Physicochemical Properties of 20 Ionic Liquids Prepared by the Carbonate-Based IL (CBILS) Process

Ionic liquids (ILs) are an emerging materials' class with applications in areas such as energy storage, catalysis, and biomass dissolution and processing. Their physicochemical properties including surface tension, viscosity, density and their interplay between cation and anion chemistry are decisive in these applications. For many commercially available ILs, a full set of physicochemical data is not available. Here, we extend the knowledge base by providing physicochemical properties such as density (20 and 25 °C), refractive index (20 and 25 °C), surface tension (23 °C, including polar and dispersive components), and shear viscosity (ambient atmosphere, shear rate 1-200 s-1), for 20 commercial ILs. A correlation between the crystal volume, dispersive surface tension, and shear viscosity is introduced as a predictive tool, allowing for viscosity estimation. Systematic exploration of cation/anion alkyl side chain lengths reveals the impact on the IL's physicochemical attributes. Increasing the anion's headgroup decreases surface tension up to 35.7% and consequently shear viscosity. We further demonstrate that the dispersive part of the surface tension linearly correlates with the refractive index of the ionic liquid. While we provide additional physicochemical data, the screening and modeling efforts will contribute to better structure property predictions enabling faster progress in design and applications of ILs.

Ionic Pairing and Selective Solvation of Butylmethylimidazolium Chloride Ion Pairs in DMSO-Water Mixtures: A Comprehensive Examination via Molecular Dynamics Simulations and Potentials of Mean Force Analysis

Ionic liquids (ILs) with dimethyl sulfoxide (DMSO) and water act as a promising solvent medium for the dissolution of cellulose in an efficient manner. To develop a proper solvent system, it is really important to understand the thermodynamics of the molecular solutions consisting of ILs, DMSO, and water. The ion-pairing propensity of the ILs in the presence of DMSO and water plays a crucial role in governing the property of the solvent mixtures. Employing all-atom molecular dynamics simulations, we estimate the potentials of mean force between BMIM+ and Cl- ions in DMSO-water mixtures. Analysis reveals a significant increase in the thermodynamic stability of both contact ion pair (CIP) and solvent-assisted ion pair (SAIP) states with a rising DMSO mole fraction. Thermodynamic assessments highlight the entropic stabilization of CIP states and SAIP states in pure water, in DMSO-water mixtures, and in pure DMSO. The structural analysis reveals that in comparison to the DMSO local density, the local water density is relatively very high around ion pairs, more specifically in the solvation shell of a chloride ion. Preferential binding coefficients also consistently indicate exclusion of DMSO from the ion pair in DMSO-water mixtures. To enhance our understanding regarding the solvent molecules kinetics around the ion pairs, the survival probabilities of DMSO and water are computed. The calculations reveal that the water molecules prefer a prolonged stay in the solvation shell of Cl- ions.

Molecular mechanism of cellulose dissolution in N-methyl morpholine-N-oxide: A molecular dynamics simulation study

N-methyl morpholine-N-oxide (NMMO) is the only commercialised solvent to dissolve cellulose and produce lyocell. However, the molecular mechanism of NMMO-induced cellulose solubilisation is unknown which limits further process development. In this work, and for the first time the complete dissolution process of a large cellulose bunch was simulated in NMMO monohydrate using long microsecond molecular dynamic simulations. The dissolution process was also simulated in 1-ethyl-3-methylimidazolium acetate (EmimAc) as an efficient ionic liquid in cellulose dissolution and the results were compared with the aqueous conditions. While the cellulose bunch showed a stable and insoluble structure in pure water, it was completely and efficiently dissolved in both NMMO monohydrate and EmimAc. It was shown that the dissolution time of cellulose in NMMO monohydrate is almost twice that in EmimAc, which is in agreement with the experimental observations. Although it is revealed that hydrogen bonding is the main driving force of cellulose dissolution in NMMO monohydrate, one cannot explain the complete molecular mechanism of NMMO-induced cellulose dissolution only by considering hydrogen bonds. A straightforward molecular mechanism was proposed, in which the interactions of NMMO molecules, not with cellulose, but with the other NMMO molecules play a critical role in the dissolution process.

Counteraction Effects of Ammonium-Based Ionic Liquids on Urea-Induced Denaturation of α-Lactalbumin: A Comprehensive Molecular Simulation Study

Ionic liquids (ILs) are known to stabilize protein conformations in aqueous medium. Importantly, ILs can also act as refolding additives in urea-driven denaturation of proteins. However, despite the importance of the problem, detailed microscopic understanding of the counteraction effects of ILs on urea-induced protein denaturation remains elusive. In this work, atomistic molecular dynamics (MD) simulations of the protein α-lactalbumin have been carried out in pure aqueous medium, in 8 M binary urea-water solution and in ternary urea-IL-water solutions containing ammonium-based ethyl ammonium acetate (EAA) as the IL at different concentrations (1-4 M). Attempts have been made to quantify detailed molecular-level understanding of the origin behind the counteraction effects of the IL on urea-induced partial unfolding of the protein. The calculations revealed significant conformational changes of the protein with multiple free energy minima due to its partial unfolding in binary urea-water solution. The counteraction effect of the IL was evident from the enhanced structural rigidity of the protein with propensity to transform into a single native free energy minimum state in ternary urea-IL-water solutions. Such an effect has been found to be associated with preferential direct binding of the IL components with the protein and simultaneous expulsion of urea from the interface, thereby providing additional stabilization of the protein in ternary solutions. Most importantly, modified rearrangement of the hydrogen bond network at the interface due to the formation of stronger protein-cation (PC) and protein-anion (PA) hydrogen bonds by breaking relatively weaker protein-urea (PU) and protein-water (PW) hydrogen bonds has been recognized as the microscopic origin behind the counteraction effects of EAA on urea-induced partial unfolding of the protein.

Eucalyptus bleached kraft pulp-ionic liquid inks for 3D printing of ionogels and hydrogels

3D printing has been recently recognized as one of the most promising technologies due to the multiple options to fabricate cost-effective and customizable objects. However, the necessity to substitute fossil fuels as raw materials is increasing the research on bio-based inks with recyclable and eco-friendly properties. In this work, we formulated inks for the 3D printing of ionogels and hydrogels with bleached kraft pulp dissolved in [Emim][DMP] at different concentrations (1-4 wt%). We explored each ink's rheological properties and printability and compared the printability parameters with a commercial ink. The rheological results showed that the 3 % and 4 % cellulose-ionic liquid inks exhibited the best properties. Both had values of damping factor between 0.4 and 0.7 and values of yield stress between 1900 and 2500 Pa. Analyzing the printability, the 4 wt% ink was selected as the most promising because the printed ionogels and the hydrogels had the best print resolution and fidelity, similar to the reference ink. After printing, ionogels and hydrogels had values of the elastic modulus (G') between 10³ and 10⁴ Pa, and the ionogels are recyclables. Altogether, these 3D printed cellulose ionogels and hydrogels may have an opportunity in the electrochemical and medical fields, respectively.

Commercialization of Ionic Liquids in Pursuit of Green Chemistry: Must we Each Become an Entrepreneur?

There will be common challenges to scaling-up any ionic liquids separations technologies which require very large volumes of ionic liquid. Some of these challenges are illustrated in this personal account which chronicles the extraction of chitin from shrimp shell from discovery to current commercialization efforts. The road being taken from discovery in an academic laboratory, through attempts to navigate the scaling-up to commercial scale using the vehicle of a faculty startup company is rewarding, but fraught with roadblocks, detours, and unexpected challenges. The differences in 'technically feasible' and 'commercially viable' are not always evident from the beginning of the journey, however, one wonders what achievements we miss as a Society because it was assumed to not be commercially viable.

Green Routes for Bio-Fabrication in Biomedical and Pharmaceutical Applications

In the last decade, significant advances in nanotechnologies, rising from increasing knowledge and refining of technical practices in green chemistry and bioengineering, enabled the design of innovative devices suitable for different biomedical applications. In particular, novel bio-sustainable methodologies are developing to fabricate drug delivery systems able to sagely mix properties of materials (i.e., biocompatibility, biodegradability) and bioactive molecules (i.e., bioavailability, selectivity, chemical stability), as a function of the current demands for the health market. The present work aims to provide an overview of recent developments in the bio-fabrication methods for designing innovative green platforms, emphasizing the relevant impact on current and future biomedical and pharmaceutical applications.

Self-healing hydrogels for bone defect repair

Severe bone defects can be caused by various factors, such as tumor resection, severe trauma, and infection. However, bone regeneration capacity is limited up to a critical-size defect, and further intervention is required. Currently, the most common clinical method to repair bone defects is bone grafting, where autografts are the "gold standard." However, the disadvantages of autografts, including inflammation, secondary trauma and chronic disease, limit their application. Bone tissue engineering (BTE) is an attractive strategy for repairing bone defects and has been widely researched. In particular, hydrogels with a three-dimensional network can be used as scaffolds for BTE owing to their hydrophilicity, biocompatibility, and large porosity. Self-healing hydrogels respond rapidly, autonomously, and repeatedly to induced damage and can maintain their original properties (i.e., mechanical properties, fluidity, and biocompatibility) following self-healing. This review focuses on self-healing hydrogels and their applications in bone defect repair. Moreover, we discussed the recent progress in this research field. Despite the significant existing research achievements, there are still challenges that need to be addressed to promote clinical research of self-healing hydrogels in bone defect repair and increase the market penetration.

Ionic Liquids as Tools to Incorporate Pharmaceutical Ingredients into Biopolymer-Based Drug Delivery Systems

This mini-review focuses on the various roles that ionic liquids (ILs) play in the development and applications of biopolymer-based drug delivery systems (DDSs). Biopolymers are particularly attractive as drug delivery matrices due to their biocompatibility, low immunogenicity, biodegradability, and strength, whereas ILs can assist the formation of drug delivery systems. In this work, we showcase the different strategies that were explored using ILs in biopolymer-based DDSs, including impregnation of active pharmaceutical ingredients (APIs)-ILs into biopolymeric materials, employment of the ILs to simplify the process of making the biopolymer-based DDSs, and using the ILs either as dopants or as anchoring agents.

Deep Eutectic Solvents: Alternative Solvents for Biomass-Based Waste Valorization

Innovative technologies can transform what are now considered “waste streams” into feedstocks for a range of products. Indeed, the use of biomass as a source of biopolymers and chemicals currently has a consolidated economic dimension, with well-developed and regulated markets, in which the evaluation of the manufacturing processes relies on specific criteria such as purity and yield, and respects defined regulatory parameters for the process safety. In this context, ionic liquids and deep eutectic solvents have been proposed as environmentally friendly solvents for applications related to biomass waste valorization. This mini-review draws attention to some recent advancements in the use of a series of new-solvent technologies, with an emphasis on deep eutectic solvents (DESs) as key players in the development of new processes for biomass waste valorization. This work aims to highlight the role and importance of DESs in the following three strategic areas: chitin recovery from biomass and isolation of valuable chemicals and biofuels from biomass waste streams.

Understanding the Dynamics of Cellulose Dissolved in an Ionic Liquid Solvent Under Shear and Extensional Flows

Ionic liquids (ILs) hold great potential as solvents to dissolve, recycle, and regenerate cellulosic fabrics, but the dissolved cellulose material system requires greater study in conditions relevant to fiber spinning processes, especially characterization of nonlinear shear and extensional flows. To address this gap, we aimed to disentangle the effects of the temperature, cellulose concentration, and degree of polymerization (DOP) on the shear and extensional flows of cellulose dissolved in an IL. We have studied the behavior of cellulose from two sources, fabric and filter paper, dissolved in 1-ethyl-3-methylimidazolium acetate ([C₂C₁Im][OAc]) over a range of temperatures (25 to 80 °C) and concentrations (up to 4%) that cover both semidilute and entangled regimes. The linear viscoelastic (LVE) response was measured using small-amplitude oscillatory shear techniques, and the results were unified by reducing the temperature, concentration, and DOP onto a single master curve using time superposition techniques. The shear rheological data were further fitted to a fractional Maxwell liquid (FML) model and were found to satisfy the Cox-Merz rule within the measurement range. Meanwhile, the material response in the non-LVE (NLVE) regime at large strains and strain rates has special relevance for spinning processes. We quantified the NLVE behavior using steady shear flow tests alongside uniaxial extension using a customized capillary breakup extensional rheometer. The results for both shear and extensional NLVE responses were described by the Rolie-Poly model to account for flow-dependent relaxation times and nonmonotonic viscosity evolution with strain rates in an extensional flow, which primarily arise from complex polymer interactions at high concentrations. The physically interpretable model fitting parameters were further compared to describe differences in material response to different flow types at varying temperatures, concentrations, and DOP. Finally, the fitting parameters from the FML and Rolie-Poly models were connected under the same superposition framework to provide a comprehensive description within the wide measured parameter window for the flow and handling of cellulose in [C₂C₁Im][OAc] in both linear and nonlinear regimes.

Exploring the Interactions of Ionic Liquids with Bio-Organic Amphiphiles Using Computational Approaches

Bio-organic amphiphiles have been shown to effectively impart unique physicochemical properties to ionic liquids resulting in the formation of versatile hybrid composites. In this work, we utilized computational methods to probe the formation and properties of hybrids prepared by mixing three newly designed bio-organic amphiphiles with 14 ionic liquids containing cholinium or glycine betaine cations and a variety of anions. The three amphiphiles were designed such that they contain unique biological moieties found in nature by conjugating (a) malic acid with the amino acid glutamine, (b) thiomalic acid with the antiviral, antibacterial pyrazole compound [3-(3,5-dimethyl-1H-pyrazol-1-yl)benzyl]amine, and (c) Fmoc-protected valine with diphenyl amine. Conductor-like screening model for real solvents (COSMO-RS) was used to obtain sigma profiles of the hybrid mixtures and to predict viscosities and mixing enthalpies of each composite. These results were used to determine optimal ionic liquid-bio-organic amphiphile mixtures. Molecular dynamics simulations of three optimal hybrids were then performed, and the interactions involved in the formation of the hybrids were analyzed. Our results indicated that cholinium-based ILs interacted most favorably with the amphiphiles through a variety of inter- and intramolecular interactions. This work serves to illustrate important factors that influence the interactions between bio-organic amphiphiles and bio-ILs and aids in the development of novel ionic liquid-based composites for a wide variety of potential biological applications.

Theoretical study of cellulose II nanocrystals with different exposed facets

Derived from the most abundant natural polymer, cellulose nanocrystal materials have attracted attention in recent decades due to their chemical and mechanical properties. However, still unclear is the influence of different exposed facets of the cellulose nanocrystals on the physicochemical properties. Herein, we first designed cellulose II nanocrystals with different exposed facets, the hydroxymethyl conformations distribution, hydrogen bond (HB) analysis, as well as the relative structural stability of these models (including crystal facets {A, B, O} and Type-A models vary in size) are theoretically investigated. The results reveal that the HB network of terminal anhydroglucose depends on the adjacent chain's contact sites in nanocrystals exposed with different facets. Compared to nanocrystals exposed with inclined facet, these exposed with flat facet tend to be the most stable. Therefore, the strategy of tuning exposed crystal facets will guide the design of novel cellulose nanocrystals with various physicochemical properties.

Contrasting Effects of Ionic Liquids of Varying Degree of Hydrophilicity on the Conformational and Interfacial Properties of a Globular Protein

Ionic liquids (ILs), depending on their cation-anion combinations, are known to influence the conformational properties and activities of proteins in a nonuniform manner. To obtain microscopic understanding of such influence, it is important to characterize protein-IL interactions and explore the modified solvation environment around the protein. In this work, molecular dynamics (MD) simulations of the globular protein α-lactalbumin have been carried out in aqueous IL solutions containing 1-butyl-3-methylimidazolium cations (BMIM⁺) in combination with a series of anions with varying degree of hydrophilicity, namely, hexafluorophosphate (PF₆^-), ethyl sulfate (ETS^-), acetate (OAc^-), chloride (Cl^-), dicyanamide (DCA^-), and nitrate (NO₃^-) . The calculations revealed that ILs with hydrophobic and hydrophilic anions have contrasting influence on conformational flexibility of the protein. It is further observed that the BMIM⁺ cations exhibit site-specific orientations at the interface depending on the hydrophilicity of the anion component. Most importantly, the results demonstrated enhanced propensity of hydrophilic ILs to replace relatively weaker protein-water hydrogen bonds by stronger protein-IL hydrogen bonds at the protein surface as compared to the hydrophobic ILs. Such breaking of protein-water hydrogen bonds at a greater extent leads to greater loss of water hydrating the protein in the presence of hydrophilic ILs, thereby reducing the protein's stability.

A novel strategy to reduce the viscosity of cellulose-ionic liquid solution assisted by transition metal ions

The high viscosity of ionic liquids, even at relatively high temperatures, can greatly affect the production of cellulose fibers through the wet-spinning process. The high viscosity mainly by due to the hydrogen bond interaction between the cations and anions of ionic liquids. It is possible to reduce the viscosity by modulating the hydrogen bond interaction. In the present work, copper chloride (CuCl₂) was dissolved in 1-butyl-3-methylimidazolium chloride ([Bmim]Cl)-cellulose solution, followed by the formation of a complex with the chloride anions by converting it to [CuCl₄]^2- anion. Through this strategy, the extrusion velocity of the solution improved, and the produced fibers obtained smoother surfaces and shrunken diameters.

Mechanism of deep eutectic solvents enhancing catalytic function of cytochrome P450 enzymes in biosynthesis and organic synthesis

Indigo is an insoluble blue dye, which generates serious pollution in its production process. Increasing focus has come to the biosynthesis of indigo that are more environment-preserved and high-efficient. Hence, this study was designed to explore the specific role of various deep eutectic solvents (DESs) on cytochromeP45-BM-3 catalyzing indole to produce indigo. DESs were synthesized by heating and stirring. The structure of the solvent was analyzed by nuclear magnetic resonance (NMR) and fourier transform infrared spectrum (FT-IR), and the relationship between the viscosity, density and refractive index of the solvent, and the water content of the solvent was examined. Circular dichroism spectrometer was used to detect the tertiary structure of the enzyme protein. The effect of solvent type, concentration, pH, temperature, and water content on the catalytic activity and stability of P450 BM-3 was measured using an ultraviolet spectrophotometer. A new solvent biphasic system was established using DESs and buffers, and indigo was prepared using recombinant E. coli-biocatalyzed indole. DESs were low-melting eutectics formed by molecules interaction of components through hydrogen bonding. The physical properties of DESs such as density, viscosity, and refractive index varied with water content and temperature of the solvent. The pH, water content, and temperature of DESs were positively correlated with the catalytic activity of P450 BM-3. To sum up, DESs can improve the catalytic activity and thermal stability of P450 BM-3. Indigo can be efficiently prepared using the DESs-buffer biphasic system.

Macroscopic Differentiators for Microscopic Structural Nonideality in Binary Ionic Liquid Mixtures

Combining two ionic liquids to form a binary ionic liquid mixture is a simple yet effective strategy to not only expand the number of ionic liquids but also precisely control various physicochemical properties of resultant ionic liquid mixtures. From a fundamental thermodynamic point of view, it is not entirely clear whether such mixtures can be classified as ideal solutions. Given a large number of binary ionic liquid mixtures that emerge, the ability to predict the presence of nonideality in such mixtures a priori without the need for experimentation or molecular simulation-based calculations is immensely valuable for their rational design. In this research report, we demonstrate that the difference in the molar volumes (ΔV) of the pure ionic liquids and the difference in the hydrogen-bonding ability of anions (Δβ) are the primary determinants of nonideal behavior of binary ionic liquid mixtures containing a common cation and two anions. Our conclusion is derived from a comparison of microscopic structural properties expressed in terms of radial, spatial, and angular distributions for binary mixtures and those of the corresponding pure ionic liquids. Molecular dynamics simulations of 16 binary ionic liquid mixtures, containing a common cation 1-n-butyl-3-methylimidazolium [C₄mim]⁺ and combinations of (less basic) fluorinated {trifluoromethylacetate [TFA]^-, trifluoromethanesulfonate [TFS]^-, bis(trifluoromethanesulfonyl)imide [NTf₂]^-, and tris (pentafluoroethyl) trifluorophosphate [eFAP]^-} versus (more basic) nonfluorinated {chloride Cl^-, acetate [OAC]^-, methylsulfate [MeSO₄]^-, and dimethylphosphate [Me₂PO₄]^-} anions, were conducted. The large number of binary ionic liquid mixtures examined here enabled us to span a broad range of ΔV and Δβ values. The results indicate that binary mixtures of two ionic liquids for which ΔV > 60 cm³/mol and Δβ > 0.4 are expected to be microscopically nonideal. On the other hand, ΔV < 60 cm³/mol and Δβ < 0.4 will lead to molecular structures that are not differentiated from those of their pure ionic liquid counterparts.

Glucose in dry and moist ionic liquid: vibrational circular dichroism, IR, and possible mechanisms

Ionic liquids and their mixtures with water show remarkable features in cellulose processing. For this reason, understanding the behavior of carbohydrates in ionic liquids is important. In the present study, we investigated three d-glucose isomers (α, β and open-chain) in 1-ethyl-3-methylimidazolium acetate in the presence and absence of water, through ab initio molecular dynamics simulations. In the complex hydrogen bonding network of these mixtures, the most interesting observation is that upon water addition every hydrogen bond elongates, except the glucose-glucose hydrogen bond for the open-chain and the α-form which shortens, clearly showing the beginning of the crystallization process. The ring glucose rearranges from on-top to in-plane and the open form changes from a coiled to a more linear arrangement when adding water which explains the contradiction that the center of mass distances of the glucose molecules with other glucose molecules grow while the hydrogen bonds shorten. The appearance of coiled open forms indicates that the previously suggested isomerization between these forms is possible and might play a role in the solubility of the related carbohydrates. The calculated IR and VCD spectra reveal insight into the intermolecular interactions, with good to excellent agreements with experimental spectra. Investigating the role of the cation, distances between the acidic carbon atom of the cation and the glucose carbon atom where ring closure and opening occurs are found, which are way shorter than dispersion-like interactions between aliphatic hydrocarbons.

Microscopic Understanding of the Effect of Ionic Liquid on Protein from Molecular Simulation Studies

We have performed molecular dynamics (MD) simulations of the protein α-lactalbumin in aqueous solution containing the ionic liquid (IL) 1-butyl-3-methyl imidazolium tetrafluoroborate ([BMIM][BF₄]) as the cosolvent at different concentrations. Attempts have been made to obtain quantitative understanding of the effects of the IL on the conformational features of the protein as well as the distributions of the IL and water around it. The calculations revealed enhanced rigidity of the protein with reduced conformational fluctuations and increasingly correlated local motions in the presence of the IL. Nonuniform relative population of the BMIM⁺ and BF₄^- ions at the protein surface with respect to that in the bulk solution has been observed. It is demonstrated that exchange of water by the IL around the protein results in rearrangement of the hydrogen bond network at the interface with breaking of protein-water hydrogen bonds and formation of protein-IL hydrogen bonds. Importantly, it is found that the protein forms increased number of stronger salt bridges in the presence of IL. This shows that the formation of a greater number of such stronger salt bridges is the origin behind the enhanced rigidity of the protein in the presence of the IL.

Insight into Cellulose Dissolution with the Tetrabutylphosphonium Chloride-Water Mixture using Molecular Dynamics Simulations

All-atom molecular dynamics simulations are utilized to determine the properties and mechanisms of cellulose dissolution using the ionic liquid tetrabutylphosphonium chloride (TBPCl)–water mixture, from 63.1 to 100 mol % water. The hydrogen bonding between small and large cellulose bundles with 18 and 88 strands, respectively, is compared for all concentrations. The Cl, TBP, and water enable cellulose dissolution by working together to form a cooperative mechanism capable of separating the cellulose strands from the bundle. The chloride anions initiate the cellulose breakup, and water assists in delaying the cellulose strand reformation; the TBP cation then more permanently separates the cellulose strands from the bundle. The chloride anion provides a net negative pairwise energy, offsetting the net positive pairwise energy of the peeling cellulose strand. The TBP–peeling cellulose strand has a uniquely favorable and potentially net negative pairwise energy contribution in the TBPCl–water solution, which may partially explain why it is capable of dissolving cellulose at moderate temperatures and high water concentrations. The cellulose dissolution declines rapidly with increasing water concentration as hydrogen bond lifetimes of the chloride–cellulose hydroxyl hydrogens fall below the cellulose’s largest intra-strand hydrogen bonding lifetime.

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.

The role of urea in the solubility of cellulose in aqueous quaternary ammonium hydroxide

We examine the role of water and urea in cellulose solubility in tetrabutylammonium hydroxide (TBAH). Molecular dynamics simulations were performed for several different solvent compositions with a fixed cellulose fraction. For each composition, two simulations were carried out with cellulose fixed in each of the crystalline and the dissolved states. From the enthalpy and the entropy of the two states, the difference in Gibbs free energy (ΔG) and hence the spontaneity is determined. A comparison with solubility experiments showed a strong correlation between the calculated ΔG and the experimental measurements. A breakdown of the enthalpic and entropic contributions reveals the roles of water and urea in solubility. At high water concentration, a drop in solubility is attributed to both increased enthalpy and decreased entropy of dissolution. Water displaces strong IL–cellulose interactions for weaker water–cellulose interactions, resulting in an overall enthalpy increase. This is accompanied by a strong decrease in entropy, which is primarily attributed to both water and the entropy of mixing. Adding urea to TBAH(aq) increases solubility by an addition to the mixing term and by reducing losses in solvent entropy upon dissolution. In the absence of urea, the flexible [TBA]⁺ ions lose substantial degrees of freedom when they interact with cellulose. When urea is present, it partially replaces [TBA]⁺ and to a lesser extent OH⁻ near cellulose, losing less entropy because of its rigid structure. This suggests that one way to boost the dissolving power of an ionic liquid is to limit the number of degrees of freedom from the outset.

We examine the role of water and urea in cellulose solubility in tetrabutylammonium hydroxide (TBAH).

Self-Healing Hydrogels: The Next Paradigm Shift in Tissue Engineering?

Given their durability and long-term stability, self-healable hydrogels have, in the past few years, emerged as promising replacements for the many brittle hydrogels currently being used in preclinical or clinical trials. To this end, the incompatibility between hydrogel toughness and rapid self-healing remains unaddressed, and therefore most of the self-healable hydrogels still face serious challenges within the dynamic and mechanically demanding environment of human organs/tissues. Furthermore, depending on the target tissue, the self-healing hydrogels must comply with a wide range of properties including electrical, biological, and mechanical. Notably, the incorporation of nanomaterials into double-network hydrogels is showing great promise as a feasible way to generate self-healable hydrogels with the above-mentioned attributes. Here, the recent progress in the development of multifunctional and self-healable hydrogels for various tissue engineering applications is discussed in detail. Their potential applications within the rapidly expanding areas of bioelectronic hydrogels, cyborganics, and soft robotics are further highlighted.

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].

Structural changes in lignocellulosic biomass during activation with ionic liquids comprising 3-methylimidazolium cations and carboxylate anions

BACKGROUND

Lignocellulosic biomass requires either pretreatment and/or fractionation to recover its individual components for further use as intermediate building blocks for producing fuels, chemicals, and products. Numerous ionic liquids (ILs) have been investigated for biomass pretreatment or fractionation due to their ability to activate lignocellulosic biomass, thereby reducing biomass recalcitrance with minimal impact on its structural components. In this work, we studied and compared 1-allyl-3-methylimidazolium formate ([AMIM][HCOO]) to the commonly used 1-ethyl-3-methylimidazolium acetate ([EMIM][CH₃COO]) for its potential to activate hybrid poplar biomass and enable high cellulose and hemicellulose enzymatic conversion. Although [EMIM][CH₃COO] has been widely used for activation, [AMIM][HCOO] was recently identified to achieve higher biomass solubility, with an increase of 40% over [EMIM][CH₃COO].

RESULTS

Since IL activation is essentially an early stage of IL dissolution, we assessed the recalcitrance of [EMIM][CH₃COO] and [AMIM][HCOO]-activated biomass through a suite of analytical tools. More specifically, Fourier transform infrared spectroscopy and X-ray diffraction showed that activation using [AMIM][HCOO] does not deacetylate hybrid poplar as readily as [EMIM][CH₃COO] and preserves the crystallinity of the cellulose fraction, respectively. This was supported by scanning electron microscopy and enzymatic saccharification experiments in which [EMIM][CH₃COO]-activated biomass yielded almost twice the cellulose and hemicellulose conversion as compared to [AMIM][HCOO]-activated biomass.

CONCLUSION

We conclude that the IL [AMIM][HCOO] is better suited for biomass dissolution and direct product formation, whereas [EMIM][CH₃COO] remains the better IL for biomass activation and fractionation.

Towards a molecular understanding of cellulose dissolution in ionic liquids: anion/cation effect, synergistic mechanism and physicochemical aspects

This perspective summarizes mechanistic studies on cellulose dissolution in ionic liquids, highlighting the synergistic mechanism, physicochemical aspects and future research trends.

Cellulose is one of the most abundant bio-renewable materials on the earth and its conversion to biofuels provides an appealing way to satisfy the increasing global energy demand. However, before carrying out the process of enzymolysis to glucose or polysaccharides, cellulose needs to be pretreated to overcome its recalcitrance. In recent years, a variety of ionic liquids (ILs) have been found to be effective solvents for cellulose, providing a new, feasible pretreatment strategy. A lot of experimental and computational studies have been carried out to investigate the dissolution mechanism. However, many details are not fully understood, which highlights the necessity to overview the current knowledge of cellulose dissolution and identify the research trend in the future. This perspective summarizes the mechanistic studies and microscopic insights of cellulose dissolution in ILs. Recent investigations of the synergistic effect of cations/anions and the distinctive structural changes of cellulose microfibril in ILs are also reviewed. Besides, understanding the factors controlling the dissolution process, such as the structure of anions/cations, viscosity of ILs, pretreatment temperature, heating rate, etc., has been discussed from a structural and physicochemical viewpoint. At the end, the existing problems are discussed and future prospects are given. We hope this article would be helpful for deeper understanding of the cellulose dissolution process in ILs and the rational design of more efficient and recyclable ILs.

NMR and Rheological Study of Anion Size Influence on the Properties of Two Imidazolium-based Ionic Liquids

NMR self-diffusion and relaxation, coupled with viscosity, were used to study the properties and structure of two imidazolium-based ionic liquids, 1-ethyl-3-methylimidazolium acetate [C₂MIM][OAc] and 1-ethyl-3-methylimidazolium octanoate [C₂MIM][OOct]. The experimental results point to the formation of different types of aggregates in each ionic liquid. These aggregates are small and stable under flow and temperature in [C₂MIM][OAc], whereas the aggregates are large and sensitive to flow and temperature in [C₂MIM][OOct]. In the latter case the size of aggregates decreases both under flow and temperature increase.

Combining random walk and regression models to understand solvation in multi-component solvent systems

Polysaccharides, such as cellulose, are often processed by dissolution in solvent mixtures, e.g. an ionic liquid (IL) combined with a dipolar aprotic co-solvent (CS) that the polymer does not dissolve in. A multi-walker, discrete-time, discrete-space 1-dimensional random walk can be applied to model solvation of a polymer in a multi-component solvent mixture. The number of IL pairs in a solvent mixture and the number of solvent shells formable, x, is associated with n, the model time-step, and N, the number of random walkers. The mean number of distinct sites visited is proportional to the amount of polymer soluble in a solution. By also fitting a polynomial regression model to the data, we can associate the random walk terms with chemical interactions between components and probe where the system deviates from a 1-D random walk. The 'frustration' between solvents shells is given as ln x in the random walk model and as a negative IL:IL interaction term in the regression model. This frustration appears in regime II of the random walk model (high volume fractions of IL) where walkers interfere with each other, and the system tends to its limiting behaviour. In the low concentration regime, (regime I) the solvent shells do not interact, and the system depends only on IL and CS terms. In both models (and both regimes), the system is almost entirely controlled by the volume available to solvation shells, and thus is a counting/space-filling problem, where the molar volume of the CS is important. Small deviations are observed when there is an IL-CS interaction. The use of two models, built on separate approaches, confirm these findings, demonstrating that this is a real effect and offering a route to identifying such systems. Specifically, the majority of CSs - such as dimethylformide - follow the random walk model, whilst 1-methylimidazole, dimethyl sulfoxide, 1,3-dimethyl-2-imidazolidinone and tetramethylurea offer a CS-mediated improvement and propylene carbonate results in a CS-mediated hindrance. It is shown here that systems, which are very complex at a molecular level, may, nonetheless, be effectively modelled as a simple random walk in phase-space. The 1-D random walk model allows prediction of the ability of solvent mixtures to dissolve cellulose based on only two dissolution measurements (one in neat IL) and molar volume.

Pretreatment of eucalyptus with recycled ionic liquids for low-cost biorefinery

It is urgent to develop recycled ionic liquids (ILs) as green solvents for sustainable biomass pretreatment. The goal of this study is to explore the availability and performance of reusing 1-allyl-3-methylimidazolium chloride ([amim]Cl) and 1-butyl-3-methylimidazolium acetate ([bmim]OAc) for pretreatment, structural evolution, and enzymatic hydrolysis of eucalyptus. Cellulose enzymatic digestibility slightly decreased with the increased number of pretreatment recycles. The hydrolysis efficiencies of eucalyptus pretreated via 4th recycled ILs were 54.3% for [amim]Cl and 72.8% for [bmim]OAc, which were 5.0 and 6.7-folds higher than that of untreated eucalyptus. Deteriorations of ILs were observed by the relatively lower sugar conversion and lignin removal from eucalyptus after 4th reuse. No appreciable changes in fundamental framework and thermal stability of [amim]Cl were observed even after successive pretreatments, whereas the anionic structure of [bmim]OAc was destroyed or replaced. This study suggested that the biomass pretreatment with recycled ILs was a potential alternative for low-cost biorefinery.

Understanding the role of water in the interaction of ionic liquids with wood polymers

Hemicellulose lean pulps are a raw material source for numerous high value products. We have previously presented the IONCELL-P(ulp) process, a hemicellulose extraction method, based on a binary mixture of ionic liquid and water. The IONCELL-P process does not suffer from yield losses or polymer degradation and retains the Cellulose I crystalline form. In this paper, a selection of cellulose dissolving ionic liquids is tested, in order to compare their applicability in the process. We demonstrate that the extraction selectivity towards low molar mass polymers is related to the anion's ability to accept hydrogen bonds (Kamlet-Taft β-value), if divided by the water molar fraction of the solvent system. Pulp consistency, solvent system viscosity and pH are investigated in order to identify the factors affecting the extraction efficiency. The results show that all the tested ionic liquid-water mixtures were able to dissolve hemicelluloses, but there were differences in their efficiency, selectivity and the ability to process high pulp consistencies.

Efficiency of hydrophobic phosphonium ionic liquids and DMSO as recyclable cellulose dissolution and regeneration media

Hydrophobic, long-chain tetraalkylphosphonium acetate salts (ionic liquids) were combined with DMSO and the feasibility of these solvent systems for cellulose dissolution and regeneration was studied.

Anion Bridging-Induced Structural Transformation of Cellulose Dissolved in Ionic Liquid

We performed structural investigations of cellulose mixed with 1-ethyl-3-methylimidazolium acetate ([Emim][OAc]) in the entire concentration range (0-100 mol %) by wide-angle X-ray scattering with the aid of quantum chemical calculations and ¹³C solid-state NMR spectroscopy. We particularly focused on a highly concentrated region (≥30 mol %), which has previously been overlooked. At concentrations of 15-30 mol %, a periodic peak corresponding to cellulose chain alignment emerged; this is associated with a lyotropic cholesteric liquid-crystalline phase. At concentrations of ≥30 mol %, the structure is transformed into ordered layers where OAc anions and Emim cations intercalate. This transformation is found to be driven by a change in the interaction between the IL anions and the OH groups of cellulose. At low concentrations, the anion mainly interacts with the OH group of cellulose in a 1:1 ratio, as previously reported; at high concentrations, the anions bridge the OH groups of two cellulose chains.

Why are some cyano-based ionic liquids better glucose solvents than water?

Among different classes of ionic liquids (ILs), those with cyano-based anions have been of special interest due to their low viscosity and enhanced solvation ability for a large variety of compounds. Experimental results from this work reveal that the solubility of glucose in some of these ionic liquids may be higher than in water - a well-known solvent with enhanced capacity to dissolve mono- and disaccharides. This raises questions on the ability of cyano groups to establish strong hydrogen bonds with carbohydrates and on the optimal number of cyano groups at the IL anion that maximizes the solubility of glucose. In addition to experimental solubility data, these questions are addressed in this study using a combination of density functional theory (DFT) and molecular dynamics (MD) simulations. Through the calculation of the number of hydrogen bonds, coordination numbers, energies of interaction and radial and spatial distribution functions, it was possible to explain the experimental results and to show that the ability to favorably interact with glucose is driven by the polarity of each IL anion, with the optimal anion being dicyanamide.