Role of the Hofmeister series in the formation of ionic-liquid-based aqueous biphasic systems

Liquid-Liquid Phase Separation of Aqueous Ionic Liquids in Covalent Organic Frameworks for Thermal Switchable Proton Conductivity

Covalent organic frameworks (COFs) have regular channels that can accommodate guest molecules to provide highly conductive solid electrolytes. However, designing smart, conductive COFs remains a great challenge. Herein, we report the first example of PEG-functionalized ionic liquids (ILs) anchored on the COF walls by strong hydrogen bonding to fabricate thermally responsive COFs (ILm@COF). We found that similar to the traditional IL/water mixture, the ILs undergo lower critical solution temperature (LCST)-type phase behavior within COF nanopores under high moisture levels. However, the phase separation temperature of aqueous IL decreases in COF channels due to the strong interaction between the IL and COF. Thus, the proton conductivity of ILm@COF can be reversibly switched by phase miscibility and separation in COF nanopores, and there is no obvious decrease even after 20 switching cycles. Our work provides important clues for understanding liquid-liquid phase separation in a confined nanospace and opens a new pathway to switchable proton conductivity.

Efficiently Removing Heavy Metals from High-Salinity Wastewater via Ionic Liquid-Based Aqueous Biphasic Systems

For the separation of metal ions, ionic liquid-based aqueous biphasic systems (IL-ABSs) offer a promising alternative to solvent extraction. However, the incorporation of an extensive quantity of inorganic salts restricts their practical application. Because heavy metal wastewater often contains high concentrations of inorganic salts, it offers good prospects for the application of IL-ABSs in the separation of heavy metals. In this work, an IL-ABS was formed by tributyltetradecylphosphonium chloride ([P44414]Cl), and simulated high-salinity wastewater (NaCl and Na2SO4 as the main inorganic salts) was used for the separation of heavy metals. The phase diagram results indicated that the formation of a two-phase system required a relatively high salt concentration. The extraction process followed the mechanism of anion exchange; thus, heavy metals such as zinc and cadmium that formed complexes with chloride ions could be effectively extracted (extraction rate >99.5%) with a very fast rate (extraction time <1 min) at a wide pH range (pH = 2-7). After extraction, the metals could be stripped well (stripping rate >99.5%) after contact with the NaOH solution. This research provided a new approach for treating heavy metals in high-salinity effluents, which has the advantages of IL-ABS and avoids the disadvantages of adding large amounts of inorganic salts at the same time.

Outstanding Performance of the Deep Eutectic Solvent-Based Aqueous Biphasic System Constructed with Sodium Citrate for a Green Gold Separation

Aqueous biphasic systems (ABSs) that are based on deep eutectic solvents (DESs) are environmentally benign systems to use for metal ion separation. In this work, a series of DESs was synthesized for the first time with PEG 400 as hydrogen bond donors and tetrabutylphonium bromide (P₄Br), tetrabutylammonium bromide (N₄Br), or tetrabutylammonium chloride (N₄Cl) as hydrogen bond acceptors, and then they were combined with citrate (Na₃C₆H₅O₇), which is eco-friendly, to construct an ABS for use in the separation of Au(I) from an aurocyanide solution. Phase diagrams of DESs + Na₃C₆H₅O₇ + H₂O systems were constructed using the experimentally measured data. Multiple factors that affect the efficiency of the gold extraction were studied; these factors were the species of salt or DES and their content, the equilibrium pH, the oscillation time, and the initial gold concentration. Gold(I) is preferentially retained in the DES-rich phase, and the P₄Br:PEG 1:2 + Na₃C₆H₅O₇ + H₂O system has a high extraction efficiency of 100.0% under optimized conditions. FT-IR, NMR, and TEM characterizations and DFT calculations show that the migration of Au(I) from the salt-rich to the DES-rich phase follows an ion exchange mechanism. Specifically, Au(CN)₂^- replaces Br^- in the original P₄Br and generates a stable ion pair with the quaternary phosphonium salt cation, P⁺, and this replacement is driven by electrostatic attractions. A new strong hydrogen bond network simultaneously forms between the anionic Au(CN)₂^- and the -OH group in the PEG 400 component. Finally, the gold of Au(I)-loaded P₄Br:PEG 1:2 can be successfully reduced by sodium borohydride with an efficiency of 100.0%. The strategy to extract gold(I) from alkaline cyanide solutions using an ABS based on DESs as proposed in this work provides a potential platform for developing green technology for recovering gold.

Phase Behavior of Ionic Liquid-Based Aqueous Two-Phase Systems

As an environmentally friendly separation medium, the ionic liquid (IL)-based aqueous two-phase system (ATPS) is attracting long-term attention from a growing number of scientists and engineers. Phase equilibrium data of IL-based ATPSs are an important basis for the design and optimization of chemical reactions and separation processes involving ILs. This article provides the recent significant progress that has been made in the field and highlights the possible directions of future developments. The effects of each component (such as salting-out agents and ILs) on the phase behavior of IL-based ATPSs are summarized and discussed in detail. We mainly focus on the phase behavior of ATPSs by using ILs, expecting to provide meaningful and valuable information that may promote further research and application.

Thermoresponsive Ionic Liquid/Water Mixtures: From Nanostructuring to Phase Separation

The thermodynamics, structures, and applications of thermoresponsive systems, consisting primarily of water solutions of organic salts, are reviewed. The focus is on organic salts of low melting temperatures, belonging to the ionic liquid (IL) family. The thermo-responsiveness is represented by a temperature driven transition between a homogeneous liquid state and a biphasic state, comprising an IL-rich phase and a solvent-rich phase, divided by a relatively sharp interface. Demixing occurs either with decreasing temperatures, developing from an upper critical solution temperature (UCST), or, less often, with increasing temperatures, arising from a lower critical solution temperature (LCST). In the former case, the enthalpy and entropy of mixing are both positive, and enthalpy prevails at low T. In the latter case, the enthalpy and entropy of mixing are both negative, and entropy drives the demixing with increasing T. Experiments and computer simulations highlight the contiguity of these phase separations with the nanoscale inhomogeneity (nanostructuring), displayed by several ILs and IL solutions. Current applications in extraction, separation, and catalysis are briefly reviewed. Moreover, future applications in forward osmosis desalination, low-enthalpy thermal storage, and water harvesting from the atmosphere are discussed in more detail.

Aqueous Biphasic Systems Comprising Natural Organic Acid-Derived Ionic Liquids

Despite the progress achieved by aqueous biphasic systems (ABSs) comprising ionic liquids (ILs) in extracting valuable proteins, the quest for bio-based and protein-friendly ILs continues. To address this need, this work uses natural organic acids as precursors in the synthesis of four ILs, namely tetrabutylammonium formate ([N4444][HCOO]), tetrabutylammonium acetate ([N4444][CH3COO]), tetrabutylphosphonium formate ([P4444][HCOO]), and tetrabutylphosphonium acetate ([P4444][CH3COO]). It is shown that ABSs can be prepared using all four organic acid-derived ILs paired with the salts potassium phosphate dibasic (K2HPO4) and tripotassium citrate (C6H5K3O7). According to the ABSs phase diagrams, [P4444]-based ILs outperform their ammonium congeners in their ability to undergo liquid–liquid demixing in the presence of salts due to their lower hydrogen-bond acidity. However, deviations to the Hofmeister series were detected in the salts’ effect, which may be related to the high charge density of the studied IL anions. As a proof of concept for their extraction potential, these ABSs were evaluated in extracting human transferrin, allowing extraction efficiencies of 100% and recovery yields ranging between 86 and 100%. To further disclose the molecular-level mechanisms behind the extraction of human transferrin, molecular docking studies were performed. Overall, the salting-out exerted by the salt is the main mechanism responsible for the complete extraction of human transferrin toward the IL-rich phase, whereas the recovery yield and protein-friendly nature of these systems depend on specific “IL-transferrin” interactions.

Assessing the hydrotropic effect in the presence of electrolytes: competition between solute salting-out and salt-induced hydrotrope aggregation

Water solubility enhancement is a long-standing challenge in a multitude of chemistry-related fields. Hydrotropy is a simple and efficient method to improve the solubility of hydrophobic molecules in aqueous media....

Current Topics in Ionic Liquid Crystals

Ionic liquid crystals (ILCs), that is, ionic liquids exhibiting mesomorphism, liquid crystalline phases, and anisotropic properties, have received intense attention in the past years. Among others, this is due to their special properties arising from the combination of properties stemming from ionic liquids and from liquid crystalline arrangements. Besides interesting fundamental aspects, ILCs have been claimed to have tremendous application potential that again arises from the combination of properties and architectures that are not accessible otherwise, or at least not accessible easily by other strategies. The current review highlights recent developments in ILC research, starting with some key fundamental aspects. Further subjects covered include the synthesis and variations of modern ILCs, including the specific tuning of their mesomorphic behavior. The review concludes with reflections on some applications that may be within reach for ILCs and finally highlights a few key challenges that must be overcome prior and during true commercialization of ILCs.

Preparation of a novel environmentally friendly hydrophobic deep eutectic solvent ChCl-THY and its application in removal of hexavalent chromium from aqueous solution

A liquid-liquid extraction methodology was developed for the removal of Cr(VI) from contaminated water using a novel green hydrophobic deep eutectic solvent (DES) as an efficient sole extracting agent. The hydrophobic DES was obtained by mixing choline chloride and thymol in 1:4 molar ratio at 70°C for 10 min and was denoted as ChCl-THY(1:4). The ChCl-THY(1:4) works efficiently for removal of high (20 mg/L) and low (500 µg/L) concentration of Cr(VI) from artificially contaminated natural water with >95% extraction efficiency (E%) at optimized reaction conditions (pH 2-6, 40°C). The DES was characterized by ¹ H NMR and FTIR spectroscopy, and the data suggest that interaction occurs between Cl^- ion of choline chloride and H atoms of thymol molecules. Physicochemical properties such as density, melting point, moisture, and solubility were studied and discussed. Herein, no sharp melting point was observed for ChCl-THY(1:4) in DSC curve. DES was regenerated using 0.1 M NaOH as stripping agent, and 50%-60% extraction efficiency could be attained in the next cycle. A plausible mechanism of interaction between Cr(VI) species and DES was also explored with the help of FTIR spectroscopy. PRACTITIONER POINTS: A novel hydrophobic DES (ChCl-THY) is prepared by mixing choline chloride and thymol at 1:4 molar ratio. ChCl-THY(1:4) is employed for the first time as sole extracting agent to remove the Cr(VI) from contaminated aqueous solution. >95% extraction efficiency was achieved by ChCl-THY(1:4) in natural water conditions at µg/L and mg/L level of contamination. Both the component used to prepare the DES are naturally abundant; hence, DES is not toxic for biota. The element present in natural water did not show any interference with extraction of Cr(VI).

Elgharbawy A, Khezri R, Wilfred CD. Prediction of Potential Ionic Liquids (ILs) for the Solid-Liquid Extraction of Docosahexaenoic Acid (DHA) from Microalgae Using COSMO-RS Screening Model

This study performs a screening of potential Ionic Liquids (ILs) for the extraction of Docosahexaenoic Acid (DHA) compounds by the calculation of capacity values. For this purpose, a Conductor-Like Screening Model for Real Solvents (COSMO-RS) was employed to study the molecular structures of the ILs, and therefore, predict their extraction potential. The capacity values of 22 anions combined with 16 cations based ILs, were investigated to evaluate the effectiveness of ILs in the extraction of DHA. It was found that among the investigated ILs, a combination of tetramethyl ammonium with SO₄ or Cl was the best fit for DHA extraction, followed by pyrrolidinium, imidazolium, pyridinium and piperidinium. Furthermore, it was observed that the extraction capacity and the selectivity of ILs decreased with an increase in alkyl chain length; therefore, ethyl chain-ILs, with the shortest chain lengths, were found to be most suitable for DHA extraction. The predicted results were validated through the experimentally calculated extraction yield of a DHA compound from Nannochloropsis sp. Microalgae. Five selected ILs, namely [EMIM][Cl], [BMIM][Cl], [TMAm][Cl], [EMPyr][Br] and [EMPyrro][Br], were selected from COSMO-RS for empirical extraction purposes, and the validation results pinpointed the good prediction capabilities of COSMO-RS. The findings in this study can simplify the process of selecting suitable ILs for DHA extraction and reduce the number of required empirical evaluations.

A Novel Approach in Sorting Chirality Species of Single-Wall Carbon Nanotubes Based on an Aqueous Two-Phase System of Polymer-Salt

Sorting of distinct (n, m) chirality species of single-wall carbon nanotubes (SWCNTs) is essential for progress in technical applications in the field of electronic and optic devices. The purpose of this study is to investigate the isolation of single-wall carbon nanotubes based on diameters/chirality in a polymer-salt (polyethylene glycol and sodium citrate) aqueous two-phase system (ATPS) a substitute for common polymer-polymer (polyethylene glycol and dextran) system. The ATPS based on polymer-salt used instead of the common polymer-polymer system due to low viscosity, reduced surface tension, and lower cost of sodium citrate compared to the dextran. For this purpose, the ratio of concentrations of polyethylene glycol to sodium citrate as well as the effect of temperature on the isolation are both investigated and the selectivity and the recovery estimated approximately. The absorbance spectra from both top and bottom phases at different polymer and salt contents and at different temperatures show that by using this system in optimal conditions of polymer to salt ratio of 2:1 at temperature of 20 °C, a suitable separation of nanotubes with 85% yield of the chiral groups of 9 and 10 can be obtained.

Exploring ion-ion preferences through structure-property correlations: amino acid-derived, bis(guanidinium) disiloxane salts

In a more synthetical approach to the study of ion-specific phenomena, four dipodal bis(guanidinium) siloxanes have been synthesized starting from glycine, β-alanine, γ-aminobutanoic acid, L-proline and 1,3-bis(3-aminopropyl)tetramethyldisiloxane. Together with their non-amide progenitor they were comparatively studied in regards to their interactions with nine different anions: sulphate, chromate, molybdate, benzoate, chloride, azide, nitrite, nitrate and thiocyanate. Their aqueous solubilities, form, 1H NMR and FT-IR spectra were examined while searching for anion-specific interactions falling in- or outside of the Hofmeister series. We show that although the "chao-" and "kosmotropic" ions affect the properties of solutions in a predictable way, more selective cation-anion pairing is responsible for phase separation and crystallinity. As a prominent example, crystal structure of one of the benzoate salts was successfully obtained and reveals a synergy of hydrophobic packing, ionic and hydrogen bonding. Immobilized but still flexible siloxane bridges give rise to crystals described by P 42/n space group and neatly segregated into hydro- and lipophilic sections.

Binary Mixtures of Ionic Liquids in Aqueous Solution: Towards an Understanding of their Salting-In/Salting-Out Phenomena

The order of the salting-in or salting-out inducing ability of ions on the aqueous solubility of macromolecules in aqueous solutions is known as the Hofmeister series. Taking into account that ionic liquids (ILs) are constituted by ions, they can exert similar effects on the solubility of other ILs in aqueous media. In order to expand the knowledge on the salting-in/-out ability of ILs, experimental studies on the solubility of 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonylimide) in water in presence of other IL/salts were conducted at 298.15 K at atmospheric pressure. Both the impact of the anion and cation of the IL were evaluated with the following ILs/salts: 1-butyl-3-methylimidazolium chloride, 1-butyl-3-methylimidazolium hydrogensulfate, cholinium bis(trifluoromethylsulfonyl)imide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide and lithium bis(trifluoromethylsulfonyl)imide, in a wide composition range. As happens with common salts, both salting-in and salting-out effects exerted by ILs were observed, with a higher impact exerted by the IL anion on the salting-out phenomenon. These data allow to better understand the ILs impact when designing liquid-liquid separation processes.

Characterization of Ionic Liquid Aqueous Two-Phase Systems: Phase Separation Behaviors and the Hydrophobicity Index between the Two Phases

1-Allyl-3-methylimidazolium chloride [Amim][Cl] and 1-butyl-3-methylimidazolium chloride [Bmim][Cl] are water-soluble ionic liquids (ILs) that can from an aqueous two-phase system (ATPS) when mixed with specific salts. Herein, we prepared [Amim][Cl]- and [Bmim][Cl]-ATPSs by adding the salts (K₂CO₃, K₂HPO₄). To investigate the phase separation behavior of the IL-ATPSs, binodal curves were drawn at different temperatures and the length and slope of the tie lines were analyzed. The [Bmim][Cl]/K₂HPO₄ system underwent two-phase separation at lower temperature conditions, suggesting that the phase separation might depend on the salting-out effect in the bottom phase. Using the IL-ATPS, the distribution coefficients, K_aa, of amino acids were determined and used to characterize the hydrophobicity index (HF) between the top and bottom phases, which is a good indicator to understand the molecular partitioning behaviors in conventional ATPSs. The HF values of the IL-ATPSs were in the range 0.13-0.41 mol/kJ; these values were almost the same as the HF values reported for an ATPS composed of poly(ethylene glycol) and salt.

Utilization of inorganic salts as adjuvants for ionic liquid-water pretreatment of lignocellulosic biomass: enzymatic hydrolysis and ionic liquid recycle

Pretreatment of lignocellulosic biomass with ionic liquids (ILs) for the large-scale biorefinery remains challenging due to its high price. This study focused on the utilization of inorganic salts as adjuvants for ionic liquid-water pretreatment to improve the tolerance to water and the reusability of the ILs. After the pretreatment of rice straw by the mixture of 40% 1-ethyl-3-methylimidazolium chloride ([C2mim]Cl) + 53% water + 7% K2CO3 at 110 °C for 1 h, the residues became highly susceptible to enzymatic hydrolysis; 93.70% of lignin was removed, and 92.07% sugar yield was achieved. [C2mim]Cl-K2CO3 aqueous biphasic system was formed at room temperature when K2CO3 concentration increased to more than 30%, and the [C2mim]Cl recovery of 94.32% was achieved. The results indicate that the addition of inorganic salts to IL aqueous solutions can significantly reduce the cost of IL pretreatment, while maintaining an efficient enzymatic hydrolysis of lignocellulosic biomass.

Ionic Liquid Aqueous Two-Phase Systems From a Pharmaceutical Perspective

Aqueous Two-Phase Systems (ATPSs) have been extensively studied for their ability to simultaneously separate and purify active pharmaceutical ingredients (APIs) and key intermediates with high yields and high purity. Depending on the ATPS composition, it can be adapted for the separation and purification of cells, nucleic acids, proteins, antibodies, and small molecules. This method has been shown to be scalable, allowing it to be used in the milliliter scale for early drug development to thousands of liters in manufacture for commercial supply. The benefits of ATPS in pharmaceutical separations is increasingly being recognized and investigated by larger pharmaceutical companies. ATPSs use identical instrumentation and similar methodology, therefore a change from traditional methods has a theoretical low barrier of adoption. The cost of typical components used to form an ATPS at large scale, particularly that of polymer-polymer systems, is the primary challenge to widespread use across industry. However, there are a few polymer-salt examples where the increase in yield at commercial scale justifies the cost of using ATPSs for macromolecule purification. More recently, Ionic Liquids (ILs) have been used for ATPS separations that is more sustainable as a solvent, and more economical than polymers often used in ATPSs for small molecule applications. Such IL-ATPSs still retain much of the attractive characteristics such as customizable chemical and physical properties, stability, safety, and most importantly, can provide higher yield separations of organic compounds, and efficient solvent recycling to lower financial and environmental costs of large scale manufacturing.

Screening of Suitable Ionic Liquids as Green Solvents for Extraction of Eicosapentaenoic Acid (EPA) from Microalgae Biomass Using COSMO-RS Model

Omega-3 poly unsaturated fatty acids (PUFA) particularly eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), have many health benefits including reducing the risk of cancer and cardiovascular disease. Recently, the use of ionic liquids (ILs) in lipid extraction from microalgae provides the potential to overcome common drawbacks, offers several other benefits. To date, very limited researches are available to focus on extracting microalgae lipid and PUFA in particular by using ILs. The objective of current work is to screen the potential ILs that can be applied in EPA extraction. In this study, fast ILs screening was performed with the help of a conductor like screening model for real solvents (COSMO-RS) and the ILs with higher capacity values for use in extraction of EPA were compared. According to the results, the highest capacity for EPA extraction among 352 screened cation/anion combinations belongs to [TMAm][SO₄]. It is expected to achieve a higher yield of EPA once applying this combination as the solvent in the process of extraction. ILs with small anions were observed to have higher capacities, as well possessing higher charge density compared to larger ones, and therefore, they are more preferable for extraction purposes. Moreover, shorter alkyl chain cations are preferred when using imidazolium-based IL, which agrees with experimental data.

Understanding the Effect of Ionic Liquids as Adjuvants in the Partition of Biomolecules in Aqueous Two-Phase Systems Formed by Polymers and Weak Salting-Out Agents

Ionic liquids (ILs) as adjuvants in polymer-salt aqueous two-phase systems (ATPS) have been used to improve the extraction of biomolecules. However, the impact of ILs as adjuvants on the partition of biomolecules is still poorly understood. Previous works mostly focused on ATPS based on strong salting-out agents, which may mask the IL effect. In this work, ATPS formed by polyethylene glycol (PEG 400) and a weak salting-out salt ((NH₄)₂SO₄) with a wide number of ILs as adjuvants (chloride-based combined with cholinium, imidazolium, pyrrolidinium, piperidinium, tetralkylammonium and tetralkylphosphonium cations) were investigated. The respective phase diagrams were determined, and the systems extraction performance for a wide range of biomolecules (phenolic compounds, alkaloids and amino acids) was investigated. The results obtained show that ILs as adjuvants in polymer-salt ATPS modulate the partition of biomolecules. In particular, more hydrophobic ILs significantly enhance the partition of more hydrophobic biomolecules to the PEG-rich phase (where the IL is enriched). Furthermore, the intensity of the IL effect is more pronounced when using weak salting-out agents. A linear correlation between the biomolecules and the ILs partition coefficients, and with the biomolecules octanol-water partition coefficients, was found. In most ATPS formed by polymers and salts using ILs as adjuvants, the biomolecules partition is driven by the ILs partition and by the difference in hydrophobicity between the coexisting phases.