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.

Ion Steric Effect Induces Giant Enhancement of Thermoelectric Conversion in Electrolyte-Filled Nanochannels

Ionic thermoelectricity in nanochannels has received increasing attention because of its advantages, such as high Seebeck coefficient and low cost. However, most studies have focused on dilute simple electrolytes that neglect the effects of finite ion sizes and short-range electrostatic correlation. Here, we reveal a new thermoelectric mechanism arising from the coupling of the ion steric effect due to finite ion sizes and ion thermodiffusion in electric double layers, using both theoretical and numerical methods. We show that this mechanism can significantly enhance the thermoelectric response in nanoconfined electrolytes depending on the properties of electrolytes and nanochannels. Compared to the previously known mechanisms, the new mechanism can increase the Seebeck coefficient by 100% or even 1 order of magnitude enhancement under optimal conditions. Moreover, we demonstrate that the short-range electrostatic correlation can help preserve the Seebeck coefficient enhancement in a weaker confinement or in more concentrated electrolytes.

Insights into the Absorption of Hydrocarbon Gases in Phosphorus-Containing Ionic Liquids

The solubility of ethane, ethylene, propane, and propylene was measured in two phosphorus-containing ionic liquids, trihexyltetradecylphosphonium bis(2,4,4-trimethylpentyl)phosphinate, [P_6,6,6,14][DiOP], and 1-butyl-3-methylimidazolium dimethylphosphate, [C₄C₁Im][DMP], using an isochoric saturation method. The ionic liquid [C₄C₁Im][DMP] absorbed between 1 and 20 molecules of gas per 1000 ion pairs, at 313 K and 0.1 MPa, while [P_6,6,6,14][DiOP] absorbed up to 169 molecules of propane per 1000 ion pairs under the same conditions. [C₄C₁Im][DMP] had a higher capacity to absorb olefins than paraffins, while the opposite was true for [P_6,6,6,14][DiOP], with the former being slightly more selective than the later. From the analysis of the thermodynamic properties of solvation, we concluded that in both ionic liquids and for all of the studied gases the solvation is ruled by the entropy, even if its contribution is unfavorable. These results, together with density measurements, 2D NMR studies, and self-diffusion coefficients suggest that the gases' solubility is ruled mostly by nonspecific interactions with the ionic liquids and that the looser ion packing in [P_6,6,6,14][DiOP] makes it easier to accommodate the gases compared to [C₄C₁Im][DMP].

Molecular simulations explain the exceptional thermal stability, solvent tolerance and solubility of protein-polymer surfactant bioconjugates in ionic liquids

Proteins complexed electrostatically with polymer surfactants constitute a viscous liquid by themselves, called the solvent-free protein liquid (SFPL). A solution of SFPL in a room temperature ionic liquid (PS-IL) offers the protein hyperthermal stability, higher solubility and greater IL tolerance. A generic understanding of these protein-polymer systems is obtained herein through extensive atomistic molecular dynamics simulations of three different enzymes (lipase A, lysozyme and myoglobin) under various conditions. Along with increased intra-protein hydrogen bonding, the surfactant coating around the proteins imparts greater thermal stability, and also aids in screening protein-IL interactions, endowing them IL tolerance. The reduced surface polarity of the protein-polymer bioconjugate and hydrogen bonding between the ethylene glycol groups of the surfactant and the IL cation contribute to the facile solvation of the protein in its PS-IL form. The results presented here rationalize several experimental observations and will aid in the improved design of such hybrid materials for sustainable catalysis.

Structural, dynamical, and electronic properties of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide

We provide a microscopic insight, both structural and electronic, into the multifold interactions occurring in the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [EMIM][TFSI] currently targeted for applications in next-generation low-power electronics and optoelectronic devices. To date, practical applications have remained hampered by the lack of fundamental understanding of the interactions occurring both inside the IL and at the interface with the substrate. Our first principles dynamical simulations provide accurate insights into the nature of bonding and non-bonding interactions, dynamical conformational changes and induced dipole moments, along with their statistical distributions, of this ionic liquid, that have so far not been completely unraveled. The mobilities of the two ionic species are obtained by long-lasting dynamical simulations at finite temperature, allowing simultaneous monitoring and quantification of the isomerization occurring in the IL. Moreover, a thorough analysis of the electronic structure and partial charge distributions characterizing the two components, the cation and anion, allow rationalization of the nature of the electrostatic interactions, hydrogen bonding properties of the two ionic counterparts, and the infra-red and dielectric response of the system, especially in the low frequency range, for the full characterization of the IL.

Evolution of microscopic heterogeneity and dynamics in choline chloride-based deep eutectic solvents

Deep eutectic solvents (DESs) are an emerging class of non-aqueous solvents that are potentially scalable, easy to prepare and functionalize for many applications ranging from biomass processing to energy storage technologies. Predictive understanding of the fundamental correlations between local structure and macroscopic properties is needed to exploit the large design space and tunability of DESs for specific applications. Here, we employ a range of computational and experimental techniques that span length-scales from molecular to macroscopic and timescales from picoseconds to seconds to study the evolution of structure and dynamics in model DESs, namely Glyceline and Ethaline, starting from the parent compounds. We show that systematic addition of choline chloride leads to microscopic heterogeneities that alter the primary structural relaxation in glycerol and ethylene glycol and result in new dynamic modes that are strongly correlated to the macroscopic properties of the DES formed.

Tailoring the macroscopic properties of deep eutectic solvents requires knowing how these depend on the local structure and microscopic dynamics. The authors, with computational and experimental tools spanning a wide range of space- and timescales, shed light into the relationship between micro and macroscopic properties in glyceline and ethaline.

Giant and bidirectionally tunable thermopower in nonaqueous ionogels enabled by selective ion doping

Ionic thermoelectrics show great potential in thermal sensing owing to their ultrahigh thermopower, low cost, and ease in production. However, the lack of effective n-type ionic thermoelectric materials seriously hinders their applications. Here, we report giant and bidirectionally tunable thermopowers within an ultrawide range from −15 to +17 mV K⁻¹ in solid ionic liquid–based ionogels. Particularly, a record high negative thermopower of −15 mV K⁻¹ is achieved in the ternary ionogel, rendering it among the best n-type ionic thermoelectric materials under the same condition. A thermopower regulation strategy through ion doping to selectively induce ion aggregates to enhance ion-ion interactions is proposed. These selective ion interactions are found to be decisive in modulating the sign and magnitude of the thermopower in the ionogels. A prototype wearable device integrated with 12 p-n pairs is demonstrated with a total thermopower of 0.358 V K⁻¹, showing promise for ultrasensitive thermal detection.

Insights into the intermolecular interactions and temperature-concentration dependence of transport in ionic liquid-based EMI–TFSI/LiTFSI electrolytes

DFT calculations and MD simulations show that the EMI–TFSI/LiTFSI system is stabilized by strong nonbonded attractions, and the lithium-ion conductivity depends on the LiTFSI concentration and system temperature.

Ion Pair Supramolecular Structure Identified by ATR-FTIR Spectroscopy and Simulations in Explicit Solvent*

The present work uses ATR-FTIR spectroscopy assisted by simulations in explicit solvent and frequency calculations to investigate the supramolecular structure of carboxylate alkali-metal ion pairs in aqueous solutions. ATR-FTIR spectra in the 0.25-4.0 M concentration range displayed cation-specific behaviors, which enabled the measurement of the appearance concentration thresholds of contact ion pairs between 1.9 and 2.6 M depending on the cation. Conformational explorations performed using a non-local optimization method associated to a polarizable force-field (AMOEBA), followed by high quantum chemistry level (RI-B97-D3/dhf-TZVPP) optimizations, mode-dependent scaled harmonic frequency calculations and electron density analyses, were used to identify the main supramolecular structures contributing to the experimental spectra. A thorough analysis enables us to reveal the mechanisms responsible for the spectroscopic sensitivity of the carboxylate group and the respective role played by the cation and the water molecules, highlighting the necessity of combining advanced experimental and theoretical techniques to provide a fair and accurate description of ion pairing.

The multiscale solvation effect on the reactivity of β-O-4 of lignin dimers in deep eutectic solvents

Deep eutectic solvents (DESs) emerge as a medium to enhance the depolymerization of lignin. One critical question is how the solvation of lignin in DESs may affect the reactivity of lignin. To shed light on this question, we investigate the solvation of four lignin dimers in three DES solutions using molecular dynamics simulations and quantum mechanical calculations. The four lignin dimers are composed of guaiacyl and syringyl units and are used as the models for lignin. The three DES solutions are composed of choline, Cl^- and three acids: lactic acid, levulinic acid and oxalic acid. We investigate the preferential accumulation of individual DES components in the solvation shells and the exposure area and electrostatic potential of the β-O-4 linkage of the four lignin dimers in the three DESs. The results show that DESs could influence the affinity and nucleophilicity of the β-O-4 linkage through three effects: (1) forming a charged solvation shell, (2) varying the exposure of the β-O-4 linkage and (3) adjusting the electrostatic potential of the β-O-4 linkage. Our simulations indicate a comprehensive and multiscale effect of DESs on lignin decomposition.

Improved carbon dioxide absorption in double-charged ionic liquids

Four divalent ionic liquids based on imidazolium cations with alkyl or ether functionalized side-chains were synthesised and characterized: 3,3'-(tetraethyleneglycol-1,11-diyl)bis(1-methyl-1H-imidazolium)bromide, [tetraEG(mim)₂][Br]₂, 3,3'-(tetraethyleneglycol-1,11-diyl)bis(1-methyl-1H-imidazolium)acetate, [tetraEG(mim)₂][OAc]₂, 1-butyl-3-methylimidazolium malonate, [C₄mim]₂[Mal], and 3-butyl-1-methylimidazolium glutarate, [C₄mim]₂[Glut]. Their densities vary between 1.1 and 1.5 g cm^-3 and their viscosities between 0.2 and 4 Pa s at 353 K. We found that the molar volumes are not additive, especially in the case of the divalent ionic liquids based on the double-charged imidazolium cations, meaning that they cannot be predicted using common group contribution methods. The reason for this behaviour could be explained by the structure of the cations, which is dominated by intramolecular hydrogen bonding. The carboxylate-based divalent ionic liquids absorb reversibly large quantities of carbon dioxide following a chemical mechanism described before. An improved 1 : 1 stoichiometry is achieved both in a double-charged imidazolium acetate ionic liquid and in imidazolium carboxylate salts with double charged anions. This behaviour places these ionic liquids amongst the best performing for carbon dioxide absorption.

A comprehensive computational and principal component analysis on various choline chloride-based deep eutectic solvents to reveal their structural and spectroscopic properties

In this study, the quantum chemical properties, nonbonding interactions, and spectroscopic insights of a wide variety of choline chloride (ChCl)-based deep eutectic solvents were investigated employing molecular dynamics (MD), density functional theory, and spectroscopic analyses. Nine experimentally reported ChCl-based deep eutectic solvents (DESs) were selected for this study where ChCl was common in all the DESs and the hydrogen bond donors (HBDs) were varied. The most energetically favorable cluster was selected using MD simulation followed by density functional theory calculation. The most stable cluster structures were fully optimized, and their quantum chemical properties and IR spectra were computed at the ωB97XD/6-31G++(d,p) level of theory. Principal component analysis was performed to distinguish their behavioral differences and to find out if any correlation exists among the 1:1 and 1:2 clusters. The atom-atom radial distribution functions based on MD simulations revealed that several hydrogen bonds were formed among the donor and acceptor molecules. However, the most prominent hydrogen bonds were found to be N-H_HBD⋯Cl^- for ChCl:U, ChCl:TU, and ChCl:Ace and O-H_HBD⋯Cl^- for ChCl:Glu, ChCl:Ma, ChCl:Ox, ChCl:Gly, and ChCl:Phe. Both N-H_HBD⋯Cl^- and O-H_HBD⋯Cl^- were major interactions for ChCl:Pro, where Cl^- worked as a bridge between Ch⁺ and the respective donors. In addition, the -OH of Ch⁺ showed strong intermolecular interactions with the acceptor groups of the donor molecules, such as C=O and O-H. This study has tried to extract a pattern of the contributions of HBDs by comparing the structural, spectroscopic, and thermodynamic properties of ChCl-based DESs, which have also been successfully correlated with the intermolecular interactions.

23Na NMR T1 relaxation measurements as a probe for diffusion and dynamics of sodium ions in salt-glycerol mixtures

Mixtures of sodium salts with oxygen-containing molecules are useful from the perspective of applications such as sodium ion batteries because they fill the gap between deep eutectic solvents and molten salt hydrates. In a previous work, the physical properties (such as diffusion coefficients, conductivity, viscosity, and glass transition temperature) of four salts, namely, Na₂B₄O₇ · 10H₂O, NaOAc · 3H₂O, NaBr, and NaOAc, were measured with glycerol. Pulsed-field gradient (PFG) nuclear magnetic resonance (NMR) was also used to measure self-diffusion coefficients of ¹H-bearing species. However, the technique was not able to measure diffusion of sodium ions due to the very fast NMR relaxation rate of such species, resulting in loss of the PFG NMR signal. In the current work, this study is expanded using ²³Na T₁ relaxation measurements which, under certain assumptions, can be translated into diffusion coefficients. Analysis of the physical properties is then correlated with self-diffusion coefficient measurements to elucidate information about structure and ionic mobility. It is shown that NaOAc · 3H₂O, NaBr, and NaOAc fit models for ionic conductivity and diffusion, which are consistent with ionic liquids where charge transport is limited by ionic mobility rather than the number of charge carriers. The waters of hydration of NaOAc · 3H₂O do not appear to form a separate phase but are instead strongly coordinated to the cation. In contrast, Na₂B₄O₇ · 10H₂O appears to form a water-rich phase with enhanced sodium mobility.

Thermoelectric Ratchet Effect for Charge Carriers with Hopping Dynamics

We show that the huge Seebeck coefficients observed recently for ionic conductors arise from a ratchet effect where activated jumps between neighbor sites are rectified by a temperature gradient, thus driving mobile ions toward the cold. For complex systems with mobile molecules like water or polyethylene glycol, there is an even more efficient diffusiophoretic transport mechanism, proportional to the thermally induced concentration gradient of the molecular component. Without free parameters, our model describes experiments on the ionic liquid EMIM-TFSI and hydrated NaPSS, and it qualitatively accounts for polymer electrolyte membranes with Seebeck coefficients of hundreds of k_{B}/e.

Investigation of the Ionic Liquid Graphene Electric Double Layer in Supercapacitors Using Constant Potential Simulations

In this work, we investigate the effect of the cation structure on the structure and dynamics of the electrode–electrolyte interface using molecular dynamics simulations. A constant potential method is used to capture the behaviour of 1-ethyl-3-methylimidazolium bis (trifluoromethane)sulfonimide ([C2mim][NTf2]) and butyltrimethylammonium bis(trifluoromethane) sulfonimide ([N4,1,1,1][NTf2]) ionic liquids at varying potential differences applied across the supercapacitor. We find that the details of the structure in the electric double layer and the dynamics differ significantly, yet the charge profile and capacitance do not vary greatly. For the systems considered, charging results in the rearrangement and reorientation of ions within ∼1 nm of the electrode rather than the diffusion of ions to/from the bulk region. This occurs on timescales of O(10 ns) for the ionic liquids considered, and depends on the viscosity of the fluid.

Silica based ionogels: interface effects with aprotic and protic ionic liquids with lithium

In the frame of the development of solid ionogel electrolytes with enhanced ion transport properties, this paper investigates ionogel systems constituted by ∼80 wt% of ionic liquids (ILs) confined in meso-/macroporous silica monolith materials. The anion-cation coordination for two closely related ILs, either aprotic (AIL) butylmethylpyrrolidinium or protic (PIL) butylpyrrolidinium, both with bis(trifluoromethylsulfonyl)imide (TFSI) anions, with and without lithium cations, is studied in depth. The ILs are confined within silica with well-defined mesoporosities (8 to 16 nm). The effects of this confinement, onto melting points, onto conductivity followed by impedance spectroscopy, and onto lithium-TFSI coordination followed by Raman spectroscopy, are presented. Opposite effects have been observed on the melting temperature: it increased for the AIL (+2 °C) upon confinement, while it decreased for the PIL (-2 °C). With lithium, the confinement led to an increase of the melting temperature (+1 °C) for the PIL and AIL. Regarding ionic conductivities, a relative maximum was observed at 40 °C for a mesopore diameter of 10 nm for the AIL with 0.5 M lithium, while it was not clearly visible for the PIL. These differences are discussed in view of the charge balance at the interface between silanols and ILs: the presence of a PIL, contrary to an AIL, is expected to modify the acidity of the silica. Raman data showed that the coordination number of lithium by TFSI is reduced upon AIL confinement, although this was not observed for PILs. At last, this work highlights the impact of the acidity of a PIL on the chemistry occurring at the interface of the host network within ionogels.

Solvent Effects in the Homogeneous Catalytic Reduction of Propionaldehyde with Aluminium Isopropoxide Catalyst: New Insights from PFG NMR and NMR Relaxation Studies

Solvent effects in homogeneous catalysis are known to affect catalytic activity. Whilst these effects are often described using qualitative features, such as Kamlet‐Taft parameters, experimental tools able to quantify and reveal in more depth such effects have remained unexplored. In this work, PFG NMR diffusion and T₁ relaxation measurements have been carried out to probe solvent effects in the homogeneous catalytic reduction of propionaldehyde to 1‐propanol in the presence of aluminium isopropoxide catalyst. Using data on diffusion coefficients it was possible to estimate trends in aggregation of different solvents. The results show that solvents with a high hydrogen‐bond accepting ability, such as ethers, tend to form larger aggregates, which slow down the molecular dynamics of aldehyde molecules, as also suggested by T₁ measurements, and preventing their access to the catalytic sites, which results in the observed decrease of catalytic activity. Conversely, weakly interacting solvents, such as alkanes, do not lead to the formation of such aggregates, hence allowing easy access of the aldehyde molecules to the catalytic sites, resulting in higher catalytic activity. The work reported here is a clear example on how combining traditional catalyst screening in homogeneous catalysis with NMR diffusion and relaxation time measurements can lead to new physico‐chemical insights into such systems by providing data able to quantify aggregation phenomena and molecular dynamics.

Anions as Dynamic Probes for Ionic Liquid Mixtures

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

Ionicity of deep eutectic solvents by Walden plot and pulsed field gradient nuclear magnetic resonance (PFG-NMR)

The transport properties and ionicity of DESs were investigated by using Walden plot and the pulsed field gradient NMR method.

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

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

Self-assembled structure and dynamics of imidazolium-based protic salts in water solution

Protic ionic liquids containing cations with long alkyl chains can form self-assembled micelles, vesicles, microemulsions, and lyotropic liquid crystal structures in water, acid water or tetrahydrofuran, etc. As a result of this unique property, they are regarded as a novel category of amphiphiles, and are gaining importance in the field of colloid and interface chemistry. The critical micelle concentration (CMC) of protic salts, e.g., alkyl-ammonium nitrates in water, was found to increase with decreasing chain length. It is generally believed that a long alkyl chain length is essential for the formation of self-assembled structures. So far, no self-assembled structure has been reported for protic ionic liquids with an alkyl chain length of n < 4. This paper reports on the structure and dynamics of two imidazolium based protic organic salts with no alkyl chain or a methyl group (n = 1) attached to the cation in water solution, determined through a detailed analysis of NMR spectra and pulsed-field gradient NMR data. We demonstrate that these imidazolium cations with no or a short alkyl chain (n = 1) can form a self-assembled clustering structure in water solution, which has a strong influence on the diffusion behavior of imidazolium molecular ions. It is speculated that this self-assembled structure is likely to be present in other similar solutions of ionic liquids with short alkyl chains.

Key factor governing the physicochemical properties and extent of proton transfer in protic ionic liquids: ΔpKa or chemical structure?

A series of protic ionic liquids (PILs) are prepared by neutralisation of bis(trifluoromethanesulfonyl)amide acid (H[NTf₂]) with various amines, and the properties (especially thermal stability and ionicity) are compared with those of PILs from 1,8-diazabicyclo[5.4.0]-7-undecene (DBU) and various acids.