Mixing Enthalpy for Binary Mixtures Containing Ionic Liquids

Ionic Liquids in Metal, Photo-, Electro-, and (Bio) Catalysis

Ionic liquids (ILs) have unique physicochemical properties that make them advantageous for catalysis, such as low vapor pressure, non-flammability, high thermal and chemical stabilities, and the ability to enhance the activity and stability of (bio)catalysts. ILs can improve the efficiency, selectivity, and sustainability of bio(transformations) by acting as activators of enzymes, selectively dissolving substrates and products, and reducing toxicity. They can also be recycled and reused multiple times without losing their effectiveness. ILs based on imidazolium cation are preferred for structural organization aspects, with a semiorganized layer surrounding the catalyst. ILs act as a container, providing a confined space that allows modulation of electronic and geometric effects, miscibility of reactants and products, and residence time of species. ILs can stabilize ionic and radical species and control the catalytic activity of dynamic processes. Supported IL phase (SILP) derivatives and polymeric ILs (PILs) are good options for molecular engineering of greener catalytic processes. The major factors governing metal, photo-, electro-, and biocatalysts in ILs are discussed in detail based on the vast literature available over the past two and a half decades. Catalytic reactions, ranging from hydrogenation and cross-coupling to oxidations, promoted by homogeneous and heterogeneous catalysts in both single and multiphase conditions, are extensively reviewed and discussed considering the knowledge accumulated until now.

A Comprehensive Review on Imperative Role of Ionic Liquids in Pharmaceutical Sciences

Ionic liquids (ILs) are poorly-coordinated ionic salts that can exist as a liquid at room temperatures (or <100 °C). ILs are also referred to as "designer solvents" because so many of them have been created to solve particular synthetic issues. ILs are regarded as "green solvents" because they have several distinctive qualities, including better ionic conduction, recyclability, improved solvation ability, low volatility, and thermal stability. These have been at the forefront of the most innovative fields of science and technology during the past few years. ILs may be employed in new drug formulation development and drug design in the field of pharmacy for various functions such as improvement of solubility, targeted drug delivery, stabilizer, permeability enhancer, or improvement of bioavailability in the development of pharmaceutical or vaccine dosage formulations. Ionic liquids have become a key component in various areas such as synthetic and catalytic chemistry, extraction, analytics, biotechnology, etc., due to their superior abilities along with highly modifiable potential. This study concentrates on the usage of ILs in various pharmaceutical applications enlisting their numerous purposes from the delivery of drugs to pharmaceutical synthesis. To better comprehend cuttingedge technologies in IL-based drug delivery systems, highly focused mechanistic studies regarding the synthesis/preparation of ILs and their biocompatibility along with the ecotoxicological and biological effects need to be studied. The use of IL techniques can address key issues regarding pharmaceutical preparations such as lower solubility and bioavailability which plays a key role in the lack of effectiveness of significant commercially available drugs.

The Potential Role of Ionic Liquid as a Multifunctional Dental Biomaterial

In craniofacial research and routine dental clinical procedures, multifunctional materials with antimicrobial properties are in constant demand. Ionic liquids (ILs) are one such multifunctional intelligent material. Over the last three decades, ILs have been explored for different biomedical applications due to their unique physical and chemical properties, high task specificity, and sustainability. Their stable physical and chemical characteristics and extremely low vapor pressure make them suitable for various applications. Their unique properties, such as density, viscosity, and hydrophilicity/hydrophobicity, may provide higher performance as a potential dental material. ILs have functionalities for optimizing dental implants, infiltrate materials, oral hygiene maintenance products, and restorative materials. They also serve as sensors for dental chairside usage to detect oral cancer, periodontal lesions, breath-based sobriety, and dental hard tissue defects. With further optimization, ILs might also make vital contributions to craniofacial regeneration, oral hygiene maintenance, oral disease prevention, and antimicrobial materials. This review explores the different advantages and properties of ILs as possible dental material.

Mixing Ionic Liquids Affects the Kinetics and Thermodynamics of the Oxygen/Superoxide Redox Couple in the Context of Oxygen Sensing

The electrochemical oxygen reduction reaction is vital for applications such as fuel cells, metal air batteries and for oxygen gas sensing. Oxygen undergoes a 1-electron reduction process in dry ionic liquids (ILs) to form the electrogenerated superoxide ion that is solvated and stabilized by IL cations. In this work, the oxygen/superoxide (O₂/O₂ ^•-) redox couple has been used to understand the effect of mixing ILs with different cations in the context of developing designer electrolytes for oxygen sensing, by employing cyclic voltammetry at both gold and platinum electrodes. Different cations with a range of sizes, geometries and aromatic/aliphatic character were studied with a common bis(trifluoromethylsulfonyl)imide ([NTf₂]^-) anion. Diethylmethylsulfonium ([S_2,2,1]⁺), N-butyl-N-methylpyrrolidinum ([C₄mpyrr]⁺) and tetradecyltrihexylphosphonium ([P_14,6,6,6]⁺) cations were mixed with a common 1-butyl-3-methylimidazolium ([C₄mim]⁺) cation at mole fractions (x) of [C₄mim]⁺ of 0, 0.2, 0.4, 0.6, 0.8, and 1. Both the redox kinetics and thermodynamics were found to be highly dependent on the cation structure and the electrode material used. Large deviations from "ideal" mixtures were observed for mixtures of [C₄mim][NTf₂] with [C₄mpyrr][NTf₂] on gold electrodes, suggesting a much higher amount of [C₄mim]⁺ ions near the electrode surface despite the large excess of [C₄mpyrr]⁺ in the bulk. The electrical double layer structure was probed for a mixture of [C₄mim]_0.2[C₄mpyrr]_0.8[NTf₂] using atomic force microscopy measurements on Au, revealing that the first layer was more like [C₄mim][NTf₂] than [C₄mpyrr][NTf₂]. Unusually fast kinetics for O₂/O₂ ^•- in mixtures of [C₄mim]⁺ with [P_14,6,6,6]⁺ were also observed in the electrochemistry results, which warrants further follow-up studies to elucidate this promising behavior. Overall, it is important to understand the effect on the kinetic and thermodynamic properties of electrochemical reactions when mixing solvents, to aid in the creation of designer electrolytes with favorable properties for their intended application.

Why Do Liquids Mix? The Mixing of Protic Ionic Liquids Sharing the Same Cation Is Apparently Driven by Enthalpy, Not Entropy

We study hydrogen bond (HB) redistribution in mixtures of two protic ionic liquids (PILs) sharing the same cation: triethylammonium methanesulfonate ([TEA][OMs]) and triethylammonium trifluoromethanesulfonate ([TEA][OTf]). The mixtures exhibit large negative energies of mixing. Based on results obtained from atomic detail molecular dynamics (MD) simulations, we derive a lattice model, discriminating between HB and nonspecific intermolecular interactions. We demonstrate that due to the ordered structure of the PILs, mostly the HB interactions contribute to the mixing energy. This allows to us to connect the equilibrium of HBs to each of the two anion species with the corresponding excess energies and entropies. The entropy associated with HB redistribution is shown to be negative, and even overcompensating the positive entropy associated with a statistical distribution of the ions in the mixture. This is strongly suggesting that the mixing process is driven by enthalpy, not entropy.

The application of green solvent in a biorefinery using lignocellulosic biomass as a feedstock

The high dependence on crude oil for energy utilization leads to a necessity of finding alternative sustainable resources. Solvents are often employed in valorizing the biomass into bioproducts and other value-added chemicals during treatment stages. Unfortunately, despite the effectiveness of conventional solvents, hindrances such as expensive solvents, unfavourable environmental ramifications, and complicated downstream separation systems often occur. Therefore, the scientific community has been actively investigating more cost-effective, environmentally friendly alternatives and possess the excellent dissolving capability for biomass processing. Generally, 'green' solvents are attractive due to their low toxicity, economic value, and biodegradability. Nonetheless, green solvents are not without disadvantages due to their complicated product recovery, recyclability, and high operational cost. This review summarizes and evaluates the recent contributions, including potential advantages, challenges, and drawbacks of green solvents, namely ionic liquids, deep eutectic solvents, water, biomass-derived solvents and carbon dioxide in transforming the lignocellulosic biomass into high-value products. Moreover, research opportunities for future developments and potential upscale implementation of green solvents are also critically discussed.

Hydrogen Bond Redistribution Effects in Mixtures of Protic Ionic Liquids Sharing the Same Cation: Non-ideal Mixing with Large Negative Mixing Enthalpies

We report a joint experimental and theoretical study characterising the hydrogen bond (HB) redistribution in mixtures of two different protic ionic liquids (PILs) sharing the same cation: triethylammonium-methanesulfonate ([TEA][OMs]) and...

Bi(III) halometallate ionic liquids: Interactions and speciation

Bismuth containing compounds are of particular interest for optical or photo-luminescent applications in sensing, bio-imaging, telecommunications, and opto-electronics and as components in non-toxic extremely dense liquids. Bismuth(III) halometallates form highly colored novel ionic liquid based solvents for which experimental characterization and fundamental understanding are limited. In this work, Bismuth(III) halometallates incorporating chloride, bromide, and iodide have been studied via density functional theory employing B3LYP-D3BJ/aug-cc-pVDZ. Lone anions, and anions in clusters with sufficient 1-ethyl-3-methyl-imidazolium [C₂C₁Im]⁺ counter-cations to balance the charge, have been investigated in the gas- phase, and with polarizable continuum solvation. Evaluation of speciation profiles indicates that dimeric or trimeric anions are prevalent. In contrast to analogous Al systems, anions of higher charge (-2, -3) are present. Speciation profiles are similar, but not identical with respect to the halide. The Bi based anions [Bi_mX_n]^x- in the gas phase and generalized solvation environment produce multiple low energy conformers; moreover, key structural interaction patterns emerge from an analysis of ion-pair and neutral-cluster structures (Bi_mX_n)^x-(C₂C₁Im)_x ⁺ for x = 1, 2, and 3. Cation-anion interactions are weak; with Coulombic and dispersion forces predominating, anion-π structures are favored, while significant hydrogen bonding does not occur. Anion to cation charge transfer is minimal, but mutual polarization is significant, leading to local positive regions in the anion electrostatic potential surface. The key features of experimental x-ray photoelectron, UV-Vis spectra, and Raman spectra are reproduced, validating the computational results and facilitating rationalization of key features.

Balance Between Contact and Solvent-Separated Ion Pairs in Mixtures of the Protic Ionic Liquid [Et3NH][MeSO3] with Water Controlled by Water Content and Temperature

The formation of aggregates of ionic species is a crucial process in liquids and solutions. Ion speciation is particularly interesting for the case of ionic liquids (ILs) since these Coulombic fluids consist solely of ions. Most of their unique properties, such as enthalpies of vaporization and conductivities, are strongly related to ion pair formation. Here, we show that the balance of hydrogen-bonded contact ion pairs (CIP) and solvent-separated (SIP) ion pairs in protic ionic liquids (PILs) and in their mixtures with water can be well understood by a combination of far-infrared (FIR) and mid-infrared (MIR) spectroscopy, density functional theory (DFT) calculations of PIL/water aggregates, and molecular dynamics (MD) simulations of PIL/water mixtures. This combined approach is applied to mixtures of triethylammonium methanesulfonate [Et₃NH][MeSO₃] with water. It is shown that ion speciation in this mixture depends on three parameters: the relative hydrogen bond acceptor strength of the counter ion and the molecular solvent, the solvent concentration, and the temperature. For selected PIL/water mixtures, the equilibrium constants for CIPs and SIPs were determined as a function of the solvent content and temperature. Finally, for the studied PIL/water mixtures, the transition from CIPs to SIPs could be understood on enthalpic and entropic grounds. A detailed picture of this interconversion process could be described at the molecular level by means of MD simulations. In addition, the concentration dependence of ion pair formation can be well understood with help of a simplified "cartoon-like" statistical model describing hydrogen bond redistribution.

Mixing divalent ionic liquids: effects of charge and side-chains

We have prepared novel divalent ionic liquids (ILs) based on the bis(trifluoromethylsulfonyl)imide anion where two charged imidazolium groups in the cations are either directly bound to each other or linked by a single atom. We assessed the influence of the side-chain functionality and divalency on their physical properties and on the thermodynamics of mixing. The results indicate that shortening the spacer of a divalent IL reduces its thermal stability and increases its viscosity. Mixtures of divalent and monovalent ILs show small but significant deviations from ideality upon mixing. These deviations appear to depend primarily on the (mis)match of the nature and length of the cation side-chain. The non-ideality imposed by mixing ILs with different side-chains appears to be enhanced by the increase in formal charge of the cations in the mixture.

Revisiting Ionic Liquid Structure-Property Relationship: A Critical Analysis

In the last few years, ionic liquids (ILs) have been the focus of extensive studies concerning the relationship between structure and properties and how this impacts their application. Despite a large number of studies, several topics remain controversial or not fully answered, such as: the existence of ion pairs, the concept of free volume and the effect of water and its implications in the modulation of ILs physicochemical properties. In this paper, we present a critical review of state-of-the-art literature regarding structure–property relationship of ILs, we re-examine analytical theories on the structure–property correlations and present new perspectives based on the existing data. The interrelation between transport properties (viscosity, diffusion, conductivity) of IL structure and free volume are analysed and discussed at a molecular level. In addition, we demonstrate how the analysis of microscopic features (particularly using NMR-derived data) can be used to explain and predict macroscopic properties, reaching new perspectives on the properties and application of ILs.

Microstructural and Dynamical Heterogeneities in Ionic Liquids

Ionic liquids (ILs) are a special category of molten salts solely composed of ions with varied molecular symmetry and charge delocalization. The versatility in combining varied cation-anion moieties and in functionalizing ions with different atoms and molecular groups contributes to their peculiar interactions ranging from weak isotropic associations to strong, specific, and anisotropic forces. A delicate interplay among intra- and intermolecular interactions facilitates the formation of heterogeneous microstructures and liquid morphologies, which further contributes to their striking dynamical properties. Microstructural and dynamical heterogeneities of ILs lead to their multifaceted properties described by an inherent designer feature, which makes ILs important candidates for novel solvents, electrolytes, and functional materials in academia and industrial applications. Due to a massive number of combinations of ion pairs with ion species having distinct molecular structures and IL mixtures containing varied molecular solvents, a comprehensive understanding of their hierarchical structural and dynamical quantities is of great significance for a rational selection of ILs with appropriate properties and thereafter advancing their macroscopic functionalities in applications. In this review, we comprehensively trace recent advances in understanding delicate interplay of strong and weak interactions that underpin their complex phase behaviors with a particular emphasis on understanding heterogeneous microstructures and dynamics of ILs in bulk liquids, in mixtures with cosolvents, and in interfacial regions.

Nanostructure domains, voids, and low-frequency spectra in binary mixtures of N,N-dimethylacetamide and ionic liquids with varying cationic size

Classical molecular dynamics (MD) simulations were carried out on binary mixtures of N,N-dimethylacetamide (DMA) with hydroxide based ammonium ionic liquids (ILs), tetraethylammonium hydroxide (TEAH), tetrapropylammonium hydroxide (TPAH), tetrabutylammonium hydroxide (TBAH), at three different mole fractions of IL (X IL). The solvation of DMA molecules by ions of ILs was studied by the combined distribution function (CDF). CDFs show that anions have strong correlations with the DMA due to the hydrogen bonding. Increasing the DMA disrupts the nanosegregated domains and causes changes in correlations of cation-DMA and anion-DMA. Also, increased translational motion of ions, as well as the fluidity of IL and a significant improvement in self-diffusion coefficients, are observed with the presence of more DMA. The structural microheterogeneity was investigated using the Voronoi tessellation method. Domain analysis confirms the formation of discreet domains by anions at all the mole fractions. The results also complement the experimental observations, which suggest that two types of aggregations are possible in given mixtures: below and above 0.5 X IL. When the alkyl chain length on the cation increases, a notable decrease in ion translational motion was observed in the IL rich region. In the concentrated IL mixture, the self-diffusion coefficient of the cation is higher than that of the corresponding anion; further addition of IL (X IL < 0.5) results in weaker interactions between DMA and anion when compared to DMA-cation. The mean collision time of each species is found to have an inverse relation with X IL. The analysis of the vibrational density of states provides the low-frequency spectral feature of the mixtures.

Anomalous and Not-So-Common Behavior in Common Ionic Liquids and Ionic Liquid-Containing Systems

This work highlights unexpected, not so well known responses of ionic liquids and ionic liquid-containing systems, which are reported in a collective manner, as a short review. Examples include: (i) Minima in the temperature dependence of the isobaric thermal expansion coefficient of some ILs; (ii) Viscosity Minima in binary mixtures of IL + Molecular solvents; (iii) Anomalies in the surface tension within a family of ILs; (iv) The constancy among IL substitution of C_p/V_m at and around room temperature; (v) ILs as glass forming liquids; (vi) Alternate odd-even side alkyl chain length effects; (vii) Absolute negative pressures in ILs and IL-containing systems; (viii) Reversed-charged ionic liquid pairs; (ix) LCST immiscibility behavior in IL + solvent systems.

On the structural origin of free volume in 1-alkyl-3-methylimidazolium ionic liquid mixtures: a SAXS and 129Xe NMR study

Ionic liquid (IL) mixtures enable the design of fluids with finely tuned structural and physicochemical properties for myriad applications. In order to rationally develop and design IL mixtures with the desired properties, a thorough understanding of the structural origins of their physicochemical properties and the thermodynamics of mixing needs to be developed. To elucidate the structural origins of the excess molar volume within IL mixtures containing ions with different alkyl chain lengths, 3 IL mixtures containing 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ILs have been explored in a joint small angle X-ray scattering (SAXS) and 129Xe NMR study. The apolar domains of the IL mixtures were shown to possess similar dimensions to the largest alkyl chain of the mixture with the size evolution determined by whether the shorter alkyl chain was able to interact with the apolar domain. 129Xe NMR results illustrated that the origin of excess molar volume in these mixtures was due to fluctuations within these apolar domains arising from alkyl chain mismatch, with the formation of a greater number of smaller voids within the IL structure. These results indicate that free volume effects for these types of mixtures can be predicted from simple considerations of IL structure and that the structural basis for the formation of excess molar volume in these mixtures is substantially different to IL mixtures formed of different types of ions.

Using Thermodynamics to Assess the Molecular Interactions of Tetrabutylphosphonium Carboxylate–Water Mixtures

Densities, ρ, viscosities, η, and enthalpies of mixing, , of binary [P4 4 4 4][CnCOO]–water mixtures (with n=1, 2 or 7) were determined at atmospheric pressure as a function of temperature. The excess, , apparent, , and partial, , molar volumes were deduced from experimental data, as well as fragilities, m*, and excess Gibbs free energies of activation of viscous flow, . exhibited predominantly negative deviation from ideality, with a minimum at approximately ~0.8 for all three systems, indicating strong hydrogen-bonding interactions. All three binary systems were found to be fragile, with [P4 4 4 4][C7COO] showing the smallest deviations in fragility with the addition of water. values of the systems were exothermic over the entire composition range, having the following trend: [P4 4 4 4][C2COO]&gt;[P4 4 4 4][C7COO]&gt;[P4 4 4 4][C1COO].

Using hydrogenated and perfluorinated gases to probe the interactions and structure of fluorinated ionic liquids

Ionic liquids with perfluorinated instead of hydrogenated alkyl chains dissolve larger quantities of perfluorocarbon gases that are solvated in their apolar domains.

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.

Innovative aspects of protein stability in ionic liquid mixtures

Mixtures of ionic liquids (ILs) have attracted our attention because of their extraordinary performances in extraction technologies and in absorbing large amount of CO₂ gas. It has been observed that when two or more ILs are mixed in different proportions, a new solvent is obtained which is much better than that of each component of ILs from which the mixture is obtained. Within a mixture of ILs, several unidentified interactions occur among several ions which give rise to unique solvent properties to the mixture. Herein, in this review, we have highlighted the utilization of the advantageous properties of the IL mixtures in protein stability studies. This approach is exceptional and opens new directions to the use of ILs in biotechnology.

A theoretical study on mixtures of amino acid-based ionic liquids

Ionic liquid mixtures containing amino acid anions are studied at the microscopic level using molecular dynamics simulations.