Physicochemical Properties Of Ionic Liquids Research Articles

Advanced Uranium Removal with Pure, Nonfluorinated Ionic Liquids: Fine-Tuning Hydrophilic and Hydrophobic Properties for Enhanced Selectivity.

Ionic liquids (ILs) have significant potential for eco-friendly extraction of uranium from aqueous solutions, which is critical for nuclear technology, fuel cycle management, and environmental protection. This study examines the impact of the adjustable hydrophobic/hydrophilic properties of ILs on the removal of uranium(VI) (UO22+) from aqueous solutions utilizing both a novel hydrophilic IL (1-butoxyethyl-1-methylmorpholinium butoxyethylphosphite - Mor1-2O4-BOEP) and 1-heptyl-1-methylmorpholinium heptylphosphite (Mor1-7-HP) as an example of a hydrophobic IL with a similar structure. The transfer mechanism of uranyl ions from water to organic or solid phases closely depends on the physicochemical properties of ILs, especially their hydrophobicity. The hydrophobic Mor1-7-HP extracts uranyl via neutral complex formation as UO2(NO3)2-(Mor1-7-HP)2. Conversely, hydrophilic Mor1-2O4-BOEP induced selective precipitation as UO2(NO3)-(BOEP), transferring uranyl to the solid phase. Optimization of the working parameters, in terms of acidity of the aqueous solution and amount of ILs used, allowed the extraction of over 98% of U(VI). The stoichiometry of the organic complex and the precipitate was determined using physicochemical techniques. These tunable H-phosphonate-based ILs have advantages over traditional solvent extraction and conventional ILs, allowing easier handling, improved selectivity, and lower environmental impact. This work advances uranium separation techniques with applications in hydrometallurgy, particularly in the treatment of wastewater and radioactive waste for sustainable uranium recovery.

Physicochemical properties of ionic liquids with indole removal function

Indole exists in coal tar, gasoline, diesel oil and other oil products, in the process of using oil products, will cause serious pollution to the environment, so it is necessary to remove indole from oil products. In this paper, 1-hydroxy-3-methylimidazolserine was synthesized and characterized. Its density, surface tension, and refractive index were measured every 5 K in the range of 293.15–333.15 K using standard addition method. The relationships between temperature and water content were discussed. The mass change with time was measured by thermogravimetric analyzer, and its evaporation enthalpy was discussed. Furthermore, the polarity was predicted using these data. Finally, it was used to separate indole from simulated oil. A simulated oil solution containing 0.2 g/L indole was extracted under conditions of a mass ratio of 1:5 between ionic liquid and simulated oil, temperature of 30 °C, and time of 60 min. The extraction rate reached 88.9 %, indicating that the prepared ionic liquid can be used as an extractant for removing indole from simulated oil. In addition, the extraction mechanism was also investigated, and it was found that the indole was successfully extracted from the simulated oil because of the hydrogen bond between the indole and the ionic liquid.

Molecular Interactions between Ionic Liquid Lubricants and Silica Surfaces: An MD Simulation Study.

The unique physicochemical properties of ionic liquids (ILs) attracted interest in their application as lubricants of micro/nano-electromechanical systems. This work evaluates the feasibility of using the protic ionic liquids [4-picH][HSO4], [4-picH][CH3SO3], [MIMH][HSO4], and [MIMH][CH3SO3] and the aprotic ILs [C6mim][HSO4] and [C6mim][CH3SO3] as additives to model lubricant poly(ethylene glycol) (PEG200) to lubricate silicon surfaces. Additives based on the cation [4-picH]+ exhibited the best tribological performance, with the optimal value for 2% [4-picH][HSO4] in PEG200 (w/w). Molecular dynamics (MD) simulations of the first stages of adsorption of the ILs at the glass surface were performed to portray the molecular behavior of the ILs added to PEG200 and their interaction with the silica substrate. For the pure ILs at the solid substrates, the MD results indicated that weak specific interactions of the cation with the glass interface are lost to accommodate the larger anion in the first contact layer. For the PEG200 + 2% [4-picH][HSO4] system, the formation of a more compact protective film adsorbed at the glass surface is revealed by a larger trans population of the dihedral angle -O(R)-C-C-O(R)- in PEG200, in comparison to the same distribution for the pure model lubricant. Our findings suggest that the enhanced lubrication performance of PEG200 with [4-picH][HSO4] arises from synergistic interactions between the protic IL and PEG200 at the adsorbed layer.

Supported Ionic Liquid Membrane with Highly-permeable Polyamide Armor by In Situ Interfacial Polymerization for Durable CO2 Separation.

Supported ionic liquid membranes (SILMs), owing to their capacities in harnessing physicochemical properties of ionic liquid for exceptional CO2 solubility, have emerged as a promising platform for CO2 extraction. Despite great achievements, existing SILMs suffer from poor structural and performance stability under high-pressure or long-term operations, significantly limiting their applications. Herein, a one-step and in situ interfacial polymerization strategy is proposed to elaborate a thin, mechanically-robust, and highly-permeable polyamide armor on the SILMs to effectively protect ionic liquid within porous supports, allowing for intensifying the overall stability of SILMs without compromising CO2 separation performance. The armored SILMs have a profound increase of breakthrough pressure by 105% compared to conventional counterparts without armor, and display high and stable operating pressure exceeding that of most SILMs previously reported. It is further demonstrated that the armored SILMs exhibit ultrahigh ideal CO2 /N2 selectivity of about 200 and excellent CO2 permeation of 78 barrers upon over 150h operation, as opposed to the full failure of CO2 separation performance within 36h using conventional SILMs. The design concept of armor provides a flexible and additional dimension in developing high-performance and durable SILMs, pushing the practical application of ionic liquids in separation processes.

In-depth analysis of the effect of physicochemical properties of ionic liquids on anti-icing behavior of silicon based-coatings

This study dissects the use of room temperature ionic liquids (RT-ILs) in a dynamic surface for anti-icing applications. Anti-icing properties of ILs-containing PDMS-based coatings fabricated by two different types of ILs, are evaluated to establish a relationship between the anti-icing behavior and physicochemical properties of the ILs. The surface chemistry of coatings and the existence of distinctive groups of the ILs anions on the surface of coatings characterized by Attenuated Total Reflectance-Fourier transform infrared (FTIR) spectroscopy (ATR-FTIR) and confirmed by using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy-energy dispersive X-ray analysis (SEM/EDX). The 3D profiles and roughness parameter of the surfaces were investigated by a confocal laser microscopy profiler. Regarding the importance of correlation between wettability and ice adhesion, the wettability of coatings was studied. The results show a significant reduction 30–40% in sliding angle (SA) and contact angle hysteresis (CAH) in ILs-containing coatings. Differential scanning calorimetry (DSC), freezing delay time, push-off and centrifugal adhesion tests (CAT) served to distinguish the performance of the two ILs in regard to ice nucleation temperature and formation time, and ice adhesion strength, respectively. The 60–70% reduction in ice adhesion strength for the IL-containing coatings, obtained by push-off test, is due to the presence of strong ionic hydrogen bounded water and the formation of self-lubricating quasi liquid layer that can function at much lower temperature. Solid-state nuclear magnetic resonance (NMR) spectroscopy has confirmed the presence of nonfrozen water molecules at the interface. Consequently, incorporating ILs into coatings provides an effective approach to delay ice nucleation, restrict ice growth recrystallization and reduce ice adhesion strength. A comparison of enhanced anti-icing performance of ILs-containing coatings highlights the significant influence of minor changes in the chemical structure of ILs on their potential for anti-icing applications.

Preparation and Physicochemical Characterization of Phosphonium-Based Ionic Liquids Containing Carboxylate Anions

Ionic liquids (ILs) are organic salts composed of cation and anion with melting points below 100°C. They have unique characteristic features.1) ILs are also promising liquid materials available for various applications because their function can be easily controlled by changing the combination of cations and anions and by introducing substituents. Among the numerous ILs, considerable ILs based on carboxylate anions have been reported. On the other hand, several published papers reported ILs based on quaternary phosphonium cations, demonstrating relatively high transport properties and thermal stability2). We report here the preparation and physicochemical properties of ILs based on triethyloctylphosphonium (P2228) and tributyloctylphosphonium (P4448) cations together with various carboxylate anions.The intermediate hydroxides were obtained by adding an anion exchange resin (Amberlite IRN78 OH) into the precursor bromide aqueous solutions. Equimolar amounts of carboxylic acids (formic, acetic, propionic, butyric and octanoic acids) were added to the hydroxide solutions to form carboxylate-based phosphonium salts. The salts obtained were isolated by evaporation and then were dried under high vacuum at 50 °C. The physicochemical properties of ILs, e.g. density, viscosity, conductivity and thermal decomposition temperature were measured.Table 1 shows the physicochemical properties of the ILs at 25 °C. P2228-PrCO2 became a solid at room temperature, while P2228-HCO2, P2228-MeCO2, and P2228-EtCO2 became viscous liquids at room temperature. Furthermore, all P4448-based salts became viscous liquids at room temperature. The density of each IL was increased with decreasing the carbon numbers of the alkyl chains in carboxylate anions. The number density also showed similar tendency to the density. These results indicate that the number density of the formate was the highest since the anion size was the smallest. The viscosity of each ILs was decreased with increasing the carbon numbers of the alkyl chains in carboxylate anions. Although the details of the relationship between measured viscosity and the carbon numbers of the alkyl chains in carboxylate anions still remain unclear at present, it is thought to be the charge distribution of the anions is one of the factors. The acetate-, propionate-, butyrate-, and octanoate-based ILs showed high thermal decomposition temperatures around 270 to 300°C. However, the formate-based ionic liquid showed lower thermal stability, which might be due to the decarboxylation of the formate anion. Figure 1

Influence of anionic species on the molecular structure, nature of bonding, reactivity, and stability of ionic liquids-based on 1-butyl-3-methylimidazolium

The unique physicochemical properties of ionic liquids (ILs) have positioned them as a significant class of solvents, making them a subject of considerable interest. In this investigation, the experimental analysis involved solvothermal reflux techniques, while theoretical approaches utilized density functional theory (DFT) with the def2-SVP/ωB97XD computational approach to explore the electronic properties, reactivity, stability, and structural aspects of ILs based on the combinations of [BMIM]+ cation with [MeSO3]-, [BF4]-, [PF6]-, and [C2H3O2]- anions. The experimental and theoretical data exhibited good agreement, demonstrating that the choice of cation and anion greatly influences the stability of ILs. By employing Frontier molecular orbital analysis, the reactivity and stability of the studied ionic species were observed to follow the order of [BMIM]+[MeSO3]- > [BMIM]+[BF4]- > BMIM > [BMIM]+[PF6]- > [BMIM]+[C2H3O2]-, with corresponding energy gaps of 4.8474 > 3.1971 > 2.013 > 1.8830 and 1.3276 eV, respectively. The Natural Bond Orbital (NBO) analysis, encompassing second-order perturbation energy, revealed the strength of interactions within the investigated systems in the following order: [BMIM]+[PF6]- < [BMIM]+[MeSO3]- < [BMIM]+[BF4]- < [BMIM]+[C2H3O2]-. Topology analysis indicated a higher electron density (ρ(r)) value of 0.8966 a.u., indicating a denser distribution of electrons. Furthermore, the nature of bonding in the studied ion pairs displayed the following order: [BMIM]+[MeSO3]- > [BMIM]+[BF4]- > [BMIM]+[PF6]- > [BMIM]+[C2H3O2]-, signifying that [BMIM]+[MeSO3]- exhibited stronger interactions compared to the other combinations. The vibrational spectra and UV–vis results of the investigated ILs underscored the sensitivity of characteristic peaks in frequency and intensity to the specific cation–anion combination. These findings have the potential to guide experimental researchers in exploring the applications of the studied ionic species in diverse areas such as energy storage and catalysis.

Ligand Effect on Physicochemical Properties of Ionic Liquid.

In the solvent extraction process, the importance of an extractant (or ligand) and a diluent is inferred from their respective physicochemical properties. We have brought together all the recent results reported on the mixture of different extractants dissolved in a well-known ionic liquid diluent: 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ([C4 mim][NTf2 ]) in the form of a review and aimed to emphasize the role of ligand polarity and structure on the physicochemical properties of an ionic liquid (IL) diluent. Some of the most important properties such as dynamic viscosity (η), absolute density ( ), energy of activation (Ea ), coefficient of thermal expansion (α), phase separation time (PST), refractive index (n), etc., have been discussed meticulously in the paper. The effect of ligand structure on the aggregation behaviour of IL phase and the physicochemical properties of gamma irradiated solvent phases containing different ligands and their solution with IL phase also have been deliberated in detail.

Application and problems analysis of traditional Chinese medicine volatile oil preparation technology based on ionic liquids

Ionic liquids(ILs) are salts composed entirely of anions and cations in a liquid state at or near room temperature, which have a variety of good physicochemical properties such as low volatility and high stability. This paper mainly reviewed the research overview of ILs in the application of traditional Chinese medicine(TCM) volatile oil preparation technology. Firstly, it briefly introduced the application of TCM volatile oil preparation technology and composition classification and physicochemical properties of ILs, and then summarized the application of ILs in the extraction, separation, analysis, and preparation of TCM volatile oil. Finally, the problems and challenges of ILs in the application of TCM volatile oil were explained, and the application of ILs in TCM volatile oil in the future was prospected.

(Digital Presentation) Physicochemical Characterization of Sulfonate-Based Phosphonium Ionic Liquids

Ionic liquids (ILs) have unique physicochemical properties such as favorable solubility of organic and inorganic compounds, relatively high ionic conductivity, no measurable vapor pressure, high thermal stability, low flammability, etc. ILs are also promising liquid materials available for various applications because their function can be easily controlled by changing the combination of cations and anions and by introducing substituents. Although many kinds of ILs have already been investigated, phosphonium cation based ILs have rarely been proposed. We have previously designed and synthesized the phosphonium ILs together with typical sulfonylamide-based anions such as bis(trifluoromethylsulfonyl)amide and bis(fluorosulfonyl)amide anions.1,2) On the other hand, interests in phosphonium ILs consisting of the other anions is increasing for diverse applications. In this work, we design and prepare the ILs based on tetrabutylphosphonium cation (P4444 +) together with various sulfonate-based anions (Fig. 1), characterizing their physicochemical properties as a new family of ILs. The phosphonium IL was prepared by an aqueous neutralization reaction of tetrabutylphosphonium hydroxide with the stoichiometric amounts of various sulfonic acids. The obtained phosphonium ILs were isolated by water evaporation, and were dried in vacuo for at least 1 day. The physicochemical properties of ILs, e.g. density, viscosity, conductivity (ac impedance method) and thermal decomposition temperature (thermogravimetric analysis), were measured under argon atmosphere.The phosphonium salts based on unsubstituted sulfonate anions such as SO3CH3, SO3CH2CH3 and SO3(CH2)2CH3 were white crystalline solids at room temperature, whereas both amino- and hydroxy- substituted sulfonate salts were viscous liquids at room temperature. All phosphonium salts obtained were hydrophilic. Table 1 summarizes the physicochemical properties of phosphonium ILs based on amino- and hydroxy-substituted sulfonate anions. These phosphonium ILs exhibited considerably high viscosities when compared to the previously published viscosity value of P4444-lactate IL (415 mPa s at 25 °C)3), which suggests that the electrostatic interaction between P4444 cation and the sulfonate anions is much stronger than those in the case of carboxylate-based phosphonium ILs. It should be noted that P4444-SO3(CH2)3NH2 obviously indicated not only the highest density but also the highest viscosity and the lowest conductivity, which might be due to the fact that the IL has the largest anion to give a significant van der Waals interaction. Figure 1

Machine learning-based framework for predicting toxicity of ionic liquids

There has been an increase interest in the application of ionic liquids (ILs) in chemical, biochemical, and other industrial processes. Besides physicochemical properties of ILs, toxicity of ILs is an important parameter that must also be accessed while designing ILs for any specific applications. Consideration of toxicity levels is crucial because of the potential risks to the ecosystem and biome that come with the widespread usage of ILs. Numerous combinations of cations and anions results in a rapidly increasing new IL structures. Toxicity measurements via experiments is not only time consuming but also resource intensive. Hence, a suitable alternative is to develop a metamodel that can predict the toxicity of ILs with reasonable accuracy. The objective of this study is to use machine learning algorithms like multiple linear regression and artificial neural network (ANN) to develop a model that can aid in the prediction of toxicity for any given IL structure. Group contribution-based methods are deployed to extract relevant features for each IL, which are used as inputs and toxicity values as output to develop the metamodel. Based on our results obtained, the linear regression-based model with regularization yields a reliable and a robust metamodel. This can be used as a screening framework to design ILs with low toxicity values for targeted applications.

Micellization and aggregation approach in aqueous amphiphilic ionic liquid and anionic polymer system

In this study, we investigate the aggregation of an ionic liquid (IL), 1-allyl-3-methylimidazolium bromide ([AMIM][Br]) in aqueous solution containing variable amounts of poly(diallyldimethylammonium) chloride (PDADMAc), using conductivity, density, and viscosity techniques in combination with the molecular dynamics simulation approach. The critical micelle concentration (cmc) after micellization for IL in solutions was derived from the conductivity measurements. The effect of PDADMAc on the physicochemical properties of IL in aqueous solution was investigated using comprehensive biophysical techniques such as UV-visible absorption spectroscopy, viscosity (η) and density (ρ) measurements. With the addition of PDADMAc concentration (0, 0.25, 0.05, and 0.1 g.l-1), cmc values increased due to the interaction between IL and PDADMAc. Our UV spectroscopy results showed that the interaction between PDADMAc and [AMIM][Br] was due to the significant differences in the interactions between the amide group of the polymer, the ionic liquid ions, and the water molecules. The apparent molar volumes (V) were calculated from the experimental densities for the (AMIM)(Br)-PDADMAc systems studied. Viscosity data decreased with increasing IL concentration and increased with [PDADMAc]. A computer simulation was used to investigate the aggregate structure between (AMIM)(Br]) and PDADMAc.

Investigation of the interionic interactions and spectroscopic features of 1-Octyl-3-methylimidazolium chloride, tetrafluoroborate, and hexafluorophosphate ionic liquids: An experimental survey and DFT modeling

This study presents the spectroscopic and electronic properties of some imidazole-based ionic liquids (ILs) consisting of 1-Octyl-3-methylimidazolium cation and chloride, tetrafluoroborate, and hexafluorophosphate anions from experimental and theoretical perspectives. The ground state structural and vibrational characteristics of the ionic liquids have been achieved through Density Functional Theory (DFT) calculations at the Becke's and the Lee–Yang–Parr (B3LYP) and 6–311++G(d,p) level of theory in Gaussian 16. The electronic and magnetic features of the ILs have been examined by using electronic absorption spectra and Nuclear Magnetic Resonance spectroscopy (NMR) techniques along with the calculations from Time-Dependent Density Functional Theory (TD-DFT) and using Gauge-Including Atomic Orbital (GIAO) method.The intra- and interionic noncovalent interactions in the ionic liquids have been revealed via the electron density analysis based on the Atoms in Molecules (AIM) approach, and decomposed as hydrogen bonding, van der Waals interactions, and steric effects via Reduced Density Gradient (RDG) analysis. To gain insight into the possible effects of anion-cation interaction on the physicochemical properties of ionic liquids, interionic interactions, reactivity properties, topological critical points, and electrostatic potential surfaces were obtained. These interactional characteristics were interpreted in terms of both the anion dependency and interaction type. Notably, the results of this study were evaluated together with the results obtained from our previous study on ILs consisting of the same anions with the 1-Hexyl-3-methylimidazolium cation to achieve the effects of the extension in the chain length of the cation in the presence of the same anion on the spectroscopic and electronic properties of ionic liquids.

Reference electrodes with polymeric membranes containing ionic liquids of various physicochemical properties

In this paper, the development of reference electrodes with polyurethane membranes containing ionic liquids is described. In particular, the influence of physicochemical properties of ionic liquids on the performance of polymeric membrane reference electrodes was elucidated. It was found that the potential stability of tested electrodes strongly depends on the differences in partition coefficient values between cation and anion of the ionic liquid. Among tested ionic liquids, 1-(2-hydroxyethyl)− 3-imidazolium bis{(trifluoromethyl)sulfonyl}imide induced the best potential stability of reference electrodes. It was also shown that the amount of the ionic liquid in the polymeric membrane does not influence significantly the performance of resulting reference electrodes. Moreover, results demonstrate that the addition of the plasticizer to the polyurethane membrane is not necessary to prepare reference electrodes that show sufficiently stable EMF values in solutions of various ion concentrations. Membranes of optimal composition were used to prepare All-Solid-State reference electrodes on glassy carbon disc electrodes with acceptable potential stability.

Quantitative structure-property relationship for predicting the diffusion coefficient of ionic liquids

Quantitative structure–property relationship (QSPR) models can offer theoretical prediction of physicochemical properties of ionic liquids (ILs), and the exploration of molecular descriptors that reflect the structural characteristics of ILs is of primary importance for QSPR models. Here, three novel QSPR models, termed as Model I, Model II and DB∞-QSPR model, are developed to predict the diffusion coefficient of ILs by using conductor-like screening model for segment activity coefficient (COSMO-SAC) based descriptors. The established Model I matches well with the experimental data set for both cations and anions, as evidenced by the high values of the coefficient of determination (R2) (greater than 0.95) in the training set and the testing set. Model II can be used to calculate the self-diffusion coefficients of ILs with a fixed anion of [NTf2], and the corresponding R2 values are greater than 0.97 in the training set and the testing set. The mutual diffusion coefficients of ILs in water at infinite dilution (DB∞) can reliably be predicted by DB∞-QSPR model, which demonstrates excellent predictive power because of the high values of R2 (greater than 0.97) in the training set and the testing set. In addition, the value of average absolute relative deviation (AARD) between experimental and calculated DB∞ is 5.023% in the overall set. According to the afore-mentioned QSPR models, the diffusion coefficient of ILs is positively correlated with temperature and negatively correlated with the cavity volume of ion.

An overview of magnetic ionic liquids: From synthetic strategies to applications in microextraction techniques.

Remarkable progress has been achieved in the application of magnetic ionic liquids in microextraction-based procedures. These materials exhibit unique physicochemical properties of ionic liquids featuring additional responses to magnetic fields by incorporating a paramagnetic component within the chemical structure. This intriguing property can open new horizons in analytical extractions because the solvent manipulation is facilitated. Moreover, the tunable chemical structures of magnetic ionic liquids also allow for task-specific extractions that can significantly increase the method selectivity. This review aimed at providing an up-to-date overview of articles involving synthesis, physicochemical properties, and applications of magnetic ionic liquids highlighting recent developments and configurations. Moreover, a section containing critical evaluation and future trends in magnetic ionic liquid-based extractions is included.

Formation of stoichiometric and non-stoichiometric ionic liquid and cocrystal multicomponent phases of lidocaine with azelaic acid by changing counterion ratios

Tuning of physicochemical properties of ionic liquids and crystalline materials is a challenge that opens up unlimited possibilities for expanding applications and controlling biological activity of pharmaceutical multicomponent phases incorporating active pharmaceutical ingredients (APIs) with counterions or coformers. In this work we have investigated the effect of changing lidocaine (LID) and azelaic acid (AZE) ratios on the physicochemical properties of their corresponding multicomponent systems using a thermodynamics-based approach. Microscopy, X-ray diffraction analysis, infrared spectroscopy, nuclear magnetic resonance, and thermogravimetric analysis provided complimentary characterisation. Mechanochemical synthesis of LID:AZE systems at a range of stoichiometries yielded at least two distinct liquid phases and two distinct crystalline phases, one of which involved a unique 2:3 LID:counterion composition not observed previously. Furthermore, to the best of our knowledge the formation of oligomeric ionic liquids involving dicarboxylic acids is also being reported for the first time. This work highlights the need for a careful characterisation of multicomponent systems, especially for pharmaceutical applications.

Imidazole-Based Ionic Liquids with BF4 as the Counterion Perform Outstanding Abilities in Both Inhibiting Clay Swelling and Lowing Water Cluster Size.

Promoting fluid transportation in porous media has important applications in energy, pedology, bioscience, etc. For this purpose, one effective way is to prevent swelling through surface modification; however, it is far from enough in real cases, such as ultra-low permeability reservoirs and tight oils. In this study, we considered the comprehensive effects of inhibiting clay swelling, flocculation performance, reducing water clusters and interfacial tension and developed a series of imidazole-based tetrafluoroborate ionic liquids (ILs) with different lengths of alkyl chains. Through measurements of anti-swelling rates, XRD, SEM, 17O NMR, molecular dynamics simulation, zeta potential, flocculation evaluation, interfacial tension and a core flooding experiment based on ultra-low permeability reservoirs, the relationships between the molecular structure and physicochemical properties of ILs have been revealed. Interestingly, one of the selected ILs, imidazole-based tetrafluoroborate ILs (C8-OMImBF4), shows excellent performance, which is helpful to design an effective strategy in promoting fluid transportation in narrow spaces.

Machine learning assisted QSPR model for prediction of ionic liquid’s refractive index and viscosity: The effect of representations of ionic liquid and ensemble model development

In this study, we used four ways to represent ionic liquids (ILs), namely, molecular fingerprint (MF), molecular descriptor (MD), the addition of MF (MF + MF) and the combination of MF and MD (MF_MD), to develop quantitative structure–property relationship (QSPR) models for predicting the refractive index and viscosity of ILs. Results showed that the predictive performance of QSPR models followed the order: MD < MF + MF < MF < MF_MD, indicating combining the chemical structure information and the physicochemical properties of ILs was beneficial to enhancing the predictive performance of the QSPR model. We also investigated the effect of the data splitting way on the predictive performance of the QSPR model, and the results showed that the group-based random splitting way was more reasonable than the random splitting way. The shapely additive explanation (SHAP) method was used to interpret MF_MD-based QSPR models. Results showed that different MDs play important role in prediction of refractive index and viscosity and the effects of conditions (temperature and/or pressure) were correctly identified. The QSPR model also correctly “learned” how MF affect the viscosity but wrongly “identified” how MF affect the refractive index. Finally, we developed the ensemble models by combining these single QSPR models to develop the final more accurate QSPR model. This study demonstrated that how to represent ILs plays important role in obtaining QSPR models with high predictive performance and developing the ensemble model was the possible efficient approach to further enhance the predictive performance of the QSPR model for ILs.

Anti-electrostatic hydrogen bonding between anions of ionic liquids: a density functional theory study.

Hydrogen bonds (HBs) play a crucial role in the physicochemical properties of ionic liquids (ILs). To date, HBs between cations and anions (Ca-An) or between cations (Ca-Ca) in ILs have been reported extensively. Here, we provided DFT evidence for the existence of HBs between anions (An-An) in the IL 1-(2-hydroxyethyl)-3-methylimidazolium 4-(2-hydroxyethyl)imidazolide [HEMIm][HEIm]. The thermodynamic stabilities of anionic, cationic, and H2O dimers together with ionic pairs were studied using potential energy scans. The results show that the cation-anion pair is the most stable one, while the HB in the anionic dimer possesses similar thermodynamic stability to the water dimer. The further geometric, spectral and electronic structure analyses demonstrate that the inter-anionic HB meets the general theoretical criteria of traditional HBs. The strength order of four HBs in complexes is cation-anion pair > H2O dimer ≈ cationic dimer > anionic dimer. The energy decomposition analysis indicates that induction and dispersion interactions are the crucial factors to overcome strong Coulomb repulsions, forming inter-anionic HBs. Finally, the presence of inter-anionic HBs in the ionic cluster has been confirmed by a global minimum search for a system containing two ionic pairs. Even though hydroxyl-functionalized cations are more likely to form HBs with anions, there are still inter-anionic HBs between hydroxyl groups in the low-lying structures. Our studies broaden the understanding of HBs in ionic liquids and support the recently proposed concept of anti-electrostatic HBs.