Interactions in Water–Ionic Liquid Mixtures: Comparing Protic and Aprotic Systems

Influence of Residual Water Traces on the Electrochemical Performance of Hydrophobic Ionic Liquids for Magnesium-Containing Electrolytes

A trace amount of water is typically unavoidable as an impurity in ionic liquids, which is a huge challenge for their application in Mg-ion batteries. Here, we employed molecular sieves of different pore diameters (3, 4, and 5 Å), to effectively remove the trace amounts of water from 1-methyl-1-propylpiperidinium bis(trifluoromethylsulfonyl)imide (MPPip-TFSI) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMP-TFSI). Notably, after sieving (water content <1 mg ⋅ L-1 ), new anodic peaks arise that are attributed to the formation of different anion-cation structures induced by minimizing the influence of hydrogen bonds. Furthermore, electrochemical impedance spectroscopy (EIS) reveals that the electrolyte resistance decreases by ∼10 % for MPPip-TFSI and by ∼28 % for BMP-TFSI after sieving. The electrochemical Mg deposition/dissolution is investigated in MPPip-TFSI/tetraglyme (1 : 1)+100 mM Mg(TFSI)2 +10 mM Mg(BH4 )2 using Ag/AgCl and Mg reference electrodes. The presence of a trace amount of water leads to a considerable shift of 0.9 V vs. Mg2+/ Mg in the overpotential of Mg deposition. In contrast, drying of MPPip-TFSI enhances the reversibility of Mg deposition/dissolution and suppresses the passivation of the Mg electrode.

Condensable Gases Capture with Ionic Liquids

Condensable gases are the sum of condensable and volatile steam or organic compounds, including water vapor, which are discharged into the atmosphere in gaseous form at atmospheric pressure and room temperature. Condensable toxic and harmful gases emitted from petrochemical, chemical, packaging and printing, industrial coatings, and mineral mining activities seriously pollute the atmospheric environment and endanger human health. Meanwhile, these gases are necessary chemical raw materials; therefore, developing green and efficient capture technology is significant for efficiently utilizing condensed gas resources. To overcome the problems of pollution and corrosion existing in traditional organic solvent and alkali absorption methods, ionic liquids (ILs), known as "liquid molecular sieves", have received unprecedented attention thanks to their excellent separation and regeneration performance and have gradually become green solvents used by scholars to replace traditional absorbents. This work reviews the research progress of ILs in separating condensate gas. As the basis of chemical engineering, this review first provides a detailed discussion of the origin of predictive molecular thermodynamics and its broad application in theory and industry. Afterward, this review focuses on the latest research results of ILs in the capture of several important typical condensable gases, including water vapor, aromatic VOCs (i.e., BTEX), chlorinated VOC, fluorinated refrigerant gas, low-carbon alcohols, ketones, ethers, ester vapors, etc. Using pure IL, mixed ILs, and IL + organic solvent mixtures as absorbents also briefly expanded the related reports of porous materials loaded with an IL as adsorbents. Finally, future development and research directions in this exciting field are remarked.

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.

Ionic Liquid-Based Strategy for Predicting Protein Aggregation Propensity and Thermodynamic Stability

Novel drug candidates are continuously being developed to combat the most life-threatening diseases; however, many promising protein therapeutics are dropped from the pipeline. During biological and industrial processes, protein therapeutics are exposed to various stresses such as fluctuations in temperature, solvent pH, and ionic strength. These can lead to enhanced protein aggregation propensity, one of the greatest challenges in drug development. Recently, ionic liquids (ILs), in particular, biocompatible choline chloride ([Cho]Cl)-based ILs, have been used to hinder stress-induced protein conformational changes. Herein, we develop an IL-based strategy to predict protein aggregation propensity and thermodynamic stability. We examine three key variables influencing protein misfolding: pH, ionic strength, and temperature. Using dynamic light scattering, zeta potential, and variable temperature circular dichroism measurements, we systematically evaluate the structural, thermal, and thermodynamic stability of fresh immunoglobin G4 (IgG4) antibody in water and 10, 30, and 50 wt % [Cho]Cl. Additionally, we conduct molecular dynamics simulations to examine IgG4 aggregation propensity in each system and the relative favorability of different [Cho]Cl-IgG4 packing interactions. We re-evaluate each system following 365 days of storage at 4 °C and demonstrate how to predict the thermodynamic properties and protein aggregation propensity over extended storage, even under stress conditions. We find that increasing [Cho]Cl concentration reduced IgG4 aggregation propensity both fresh and following 365 days of storage and demonstrate the potential of using our predictive IL-based strategy and formulations to radically increase protein stability and storage.

Specific Ion Effects in Different Media: Current Status and Future Challenges

We discuss the current state of research as well as the future challenges for a deeper understanding of specific ion effects in protic and aprotic solvents as well as various additional media. Despite recent interest in solute or interfacial effects, we focus exclusively on the specific properties of ions in bulk electrolyte solutions. Corresponding results show that many mechanisms remain unknown for these simple media, although theoretical, computational, and experimental studies have provided some insights into explaining individual observations. In particular, the importance of local interactions and electronic properties is emphasized, which enabled a more consistent interpretation of specific ion effects over the past years. Despite current insufficient knowledge, we also discuss future challenges in relation to dynamic properties as well as the influence of different concentrations, different solvents, and solute contributions to gain a deeper understanding of specific ion effects for technological applications.

Deciphering Solvation Effects in Aqueous Binary Mixtures by Fluorescence Behavior of 4-Aminophthalimide: The Comparison Between Ionic Liquids and Alcohols as Cosolvents

Ionic liquids (ILs) have received attention for many years due to them being very promising as green solvent substitutes, but they are not fully understood, especially their behavior dissolved in other solvents, for example, water. Thus, the goal of this contribution is to show insight into the different IL-water mixtures interaction. In this way, two protic ILs (PILs), ethylammonium nitrate (EAN) and 1-methylimidazolium acetate (MIA), mixed with water were investigated. To study the PILs-water interaction, the unique spectroscopical behavior in water of the molecular probe 4-aminophthalimide (4-AP) was used. 4-AP emission spectra show hypsochromic shifting by changing the excitation wavelength and, using time-resolved spectroscopy, can be detected by a blue shifting with time. Also, the water mixture of an aprotic IL, 1-methyl-3-butylimidazolium tetrafluoroborate (bmimBF₄), and three alcohols, methanol (MeOH), 2-propanol (2-PrOH), and t-butanol (t-BOH), were investigated for comparison. Our results show that the water-ILs interaction is dominated by the size of the IL components, in particular, the cation size. Thus, in MIA-water and bmimBF₄-water mixtures, 4-AP is mostly solvated by the IL, even at a low IL molar fraction, as in the t-BOH-water mixture. This finding is especially interesting when ILs-water mixtures are used as a solvent in an organic reaction, where it may call attention to water probably not being the solvent that is interacting with the reactants.

Water in Protic Ionic Liquid Electrolytes: From Solvent Separated Ion Pairs to Water Clusters

The large electrochemical and cycling stability of "water-in-salt" systems have rendered promising prospective electrolytes for batteries. The impact of addition of water on the properties of ionic liquids has already been addressed in several publications. In this contribution, we focus on the changes in the state of water. Therefore, we investigated the protic ionic liquid N-butyl-pyrrolidinium bis(trifluoromethanesulfonyl)imide with varying water content at different temperatures with the aid of molecular dynamics simulations. It is revealed that at very low concentrations, the water is well dispersed and best characterized as shared solvent molecules. At higher concentrations, the water forms larger aggregates and is increasingly approaching a bulk-like state. While the librational and rotational dynamics of the water molecules become faster with increasing concentration, the translational dynamics are found to become slower. Further, all dynamics are found to be faster if the temperature increases. The trends of these findings are well in line with the experimental measured conductivities.

How sensitive are physical properties of choline chloride-urea mixtures to composition changes: Molecular dynamics simulations and Kirkwood-Buff theory

Deep eutectic solvents (DESs) have emerged as a cheaper and greener alternative to conventional organic solvents. Choline chloride (ChCl) mixed with urea at a molar ratio of 1:2 is one of the most common DESs for a wide range of applications such as electrochemistry, material science, and biochemistry. In this study, molecular dynamics simulations are performed to study the effect of urea content on the thermodynamic and transport properties of ChCl and urea mixtures. With increased mole fraction of urea, the number of hydrogen bonds (HBs) between cation-anion and ion-urea decreases, while the number of HBs between urea-urea increases. Radial distribution functions (RDFs) for ChCl-urea and ChCl-ChCl pairs shows a significant decrease as the mole fraction of urea increases. Using the computed RDFs, Kirkwood-Buff Integrals (KBIs) are computed. KBIs show that interactions of urea-urea become stronger, while interactions of urea-ChCl and ChCl-ChCl pairs become slightly weaker with increasing mole fraction of urea. All thermodynamic factors are found larger than one, indicating a non-ideal mixture. Our results also show that self- and collective diffusivities increase, while viscosities decrease with increasing urea content. This is mainly due to the weaker interactions between ions and urea, resulting in enhanced mobilities. Ionic conductivities exhibit a non-monotonic behavior. Up to a mole fraction of 0.5, the ionic conductivities increase with increasing urea content and then reach a plateau.

Understanding the dissolution of softwood lignin in ionic liquid and water mixed solvents

Lignin is the most abundant heterogeneous aromatic polymer on earth to produce a large number of value-added chemicals. Besides, the separation of lignin from the lignocellulosic biomass is essential for cellulosic biofuel production. For the first time, we report a cosolvent-based approach to understand the dissolution of lignin with 61 guaiacyl subunits at the molecular level. Atomistic molecular dynamics simulations of the lignin were performed in 0%, 20%, 50%, 80%, and 100% 1-Ethyl-3-Methylimidazolium Acetate (EmimOAc) systems. The lignin structure was significantly destabilized in both 50%, and 80% EmimOAc cosolvents, and pure EmimOAc systems leading to the breakdown of intrachain hydrogen bonds. Lignin-OAc and lignin-water hydrogen bonds were formed with increasing EmimOAc concentration, signifying the dissolution process. The OAc anions mostly solvated the alkyl chains and hydroxy groups of lignin. Besides, the imidazolium head of Emim cations contributed to solvation of methoxy groups and hydroxy groups, whereas ethyl tail interacted with the benzene ring of guaiacyl subunits. Effective dissolution was obtained in both the 50% and 80% EmimOAc cosolvent systems. Overall, our study presents a molecular view of the lignin dissolution focusing on the role of both cation and anion, which will help to design efficient cosolvent-based methods for lignin dissolution.

The transition from salt-in-water to water-in-salt nanostructures in water solutions of organic ionic liquids relevant for biological applications

Computer simulations show how nano-structural motifs in organic salts/water solutions change with salt content increasing from dilute to highly concentrated.

Synthesis, characterization and electrochemistry of triethyl ammonium sulphate ionic liquid

Protic ionic liquids (PILs) being intrinsic proton conducting ionic species are considered as potential green electrolytes for study of electrocatalytic reactions and for fabrication of IL-based fuel cells (FCs) and batteries. We have prepared a sulfate anion based protic ionic liquid (PIL), triethylammonium sulfate (TEAS) through a reaction involving transfer of proton from H2SO4 to triethylamine (TEA). 1H NMR and FT-IR spectroscopic techniques were employed for confirmation of the synthesis of TEAS and water content of the PIL was quantified using coulometric Karl–Fischer (KF) titration. 1H NMR and FT-IR analysis confirm the synthesis of the PILs and KF-titration analysis shows that TEAS contains 1.43 w/w % water. Electrical conductivity of TEAS was determined at different temperatures showing that the PIL has excellent ionic conductivity that enhances with rise in temperature of the medium. The temperature dependence of the conductivity of the PIL follows the Arrhenius equation as the logσ versus 1/T plot is linear. The electrochemical windows (EWs) of the electrolyte were found using cyclic voltammetry at Pt and Au working electrodes and found to decrease with increase in temperature of the medium. The data revealed that the surfaces of the electrodes are covered with oxide layers due to oxidation of trace water (1.43 w/w %) present in the PIL. The oxide layers growth increase and their onset potential moves to less positive values as the temperature of the PILs is increased. The data was compared with the literature and would be helpful in understanding of the surface electrochemistry in this neoteric medium for being used as potential electrolyte in industry for various electrochemical applications.

Structural investigations of molecular solutes within nanostructured ionic liquids

Ionic liquids (ILs) containing sufficiently long alkyl chains form amphiphilic nanostructures with well-defined polar and non-polar domains. Here we have explored the robustness of these amphiphilic nanostructures to added solutes and gained insight into how the nature of the solute and IL ions affect the partitioning of these solutes within the nanostructured domains of ILs. To achieve this, small angle X-ray scattering (SAXS) investigations were performed and discussed for mixtures of 9 different molecular compounds with 6 different ILs containing imidazolium cations. The amphiphilic nanostructure of ILs persisted to high solute concentrations, over 50 mol% of added solute for most 1-butyl-3-methylimidazolium ILs and above 80 mol% for most 1-decyl-3-methylimidazolium ILs. Solute partitioning within these domains was found to be controlled by the inherent polarity and size of the solute, as well as specific interactions between the solute and IL ions, with SAXS results corroborated with IR spectroscopy and molecular dynamics simulations. Molecular dynamics simulations also revealed the ability to induce π⁺-π⁺ stacking between imidazolium cations through the use of these added molecular compounds. Collectively, these results provide scope for the selection of IL ions to rationally influence and control the partitioning behaviour of given solutes within the amphiphilic nanostructure of ILs.

Ultrafast vibrational dynamics of a trigonal planar anionic probe in ionic liquids (ILs): A two-dimensional infrared (2DIR) spectroscopic investigation

A major impediment limiting the widespread application of ionic liquids (ILs) is their high shear viscosity. Incorporation of a tricyanomethanide (TCM^-) anion in ILs leads to low shear viscosity and improvement of several characteristics suitable for large scale applications. However, properties including interactions of TCM^- with the local environment and dynamics of TCM^- have not been thoroughly investigated. Herein, we have studied the ultrafast dynamics of TCM^- in several imidazolium ILs using linear IR and two-dimensional infrared spectroscopy techniques. The spectral diffusion dynamics of the CN stretching modes of TCM^- in all ILs exhibit a nonexponential behavior with a short time component of ∼2 ps and a long time component spanning ∼9 ps to 14 ps. The TCM^- vibrational probe reports a significantly faster relaxation of ILs compared to those observed previously using linear vibrational probes, such as thiocyanate and selenocyanate. Our results indicate a rapid relaxation of the local ion-cage structure embedding the vibrational probe in the ILs. The faster relaxation suggests that the lifetime of the local ion-cage structure decreases in the presence of TCM^- in the ILs. Linear IR spectroscopic results show that the hydrogen-bonding interaction between TCM^- and imidazolium cations in ILs is much weaker. Shorter ion-cage lifetimes together with weaker hydrogen-bonding interactions account for the low shear viscosity of TCM^- based ILs compared to commonly used ILs. In addition, this study demonstrates that TCM^- can be used as a potential vibrational reporter to study the structure and dynamics of ILs and other molecular systems.

Restructuring a Deep Eutectic Solvent by Water: The Nanostructure of Hydrated Choline Chloride/Urea

Deep eutectic mixtures are a promising sustainable and diverse class of tunable solvents that hold great promise for various green chemical and technological processes. Many deep eutectic solvents (DES) are hygroscopic and find use in applications with varying extents of hydration, hence urging a profound understanding of changes in the nanostructure of DES with water content. Here, we report on molecular dynamics simulations of the quintessential choline chloride-urea mixture, using a newly parametrized force field with scaled charges to account for physical properties of hydrated DES mixtures. These simulations indicate that water changes the nanostructure of solution even at very low hydration. We present a novel approach that uses convex constrained analysis to dissect radial distribution functions into base components representing different modes of local association. Specifically, DES mixtures can be deconvoluted locally into two dominant competing nanostructures, whose relative prevalence (but not their salient structural features) change with added water over a wide concentration range, from dry up to ∼30 wt % hydration. Water is found to be associated strongly with several DES components but remarkably also forms linear bead-on-string clusters with chloride. At high water content (beyond ∼50 wt % of water), the solution changes into an aqueous electrolyte-like mixture. Finally, the structural evolution of the solution at the nanoscale with extent of hydration is echoed in the DES macroscopic material properties. These changes to structure, in turn, should prove important in the way DES acts as a solvent and to its interactions with additive components.

Short-Range Order and Transport Properties in Mixtures of the Protic Ionic Liquid [C2HIm][TFSI] with Water or Imidazole

We investigate the effect of adding different molecular cosolvents, water or imidazole, to the protic ionic liquid 1-ethylimidazolium bis(trifluoromethanesulfonyl)imide, i.e., [C₂HIm][TFSI]. We explore how the added cosolvent distributes within the ionic liquid by means of molecular dynamics simulations and X-ray scattering. We also analyze the degree of short-range heterogeneity in the resulting mixtures, finding that while imidazole easily mixes with the protic ionic liquid, water tends to form small clusters in its own water-rich domains. These differences are rationalized by invoking the nature of intermolecular interactions. In aqueous mixtures water-water hydrogen bonds are more likely to form than water-ion hydrogen bonds (water-TFSI bonds being particularly weak), while imidazole can interact with both cations and anions. Hence, the cation-anion association is negligibly influenced by the presence of water, whereas the addition of imidazole creates solvent-separated ion pairs and is thus able to also increase the ionicity. As a consequence of these structural and interactional features, transport properties like self-diffusion and ionic conductivity also show different composition dependencies. While the mobility of both ions and solvent is increased considerably by the addition of water, upon adding imidazole this property changes significantly only for molar fractions of imidazole above 0.6. At these molar fractions, which correspond to a base-excess composition, the imidazole/[C₂HIm][TFSI] mixture behaves as a glass-forming liquid with suppressed phase transitions, while homomixtures such as imidazole/[HIm][TFSI] can display a eutectic point.

Dielectric Relaxation and Hydration Interactions for Protic and Aprotic Ionic Liquids using Time Domain Reflectometry

H-Bonding abilities of ionic liquids (ILs) along with hydrophobicity and cooperativity effects increases their hydration numbers making them capable for dissolving sparingly soluble organic molecules in aqueous or polar nonaqueous media, and hence ILs are potential candidates in pharmaceutical and medicinal sciences besides the different technological and academic interests. In this work, dielectric spectra were measured and analyzed for diethylammonium-based protic ionic liquids (PILs), imidazolium-based aprotic ionic liquids (APILs), and their aqueous solutions (∼0.02 to ∼0.8 mol·dm^-3) over a frequency range from 0.01 to 50 GHz using time domain reflectometry at 298.15 K. The Cole-Cole (CC) model for neat ILs and a combination of the Debye and Cole-Cole (D+CC) models for their aqueous solutions best describes the experimental dielectric relaxation spectra. Higher values of static permittivity and relaxation time were observed for less viscous PILs compared to more viscous APILs due to the existence of hydrogen bonding in PILs, ionic translational motion, and the existence of transient, short-lived proton transfer responsible for solvent polarization. For aqueous solutions of ionic liquids, the fast collective relaxation of solvent (bulk water) observed at higher frequencies (∼20 GHz) and slow relaxation is detected at lower frequency (∼5 to ∼10 GHz) due to hydrophobic hydration with or without cooperative H-bonding effect. The apparent concentrations of bulk water, c_bw^ap, and slow water, c_sw^ap, were used to obtain effective hydration numbers to understand the ion solvation. Hydration numbers revealed that imidazolium-based APILs are weakly hydrated than the diethylammonium-based PILs. Static permittivity and relaxation time of pure ILs and of aqueous solutions of studied ILs are discussed in terms of effect on alkyl chain length of cation/anion, H-bonding abilities of ions, dipole moments of ions, viscosity, hydrophobic effects, etc.

Spectroscopic and computational insights into the ion-solvent interactions in hydrated aprotic and protic ionic liquids

Ionic liquids (ILs) and their aqueous solutions are emerging media for solving and manipulating biochemical molecules such as proteins. Unleashing the full potential however requires a detailed mechanistic understanding of how suitable protic and aprotic ILs behave in the presence of water in the first place. The present work aims at making an important step by performing a combined experimental and computational study of two selected ILs and their mixtures with water: the aprotic cholinium propionate ([Chl][Pro]) and the protic N-methyl-2-pyrrolidonium propionate ([NMP][Pro]). IR and Raman spectroscopy reveal stronger ion-solvent interactions in [Chl][Pro]-H₂O systems compared to [NMP][Pro]-H₂O mixtures. This can be explained by the tightly packed ion-pair associations in [NMP][Pro] comprising the protic -N⁺-H counterpart, which allows the establishment of highly directional and strong interionic hydrogen bonds. The spectral decomposition of the O-D stretching band into three sub-peaks showed that the protic [NMP][Pro] favors the self-association of water molecules. On the other hand, the predominant fraction of water-anion/cation aggregates exists in aprotic [Chl][Pro]. These hydrated systems can be envisaged using quantum-chemical calculations in the following way: H₂O[Chl]⁺H₂O[Pro]^-H₂O and H₂O[NMP]⁺[Pro]^-H₂O, which implied preferable solvent-shared ion-pair (SIP) configurations for [Chl][Pro]-H₂O systems, whereas the contact ion-pair (CIP) state prevails for the [NMP][Pro]-H₂O systems. The latter holds even in the water-rich regime. In future work, these findings will be the basis for an understanding of the underlying principles that govern the interactions of ions with bio-molecules in aqueous solutions.

Quantifying non-specific interactions via liquid chromatography

Determinations of solute-cosolute interactions from chromatography have often resulted in problems, such as the "antibinding" (or a negative binding constant) between the solute and micelle in micellar liquid chromatography (MLC) or indeterminacy of salt-ligand binding strength in high-performance affinity chromatography (HPAC). This shows that the stoichiometric binding models adopted in many chromatographic analyses cannot capture the non-specific nature of solvation interactions. In contrast, an approach using statistical thermodynamics handles these complexities without such problems and directly links chromatographic data to, for example, solubility data via a universal framework based on Kirkwood-Buff integrals (KBI) of the radial distribution functions. The chromatographic measurements can now be interpreted within this universal theoretical framework that has been used to rationalize small solute solubility, biomolecular stability, binding, aggregation and gelation. In particular, KBI analysis identifies key solute-cosolute interactions, including excluded volume effects. We present (i) how KBI can be obtained directly from the cosolute concentration dependence of the distribution coefficient, (ii) how the classical binding model, when used solely as a fitting model, can yield the KBIs directly from the literature data, and (iii) how chromatography and solubility measurements can be compared in the unified theoretical framework provided via KBIs without any arbitrary assumptions about the stationary phase. To perform our own analyses on multiple datasets we have used an "app". To aid readers' understanding and to allow analyses of their own datasets, the app is provided with many datasets and is freely available on-line as an open-source resource.

The peculiar effect of water on ionic liquids and deep eutectic solvents

Ionic liquids (ILs) and deep eutectic solvents (DESs) have been suggested as eco-friendly alternatives to organic solvents. A trace amount of water is often unavoidable as impurity, and water is also added on purpose to reduce their problematically high viscosity and lower their high price. Understanding the distinct effects of water on the properties of ILs/DESs is highly important. In this review, we collect published experimental and theoretical results for IL/DES-H2O systems at varied water concentrations and analyze them. Results from mechanistic studies, thermodynamic modelling and advanced experiments are collected and critically discussed. Six commonly studied IL/DES-H2O systems were selected to map experimental observations onto microscopic results obtained in mechanistic studies. A great variety of distinct contours of the excess properties can be observed over the entire compositional range, indicating that the properties of IL/DES-H2O systems are highly unpredictable. Mechanistic studies clearly demonstrate that the added H2O rapidly changes the heterogeneous 3D structures of pure ILs/DESs, leading to very different properties and behaviour. There are similarities between aqueous electrolytes and IL/DES solutions but the bulky and asymmetric organic cations in ILs/DESs do not conform to the standard salt dissolution and hydration concepts. Thermodynamic modelling previously assumes ILs/DESs to be either a neutral ion-pair or completely dissociated ions, neglecting specific ion hydration effects. A new conceptual framework is suggested for thermodynamic modelling of IL/DES-H2O binary systems to enable new technologies for their practical applications.

Aqueous ionic liquids in comparison with standard co-solutes : Differences and common principles in their interaction with protein and DNA structures

Ionic liquids (ILs) are versatile solvents for a broad range of biotechnological applications. Recent experimental and simulation results highlight the potential benefits of dilute ILs in aqueous solution (aqueous ILs) in order to modify protein and DNA structures systematically. In contrast to a limited number of standard co-solutes like urea, ectoine, trimethylamine-N-oxide (TMAO), or guanidinium chloride, the large amount of possible cation and anion combinations in aqueous ILs can be used to develop tailor-made stabilizers or destabilizers for specific purposes. In this review article, we highlight common principles and differences between aqueous ILs and standard co-solutes with a specific focus on their underlying macromolecular stabilization or destabilization behavior. In combination with statistical thermodynamics theories, we present an efficient framework, which is used to classify structure modification effects consistently. The crucial importance of enthalpic and entropic contributions to the free energy change upon IL-assisted macromolecular unfolding in combination with a complex destabilization mechanism is described in detail. A special focus is also set on aqueous IL-DNA interactions, for which experimental and simulation outcomes are summarized and discussed in the context of previous findings.

Structure-property relationships in protic ionic liquids: a study of solvent-solvent and solvent-solute interactions

The ionic nature of a functionalized protic ionic liquid cannot be rationalized simply through the differences in aqueous proton dissociation constants between the acid precursor and the conjugate acid of the base precursor. The extent of proton transfer, i.e. the equilibrium ionicity, of a tertiary ammonium acetate protic ionic liquid can be significantly increased by introducing an hydroxyl functional group on the cation, compared to the alkyl or amino-functionalized analogues. This increase in apparent ionic nature correlates well with variations in solvent-solute and solvent-solvent interaction parameters, as well as with physicochemical properties such as viscosity.

Aqueous ionic liquids and their effects on protein structures: an overview on recent theoretical and experimental results

Ionic liquids (ILs) are used in a variety of technological and biological applications. Recent experimental and simulation results reveal the influence of aqueous ionic liquids on the stability of protein and enzyme structures. Depending on different parameters like the concentration and the ion composition, one can observe distinct stabilization or denaturation mechanisms for various ILs. In this review, we summarize the main findings and discuss the implications with regard to molecular theories of solutions and specific ion effects. A preferential binding model is introduced in order to discuss protein-IL effects from a statistical mechanics perspective. The value of the preferential binding coefficient determines the strength of the ion influence and indicates a shift of the chemical equilibrium either to the native or the denatured state of the protein. We highlight the role of water in order to explain the self-association behavior of the IL species and discuss recent experimental and simulation results in the light of the observed binding effects.

Structure–property relationships in protic ionic liquids: a thermochemical study

How does cation functionality influence the strength of intermolecular interactions in protic ionic liquids (PILs)? Quantifying the energetics of PILs can be an invaluable tool to answer this fundamental question.

The properties of residual water molecules in ionic liquids: a comparison between direct and inverse Kirkwood–Buff approaches

Atomistic molecular dynamics simulations of aqueous ionic liquid mixtures were performed in order to compare the resulting Kirkwood–Buff integrals with experimental data and the corresponding integrals derived by an inverse Kirkwood–Buff approach.