-term storage are problematic so finding more suitable candidates which retain its advantageous properties is crucial. As a first step towards this we have conducted a targeted modification study to understand the effects of specific functional groups on the properties of CAGE. 1 : 2 Choline-and-octanoate and 1 : 2 butyltrimethylammonium-and-octanoate were synthesised and their thermal and rheological properties examined in comparison to those of CAGE. Using differential scanning calorimetry and polarising microscopy, the model compound was found to be an isotropic liquid, while the analogues were room-temperature liquid-crystals which transition to isotropic liquids upon heating. Dynamic mechanical analysis showed that the thermal behaviour of the studied systems was even more complex, with the ILs also undergoing a thermally-activated relaxation process. Furthermore, we have used electron paramagnetic resonance (EPR) spectroscopy, along with a variety of spin probes with different functional groups, in order to understand the chemical environment experienced by solutes in each system. The EPR spectra indicate that the radicals experience two distinct environments (polar and nonpolar) in the liquid-crystalline phase, but only one average environment in the isotropic phase. The liquid-crystalline phase experiments also showed that the relative populations of the two domains depend on the nature of the solutes, with polar or strongly hydrogen-bonding solutes preferring the polar domain. For charged solutes, the EPR spectra showed line-broadening, suggesting that their ionic nature leads to complex, unresolved interactions.

Carbon-Induced Changes in the Morphology and Wetting Behavior of Ionic Liquids on the Mesoscale

Thin films of ionic liquids (ILs) have gained significant attention due to their unique properties and broad applications. Extensive research has focused on studying the influence of ILs' chemical composition and substrate characteristics on the structure and morphology of IL films at the nano- and mesoscopic scales. This study explores the impact of carbon-coated surfaces on the morphology and wetting behavior of a series of alkylimidazolium-based ILs. Specifically, this work investigates the effect of carbon coating on the morphology and wetting behavior of short-chain ([C2C1im][NTf2] and [C2C1im][OTf]) and long-chain ([C8C1im][NTf2] and [C8C1im][OTf]) ILs deposited on indium tin oxide (ITO), silver (Ag), and gold (Au) substrates. A reproducible vapor deposition methodology was utilized for the deposition process. High-resolution scanning electron microscopy, atomic force microscopy, and X-ray photoelectron spectroscopy were used to analyze the morphological and structural characteristics of the substrates and obtained IL films. The experimental data revealed that the IL films deposited on carbon-coated Au substrates showed minor changes in their morphology compared to that of the films deposited on clean Au surfaces. However, the presence of carbon coatings on the ITO and Ag surfaces led to significant morphological alterations in the IL films. Specifically, for short-chain ILs, the carbon film surface induced 2D growth of the IL film, followed by subsequent island growth. In contrast, for long-chain ILs deposited on carbon surfaces, layer-by-layer growth occurred without island formation, resulting in highly uniform and coalesced IL films. The extent of morphological changes observed in the IL films was found to be influenced by two crucial factors: the thickness of the carbon film on the substrate surface and the amount of IL deposition.

Synthesis, Characterization, and Physicochemical Properties of New [Emim][BF3X] Complex Anion Ionic Liquids

A new series of complex anion ionic liquids (ILs) [Emim][BF3X] (X = CH3SO3, EtSO4, HSO4, Tosylate) were synthesized and characterized by nuclear magnetic resonance, elemental analysis, differential scanning calorimetry analysis, and thermogravimetry. The physicochemical properties of these ILs, such as density, viscosity, conductivity, and surface tension, were measured and correlated with thermodynamic and empirical equations in the temperature range of 293.15-358.15 K under ambient conditions, and the thermal expansion coefficient, standard molar entropy, lattice potential energy, viscosity activation energy, surface enthalpy, and surface entropy were further calculated from experimental values. According to the temperature-dependent viscosity and conductivity, [Emim][BF3X] ILs follow the Walden rule, and they are classified as "good (or super) ionic liquids".

High-concentration LiPF6/sulfone electrolytes: structure, transport properties, and battery application

Non-flammable and oxidatively stable sulfones are promising electrolyte solvents for thermally stable high-voltage Li batteries. In addition, sulfolane-based high-concentration electrolytes (HCEs) show high Li+ ion transference numbers. However, LiPF6 has not yet been investigated as the main salt in sulfone-based HCEs for Li batteries. In this study, we investigated the phase behaviors, solvate structures, and transport properties of binary and ternary mixtures of LiPF6 and the following sulfone solvents: sulfolane (SL), dimethyl sulfone (DMS), ethyl methyl sulfone (EMS), and 3-methyl sulfolane (MSL). The stable crystalline solvates Li(SL)4PF6 and Li(DMS)2.5PF6 with high melting points were formed in the LiPF6/SL and LiPF6/DMS mixtures, respectively. In contrast, LiPF6/EMS, LiPF6/MSL, and LiPF6/SL/another sulfone mixtures remained liquids over a wide temperature range. Raman spectroscopy revealed that SL and another sulfone are competitively coordinated to Li+ ions to dissociate LiPF6 in the ternary mixtures. Although the ionic conductivity decreased with increasing LiPF6 concentration due to an increase in viscosity, Li+ ions diffused faster than PF6-via exchanging ligands in the HCE [LiPF6]/[SL]/[DMS] = 1/2/2, resulting in a higher Li ion transference number than that in conventional Li battery electrolytes.

Ionic Liquids: New Forms of Active Pharmaceutical Ingredients with Unique, Tunable Properties

This Review aims to summarize advances over the last 15 years in the development of active pharmaceutical ingredient ionic liquids (API-ILs), which make up a prospective game-changing strategy to overcome multiple problems with conventional solid-state drugs, for example, polymorphism. A critical part of the present Review is the collection of API-ILs and deep eutectic solvents (DESs) prepared to date. The Review covers rules for rational design of API-ILs and tools for API-IL formation, syntheses, and characterization. Nomenclature and ionic speciation, and the confusion that these may cause, are highlighted, particularly for speciation in both ILs and DESs of intermediate ionicity. We also highlight in vivo and in vitro pharmaceutical activity studies, with differences in pharmacokinetic/pharmacodynamic depending on ionicity of API-ILs. A brief overview is provided for the ILs used to deliver drugs, and the Review concludes with key prospects and roadblocks in translating API-ILs into pharmaceutical manufacturing.

Alkylcyanoborate Anions: Building Blocks for Fluorine-Free Low-Viscosity, Electrochemically and Thermally Stable Ionic Liquids

A set of mixed-substituted potassium alkylcyano- and alkylcyanofluoroborates has been synthesized using easily accessible starting compounds and characterized by elemental analysis, NMR and vibrational spectroscopy, and mass spectrometry. In addition, single-crystal structures of salts of the cyanoborate anions have been derived from X-ray diffraction experiments. The 1-ethyl-3-methylimidazolium room temperature ionic liquids ([EMIm]+ -RTILs) with the new borate anions have been synthesized and their physicochemical properties, that is, high thermal and electrochemical stability, low viscosity, and high conductivity, have been compared to the properties of related [EMIm]+ -RTILs. The influence of the different alkyl substituents at boron has been assessed. The exemplary study on the properties with the [EMIm]+ -ILs with the mixed water stable alkylcyanoborate anions points towards the potential of these fluorine-free borate anions, in general.

Investigating the Inhibitory Properties of Cupressus sempervirens Extract against Copper Corrosion in 0.5 M H2SO4: Combining Quantum (Density Functional Theory Calculation-Monte Carlo Simulation) and Electrochemical-Surface Studies

The study investigates the potential of Cupressus sempervirens (EO) as a sustainable and eco-friendly inhibitor of copper corrosion in a 0.5 M sulfuric acid medium. The electrochemical impedance spectroscopy analysis shows that the effectiveness of corrosion inhibition rises with increasing inhibitor concentrations, reaching 94% with the application of 2 g/L of EO, and potentiodynamic polarization (PDP) studies reveal that EO functions as a mixed-type corrosion inhibitor. In addition, the Langmuir adsorption isotherm is an effective descriptor of its adsorption. Scanning electron microscopy/energy-dispersive X-ray spectroscopy, atomic force microscopy surface examination, and contact angle measurement indicate that EO may form a barrier layer on the metal surface. Density functional theory calculations, Monte Carlo simulation models, and the radial distribution function were also used to provide a more detailed understanding of the corrosion protection mechanism. Overall, the findings suggest that Cupressus sempervirens (EO) has the potential to serve as an effective and sustainable corrosion inhibitor for copper in a sulfuric acid medium, contributing to the development of green corrosion inhibitors for environmentally friendly industrial processes.

Structure-Property Relationships of CO2 Absorbing Core-Shell Microparticles with Encapsulated Ionic Liquid

The demand for new ionic liquid (IL)-based systems to selectively sequester carbon dioxide from gas mixtures has prompted the development of individual components involving the tailored design of IL themselves or solid-supported materials that provide excellent gas permeability of the overall material as well as the ability to incorporate large amounts of ionic liquid. In this work, novel IL-encapsulated microparticles comprising a cross-linked copolymer shell of β-myrcene and styrene and a hydrophilic core of the ionic liquid 1-ethyl-3-methylimidazolium dicyanamide ([EMIM][DCA]) are proposed as viable materials for CO2 capture. Water-in-oil (w/o) emulsion polymerization of different mass ratios of β-myrcene to styrene (i.e. 100/0, 70/30, 50/50, 0/100) yielded IL-encapsulated microparticles, where the encapsulation efficiency of [EMIM][DCA] was dependent on the copolymer shell composition. Thermal analysis using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) revealed that both thermal stability and glass transition temperatures depend on the mass ratio of β-myrcene to styrene. Images from scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the microparticle shell morphology as well as measure the particle size perimeter. Particle sizes were found to be between 5 and 44 μm. CO2 sorption experiments were conducted gravimetrically using TGA instrumentation. Interestingly, a trade-off between CO2 absorption capacity and ionic liquid encapsulation was observed. While increasing the β-myrcene content within the microparticle shell increases the amount of encapsulated [EMIM][DCA], the observed CO2 absorption capacity did not increase as expected due to reduced porosity compared to microparticles with higher styrene content in the microparticle shell. [EMIM][DCA] microcapsules with a 50/50 weight ratio of β-myrcene/styrene showed the best synergistic effect between spherical particle diameter (32.2 μm), pore size (0.75 μm), and high CO2 sorption capacity of ∼0.5 mmol CO2/g sample within a short absorption period of 20 min. Therefore, core-shell microcapsules composed of β-myrcene and styrene are envisioned as a promising material for CO2 sequestration applications.

Dynamics in Quaternary Ionic Liquids with Non-Flexible Anions: Insights from Mechanical Spectroscopy

The present work investigates how mechanical properties and ion dynamics in ionic liquids (ILs) can be affected by ILs' design while considering possible relationships between different mechanical and transport properties. Specifically, we study mechanical properties of quaternary ionic liquids with rigid anions by means of Dynamical Mechanical Analysis (DMA). We are able to relate the DMA results to the rheological and transport properties provided by viscosity, conductivity, and diffusion coefficient measurements. A good agreement is found in the temperature dependence of different variables described by the Vogel-Fulcher-Tammann model. In particular, the mechanical spectra of all the measured liquids showed the occurrence of a relaxation, for which the analysis suggested its attribution to a diffusive process, which becomes evident when the ion dynamics are not affected by the fast structural reorganization of flexible anions on a local level.

Quantitative Relation between Ionic Diffusivity and Ionic Association in Ionic Liquid-Water Mixtures

Molecular dynamic simulations of aqueous mixtures of imidazolium ionic liquids (ILs) were performed to elucidate the dependence of the ionic diffusivity on the microscopic structures changed by water. Two distinct regimes of the average ionic diffusivity (D_ave) were identified with the increased water concentrations: the jam regime with slowly increased D_ave and the exponential regime with rapidly increased D_ave, which are found to be directly correlated to the ionic association. Further analysis leads to two general relationships independent of IL species between D_ave and the degree of ionic association: (i) a consistent linear relationship between D_ave and the inverse of ion-pair lifetimes (1/τ_IP) in the two regimes and (ii) an exponential relationship between normalized diffusivities (D̃_ave) and short-ranged interactions between cations and anions (Ẽ_ions), with different interdependent strengths in the two regimes. These findings revealed and quantified the direct correlation between dynamic properties and ionic association in IL-water mixtures.

Recent Advances in the Preparation of Delivery Systems for the Controlled Release of Scents

Scents are volatile compounds highly employed in a wide range of manufactured items, such as fine perfumery, household products, and functional foods. One of the main directions of the research in this area aims to enhance the longevity of scents by designing efficient delivery systems to control the release rate of these volatile molecules and also increase their stability. Several approaches to release scents in a controlled manner have been developed in recent years. Thus, different controlled release systems have been prepared, including polymers, metal-organic frameworks and mechanically interlocked systems, among others. This review is focused on the preparation of different scaffolds to accomplish a slow release of scents, by pointing out examples reported in the last five years. In addition to discuss selected examples, a critical perspective on the state of the art of this research field is provided, comparing the different types of scent delivery systems.

Ambient temperature liquid salt electrolytes

Alkali metal salts usually have high melting points due to strong electrostatic interactions and solvents are needed to create ambient temperature liquid electrolytes. Here, we report on six phosphate-anion-based alkali metal salts, Li/Na/K, all of which are liquids at room temperature, with glass transition temperatures ranging from -61 to -29 °C, and are thermally stable up to at least 225 °C. While the focus herein is on various physico-chemical properties, these salts also exhibit high anodic stabilities, up to 6 V vs. M/M⁺ (M = Li/Na/K), and deliver some battery performance - at elevated temperatures as there are severe viscosity limitations at room-temperature. While the battery performance arguably is sub-par, solvent-free electrolytes based on alkali metal salts such as these should pave the way for conceptually different Li/Na/K-batteries, either by refined anion design or by using several salts to create eutectic mixtures.

Insights into structure-property relationships in ionic liquids using cyclic perfluoroalkylsulfonylimides

Room temperature ionic liquids of cyclic sulfonimide anions ncPFSI (ring size: n = 4-6) with the cations [EMIm]⁺ (1-ethyl-3-methylimidazolium), [BMIm]⁺ (1-butyl-3-methylimidazolium) and [BMPL]⁺ (BMPL = 1-butyl-1-methylpyrrolidinium) have been synthesized. Their solid-state structures have been elucidated by single-crystal X-ray diffraction and their physicochemical properties (thermal behaviour and stability, dynamic viscosity and specific conductivity) have been assessed. In addition, the ion diffusion was studied by pulsed field gradient stimulated echo (PFGSTE) NMR spectroscopy. The decisive influence of the ring size of the cyclic sulfonimide anions on the physicochemical properties of the ILs has been revealed. All ILs show different properties compared to those of the non-cyclic TFSI anion. While these differences are especially distinct for ILs with the very rigid 6cPFSI anion, the 5-membered ring anion 5cPFSI was found to result in ILs with relatively similar properties. The difference between the properties of the TFSI anion and the cyclic sulfonimide anions has been rationalized by the rigidity (conformational lock) of the cyclic sulfonimide anions. The comparison of selected IL properties was augmented by MD simulations. These highlight the importance of π⁺-π⁺ interactions between pairs of [EMIm]⁺ cations in the liquid phase. The π⁺-π⁺ interactions are evident for the solid state from the molecular structures of the [EMIm]⁺-ILs with the three cyclic imide anions determined by single-crystal X-ray diffraction.

Anion and ether group influence in protic guanidinium ionic liquids

Ionic liquids are attractive liquid materials for many advanced applications. For targeted design, in-depth knowledge about their structure-property-relations is urgently needed. We prepared a set of novel protic ionic liquids (PILs) with a guanidinium cation with either an ether or alkyl side chain and different anions. While being a promising cation class, the available data is insufficient to guide design. We measured thermal and transport properties, nuclear magnetic resonance (NMR) spectra as well as liquid and crystalline structures supported by ab initio computations and were able to obtain a detailed insight into the influence of the anion and the ether substitution on the physical and spectroscopic properties. For the PILs, hydrogen bonding is the main interaction between cation and anion and the H-bond strength is inversely related to the proton affinity of the constituting acid and correlated to the increase of ¹H and ¹⁵N chemical shifts. Using anions from acids with lower proton affinity leads to proton localization on the cation as evident from NMR spectra and self-diffusion coefficients. In contrast, proton exchange was evident in ionic liquids with triflate and trifluoroacetate anions. Using imide-type anions and ether side groups decreases glass transitions as well as fragility, and accelerated dynamics significantly. In case of the ether guanidinium ionic liquids, the conformation of the side chain adopts a curled structure as the result of dispersion interactions, while the alkyl chains prefer a linear arrangement.

Cyclopropane as an Unsaturation "Effect Isostere": Lowering the Melting Points in Lipid-like Ionic Liquids

The replacement of unsaturation with a cyclopropane motif as a (bio)isostere is a widespread strategy in bacteria to tune the fluidity of lipid bilayers and protect membranes when exposed to adverse environmental conditions, e.g., high temperature, low pH, etc. Inspired by this phenomenon, we herein address the relative effect of the cyclopropanation, both cis and trans configurations, on melting points, packing efficiency, and order of a series of lipid-like ionic liquids via a combination of thermophysical analysis, X-ray crystallography, and computational modeling. The data indicate there is considerable structural latitude possible when designing highly lipophilic ionic liquids that exhibit low melting points. While cyclopropanation of the lipid-like ionic liquids provides more resistance to aerobic degradation than their olefin analogs, the impact on the melting point decrease is not as pronounced. Our results demonstrate that incorporating one or more cyclopropyl moieties in long aliphatic chains of imidazolium-based ionic liquids is highly effective in lowering the melting points of such materials relative to their counterparts bearing linear, saturated, or thioether side chains. It is shown that the cyclopropane moiety effectively disrupts packing, favoring formation of gauche conformer in the side chains, resulting in enhancement of fluidity. This was irrespective of the configuration of the methylene bridge, although marked differences in the effect of cis- and trans-monocyclopropanated ILs on the melting points were observed.

A phase variety of fluorinated ionic liquids: Molecular conformational and crystal polymorph

Crystal polymorphs of fluorinated ionic liquids (fILs) were examined at low-temperature (LT) by Raman spectroscopy. The fILs were 1-alkyl-3-methylimidazolium perfluorobutanesulfonate, [C_nmim][PFBS] (n = 4, 6, and 8). The cations and anion possess conformational degrees of freedom. Various LT phases were derived from the conformational polymorphs of the cations and the anion. Conformational flexibility depended on alkyl chain length. The crystal polymorphs in the fILs were sensitive to molecular conformations and flexibility.

Bridging the crystal and solution structure of a series of lipid-inspired ionic liquids

A series of 1,2-dimethylimidazolium ionic liquids bearing a hexadecyl alkyl chain are thoroughly examined via X-ray crystallography. The crystal structures reveal several key variations in the non-covalent interactions in the lipid-like salts. Specifically, distinct cation-cation π interactions are observed when comparing the bromide and iodide structures. Changing the anion to bis(trifluoromethane)sulfonimide (Tf₂N^-) changes these cation-cation π interactions with anion⋯π interactions. Additionally, several well-defined geometries of the cations are noted based on torsion and core-plane angles of the alkyl chains. Hirshfeld surface analysis is used to distinguish the interactions and geometries in the solid state, helping to reveal characteristic structural fingerprints for the compounds. The solid-state structures of the ionic liquids are correlated with the solution-state structures through UV-vis spectroscopic studies, further emphasizing the importance of the π interactions in the formation of aggregates. Finally, we investigated the thermal properties of the ionic liquids, revealing complex phase transitions for the iodide-containing species. These phase transitions are further rationalized via the analysis of the data gathered from the structures of the other crystallized salts.

Tracing the Effect of Replacing [Gly]- with [Ala]- and Hydroxylation of [emim]+ on the Fine-Tuning of the Transport Properties of the Corresponding Amino Acid-Based Ionic Liquids Using MD Simulation

Natural amino acid-based ionic liquids (AAILs) composed of deprotonated amino acids, [AA]^-, as anions and hydroxylated imidazolium cations provide an eco-friendly nontoxic IL family with the growing number of chemical and biochemical revolutionary applications. In this paper, the transport properties of four AAILs composed of 1-(2-hydroxyethyl)-3-methylimidazolium ([HOemim]⁺) and 1-ethyl-3-methylimidazolium ([emim]⁺) cations with alaninate and glycinate anions were studied by molecular dynamics (MD) simulations. A nonpolarizable all-atom force field with the scaled charge (±0.8e) on each of the ions was applied and compared with the unit charge model in some cases. The tunable effects of the presence of the hydroxyl group in the side chain of the imidazolium cation, the type of amino acid anion, and the varied temperature on the dynamical behavior of AAILs were investigated in detail. The experimentally compatible trends of the simulated ionic self-diffusion coefficients, ionic conductivity, and ionicity were found to be inverse to the viscosity and ionic association of these ILs as [emim][Gly] > [emim][Ala] > [HOemim][Gly] > [HOemim][Ala]. The main reason behind these trends is the higher ability of the hydroxylated cation for the hydrogen-bond formation with [AA]^-. The mean square displacement (MSD), self-diffusion, and transference number of imidazolium cations are larger than those of [AA]^- anions, except in the case of [HOemim][Gly]. It was found that the activation energy for diffusion of [AA]^- is lower than that of [HOemim]⁺ but higher than that of [emim]⁺ in [HOemim][AA] and [emim][AA] ILs, respectively. The computed velocity autocorrelation function (VACF) showed that [Gly]^-, as the lightest ion, has the shortest mean collision time and velocity randomization time among the ions, especially in the [HOemim][Gly] IL. Replacing [emim]⁺ with [HOemim]⁺, similar to the effect of decreasing temperature, causes significant decreasing of the ionic self-diffusion and increasing of the well depth of the first minimum of the ionic VACFs. Current findings show that introducing suitable functional groups in the side chain of imidazolium cations can be a viable approach for efficient engineering design and fine-tuning of the transport properties of these AAILs.

Thermal and Electrochemical Properties of Ionic Liquids Bearing Allyl Group with Sulfonate-Based Anions-Application Potential in Epoxy Resin Curing Process

Sulfonate-based ionic liquids (ILs) with allyl-containing cations have been previously obtained by us, however, the present study aims to investigate the thermal, electrochemical and curing properties of these ILs. To determine the temperature range in which ionic liquid maintains a liquid state, thermal properties must be examined using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). Melting, cold crystallization and glass transition temperatures are discussed, as well as decomposition temperatures for imidazolium- and pyridinium-based ionic liquids. The conductivity and electrochemical stability ranges were studied in order to investigate their potential applicability as electrolytes. Finally, the potential of triflate-based ILs as polymerization initiators for epoxy resins was proven.

Anticancer Agents as Design Archetypes: Insights into the Structure-Property Relationships of Ionic Liquids with a Triarylmethyl Moiety

A fundamental challenge underlying the design principles of ionic liquids (ILs) entails a lack of understanding into how tailored properties arise from the molecular framework of the constituent ions. Herein, we present detailed analyses of novel functional ILs containing a triarylmethyl (trityl) motif. Combining an empirically driven molecular design, thermophysical analysis, X-ray crystallography, and computational modeling, we achieved an in-depth understanding of structure-property relationships, establishing a coherent correlation with distinct trends between the thermophysical properties and functional diversity of the compound library. We observe a coherent relationship between melting (T _m) and glass transition (T _g) temperatures and the location and type of chemical modification of the cation. Furthermore, there is an inverse correlation between the simulated dipole moment and the T _m/T _g of the salts. Specifically, chlorination of the ILs both reduces and reorients the dipole moment, a key property controlling intermolecular interactions, thus allowing for control over T _m/T _g values. The observed trends are particularly apparent when comparing the phase transitions and dipole moments, allowing for the development of predictive models. Ultimately, trends in structural features and characterized properties align with established studies in physicochemical relationships for ILs, underpinning the formation and stability of these new lipophilic, low-melting salts.

Molecular Dynamics of Ionic Liquids from Fast-Field Cycling NMR and Molecular Dynamics Simulations

Understanding the connection between the molecular structure of ionic liquids and their properties is of paramount importance for practical applications. However, this connection can only be established if a broad range of physicochemical properties on different length and time scales is already available. Even then, the interpretation of the results often remains ambiguous due to the natural limits of experimental approaches. Here we use fast-field cycling (FFC) to access both translational and rotational dynamics of ionic liquids. These combined with a comprehensive physicochemical characterization and MD simulations provide a toolkit to give insight into the mechanisms of molecular mechanics. The FFC results are consistent with the computer simulation and conventional physicochemical approaches. We show that curling of the side chains around the positively charged cationic core is essential for the properties of ether-functionalized ionic liquids, and we demonstrate that neither geometry nor polarity alone are sufficient to explain the macroscopic properties.

Flexibility is the key to tuning the transport properties of fluorinated imide-based ionic liquids

Ionic liquids are becoming increasingly popular for practical applications such as biomass processing and lithium-ion batteries. However, identifying ionic liquids with optimal properties for specific applications by trial and error is extremely inefficient since there are a vast number of potential candidate ions. Here we combine experimental and computational techniques to determine how the interplay of fluorination, flexibility and mass affects the transport properties of ionic liquids with the popular imide anion. We observe that fluorination and flexibility have a large impact on properties such as viscosity, whereas the influence of mass is negligible. Using targeted modifications, we show that conformational flexibility provides a significant contribution to the success of fluorination as a design element. Contrary to conventional wisdom, fluorination by itself is thus not a guarantor for beneficial properties such as low viscosity.

Imidazolium Ionic Liquids as Designer Solvents Confined in Silica Nanopores

Composite silica xerogels were prepared via acid catalysed sol–gel route using tetraethoxysilan (TEOS) as silica precursor, and 1-butyl-3-methylimidazolium tetrafluoroborate [BMIM][BF₄] or 1-butyl-3-methylimidazolium chloride [BMIM][Cl] ionic liquids, used simultaneously as co-solvents, catalysts and pore templates, at various IL-to-silica ratios. Morphology of the xerogels prepared using the different IL templating agents were investigated using scanning electron microscopy (SEM), nitrogen sorption and small angle neutron scattering (SANS). The thermal behavior of the composites was analyzed by thermal gravimetry, whereas the compositions were checked by infrared spectroscopy and EDX. The differences in the morphology and thermal behavior of the composites due to the different IL additives were revealed.

Surface and Void Space Analysis of the Crystal Structures of Two Lithium Bis(pentafluoroethanesulfonyl)imide Salts

Analysis of two crystal structures of lithium bis(pentafluoroethanesulfonyl)imide is presented. Two orientations of the anion, that is a cis and trans orientation, are observed. Both structures exhibit unique interactions leading to the formation of discrete fluorous domains in the solid-state. A notable difference in the F···F interactions is seen when contrasting the two orientations wherein the trans geometry has a higher percentage of fluorine interactions than the cis orientation. The inclusion of water molecules in one of the structures also leads to the formation of a polar domain formed through a series of cyclical hydrogen bonding rings. The two structures allow for a detailed examination of the bond distances and angles involved in the formation of the two structures. Analysis of the void space in the two structures leads to the observation that the trans conformation exhibits notably higher void space as compared with the cis orientation. Hirshfeld surface analysis is used to help rationalize the interactions leading to unique changes in geometries and structure.

Machine learning investigation of viscosity and ionic conductivity of protic ionic liquids in water mixtures

Ionic liquids (ILs) are well classified as designer solvents based on the ease of tailoring their properties through modifying the chemical structure of the cation and anion. However, while many structure-property relationships have been developed, these generally only identify the most dominant trends. Here, we have used machine learning on existing experimental data to construct robust models to produce meaningful predictions across a broad range of cation and anion chemical structures. Specifically, we used previously collated experimental data for the viscosity and conductivity of protic ILs [T. L. Greaves and C. J. Drummond, Chem. Rev. 115, 11379-11448 (2015)] as the inputs for multiple linear regression and neural network models. These were then used to predict the properties of all 1827 possible cation-anion combinations (excluding the input combinations). These models included the effect of water content of up to 5 wt. %. A selection of ten new protic ILs was then prepared, which validated the usefulness of the models. Overall, this work shows that relatively sparse data can be used productively to predict physicochemical properties of vast arrays of ILs.

What quantum chemical simulations tell us about the infrared spectra, structure and interionic interactions of a bulk ionic liquid

The recently developed efficient protocol to explicit quantum mechanical modeling of the structure and IR spectra of liquids and solutions [Katsyuba et al., J. Phys. Chem. B, 2020, 124, 6664-6670] is applied to ionic liquid 1-ethyl-3-methyl-imidazolium tetrafluoroborate [Emim][BF₄], and its C2-deuterated analog [Emim-d][BF₄]. It is shown that the solvation strongly modifies the frequencies and IR intensities of both cationic and anionic components of the ionic liquids. The main features of the bulk spectra are reproduced by the simulations for cluster ([Emim][BF₄])₈, representing an ion pair solvated by the first solvation shell. The geometry of the cluster closely resembles the solid-state structure of the actual ionic liquid and is characterized by short contacts of all CH moieties of the imidazolium ring with [BF₄]^- anions. Both structural and spectroscopic analyses allow the contacts to be interpreted as hydrogen bonds of approximately equal strength. The enthalpies of these liquid-state H-bonds, estimated with the use of empirical correlations, amount to 1.2-1.5 kcal mol^-1, while the analogous estimates obtained for the gas-phase charged species [Emim][BF₄]₂^- and [Emim]₂[BF₄]⁺ increase to 3.6-3.9 kcal mol^-1.

Effect of the cation structure on the properties of homobaric imidazolium ionic liquids

In this work we investigate the structure-property relationships in a series of alkylimidazolium ionic liquids with almost identical molecular weight. Using a combination of theoretical calculations and experimental measurements, we have shown that re-arranging the alkyl side chain or adding functional groups results in quite distinct features in the resultant ILs. The synthesised ILs, although structurally very similar, cover a wide spectrum of properties ranging from highly fluid, glass forming liquids to high melting point crystalline salts. Theoretical ab initio calculations provide insight on minimum energy orientations for the cations, which then are compared to experimental X-ray crystallography measurements to extract information on hydrogen bonding and to verify our understanding of the studied structures. Molecular dynamics simulations of the simplest (core) ionic liquids are used in order to help us interpret our experimental results and understand better why methylation of C² position of the imidazolium ring results in ILs with such different properties compared to their non-methylated analogues.

Pressing matter: why are ionic liquids so viscous?

Room temperature ionic liquids are considered to have huge potential for practical applications such as batteries. However, their high viscosity presents a significant challenge to their use changing from niche to ubiquitous. The modelling and prediction of viscosity in ionic liquids is the subject of an ongoing debate involving two competing hypotheses: molecular and local mechanisms versus collective and long-range mechanisms. To distinguish between these two theories, we compared an ionic liquid with its uncharged, isoelectronic, isostructural molecular mimic. We measured the viscosity of the molecular mimic at high pressure to emulate the high densities in ionic liquids, which result from the Coulomb interactions in the latter. We were thus able to reveal that the relative contributions of coulombic compaction and the charge network interactions are of similar magnitude. We therefore suggest that the optimisation of the viscosity in room temperature ionic liquids must follow a dual approach.

Protein aggregation and crystallization with ionic liquids: Insights into the influence of solvent properties

Ionic liquids (ILs) have been used in solvents for proteins in many applications, including biotechnology, pharmaceutics, and medicine due to their tunable physicochemical and biological properties. Protein aggregation is often undesirable, and predominantly occurs during bioprocesses, while the aggregation process can be reversible or irreversible and the aggregates formed can be native/non-native and soluble/insoluble. Recent studies have clearly identified key properties of ILs and IL-water mixtures related to protein performance, suggesting the use of the tailorable properties of ILs to inhibit protein aggregation, to promote protein crystallization, and to control protein aggregation pathways. This review discusses the critical properties of IL and IL-water mixtures and presents the latest understanding of the protein aggregation pathways and the development of IL systems that affect or control the protein aggregation process. Through this feature article, we hope to inspire further advances in understanding and new approaches to controlling protein behavior to optimize bioprocesses.

Unravelling free volume in branched-cation ionic liquids based on silicon

The branching of ionic liquid cation sidechains utilizing silicon as the backbone was explored and it was found that this structural feature leads to fluids with remarkably low density and viscosity. The relatively low liquid densities suggest a large free volume in these liquids. Argon solubility was measured using a precise saturation method to probe the relative free volumes. Argon molar solubilities were slightly higher in ionic liquids with alkylsilane and siloxane groups within the cation, compared to carbon-based branched groups. The anion size, however, showed by far the dominant effect on argon solubility. Thermodynamic solvation parameters were derived from the solubility data and the argon solvation environment was modelled utilizing the polarizable CL&Pol force field. Semiquantitative analysis was in agreement with trends established from the experimental data. The results of this investigation demonstrate design principles for targeted ionic liquids when optimisation for the free volume is required, and demonstrate the utility of argon as a simple, noninteracting probe. As more ionic liquids find their way into industrial processes of scale, these findings are important for their utilisation in the capture of any gaseous solute, gas separation, or in processes involving the transformation of gases or small molecules.

The branching of ionic liquid cation sidechains utilizing silicon as the backbone was explored and it was found that this structural feature leads to fluids with remarkably low density and viscosity.

Origin of low melting point of ionic liquids: dominant role of entropy

Large structural entropy makes salts liquid at room temperature.

Role of density and electrostatic interactions in the viscosity and non-newtonian behavior of ionic liquids – a molecular dynamics study

Both viscosity and the shear-thinning of ionic liquids are determined mainly by ionic interaction, with density having a secondary effect.

Developing Structural First Principles for Alkylated Triphenylphosphonium-Based Ionic Liquids

While ionic liquids have proved to be versatile materials for a wide spectrum of applications, e.g., energy, materials, and medicine, several challenges remain concerning the rational design of novel materials. In light of this, a series of four triphenylphosphonium-based ionic liquids have been synthesized for the first time. These compounds exhibit high thermal stability with decomposition temperatures up to 450 °C. Their solid-state structures are characterized by single-crystal X-ray diffraction and the intermolecular interactions rigorously analyzed via Hirshfeld surface analysis. It was found that the unique geometries of the anions used in the study form distinct interactions with the cations. The interactions in the crystalline state are correlated with the thermal properties of the four ionic liquids to rationalize the melting points and phase transitions for each compound. The observed arrangements of the alkyl chains on the cations are investigated computationally to gain an understanding of how rotational freedom may impact the thermal properties of the compounds. By intention, each IL reported in this work offers a unique property profile and contributes to the ever-growing ionic liquid catalog.

The Structure of the Electric Double Layer of the Protic Ionic Liquid [Dema][TfO] Analyzed by Atomic Force Spectroscopy

Protic ionic liquids are promising electrolytes for fuel cell applications. They would allow for an increase in operation temperatures to more than 100 °C, facilitating water and heat management and, thus, increasing overall efficiency. As ionic liquids consist of bulky charged molecules, the structure of the electric double layer significantly differs from that of aqueous electrolytes. In order to elucidate the nanoscale structure of the electrolyte–electrode interface, we employ atomic force spectroscopy, in conjunction with theoretical modeling using molecular dynamics. Investigations of the low-acidic protic ionic liquid diethylmethylammonium triflate, in contact with a platinum (100) single crystal, reveal a layered structure consisting of alternating anion and cation layers at the interface, as already described for aprotic ionic liquids. The structured double layer depends on the applied electrode potential and extends several nanometers into the liquid, whereby the stiffness decreases with increasing distance from the interface. The presence of water distorts the layering, which, in turn, significantly changes the system’s electrochemical performance. Our results indicate that for low-acidic ionic liquids, a careful adjustment of the water content is needed in order to enhance the proton transport to and from the catalytic electrode.

Solvation Effects in Organic Chemistry: A Short Historical Overview

This short overview describes the historical development of the physics and chemistry of organic solvents and solutions from the alchemist era until the present time based on some carefully selected examples that can be considered landmarks in the history of solution chemistry.

Curled cation structures accelerate the dynamics of ionic liquids

Ionic liquids are modern liquid materials with potential and actual implementation in many advanced technologies. They combine many favourable and modifiable properties but have a major inherent drawback compared to molecular liquids - slower dynamics. In previous studies we found that the dynamics of ionic liquids are significantly accelerated by the introduction of multiple ether side chains into the cations. However, the origin of the improved transport properties, whether as a result of the altered cation conformation or due to the absence of nanostructuring within the liquid as a result of the higher polarity of the ether chains, remained to be clarified. Therefore, we prepared two novel sets of methylammonium based ionic liquids; one set with three ether substituents and another set with three butyl side chains, in order to compare their dynamic properties and liquid structures. Using a range of anions, we show that the dynamics of the ether-substituted cations are systematically and distinctly accelerated. Liquefaction temperatures are lowered and fragilities increased, while at the same time cation-anion distances are slightly larger for the alkylated samples. Furthermore, pronounced liquid nanostructures were not observed. Molecular dynamics simulations demonstrate that the origin of the altered properties of the ether substituted ionic liquids is primarily due to a curled ether chain conformation, in contrast to the alkylated cations where the alkyl chains retain a linear conformation. Thus, the observed structure-property relations can be explained by changes in the geometric shape of the cations, rather than by the absence of a liquid nanostructure. Application of quantum chemical calculations to a simplified model system revealed that intramolecular hydrogen-bonding is responsible for approximately half of the stabilisation of the curled ether-cations, whereas the other half stems from non-specific long-range interactions. These findings give more detailed insights into the structure-property relations of ionic liquids and will guide the development of ionic liquids that do not suffer from slow dynamics.