Protic Ionic Liquid Ethylammonium Nitrate Research Articles

Aggregation behaviours of sulfobetaine zwitterionic surfactants in EAN

Ionic liquids have become one kind of the most promising solvents for self-assembly. The aggregation behaviours of various surfactants have been studied in numerous ionic liquids, especially the cationic and nonionic surfactants. In this work, we would explore the aggregation behaviours of sulfobetaine zwitterionic surfactants (SB3-n, n = 12, 14, 16) in the protic ionic liquid ethylammonium nitrate (EAN). The micellization behaviour was studied by surface tension measurement and the critical micellization concentrations are much larger than those in water. The micellar phase and hexagonal lyotropic liquid crystal phase are formed by SB3-12 and SB3-14 in EAN while an additional micellar cubic lyotropic liquid crystal phase is formed in the SB3-16/EAN system. The structure of aggregates was characterized by the small angle X-ray scattering while optical and viscoelastic properties by polarized optical microscopy and rheology. The alkyl chain length effect of SB3-n on aggregation behaviours was discussed and compared with those in the other surfactant/EAN systems. Our results would provide the thorough understanding on aggregation behaviours of sulfobetaine zwitterionic surfactants in EAN.

Enthalpy of solvation of alkali metal salts in a protic ionic liquid: Effect of cation charge and size

• Experimental and theoretical study of solution and solvation of mono- and divalent alkaline metal cations. • MD simulations provided coordination numbers, which showed an interesting similarity in IL and water environments. • Solvation of alkali and alkaline earth nitrate metal salts ((LiNO 3 , KNO 3 , RbNO 3 , CsNO 3 , Mg(NO 3 ) 2 and Ca(NO 3 ) 2 ) in EAN. • Analysis of the energetics and structure of solvation confirms the well-known water-like solvation properties of EAN. An experimental and theoretical study of solution and solvation of mono- and divalent alkali metal cations in the protic ionic liquid (IL) ethylammonium nitrate (EAN) is reported. High precision solution-reaction calorimetry was used to obtain the heat of solvation, which was used for the analysis of the thermodynamics. A close relation between the structure of the salts in the crystalline phase and its enthalpy of solvation in the IL is reported. A detailed picture of the molecular environments in the solvation shells around the metal cations is provided by means of molecular dynamics simulations. The analysis of the energetics and structure of solvation confirms the well-known water-like solvation properties of EAN, with the solvation shell around the metal cations in both media being very similar. On the other hand, the results show that it is energetically more favourable to solvate smaller cations with higher valence. Indeed, the simulations show that the long-range electrostatic interactions are the main contribution to solvation interaction, with the electric field at the surface of the alkali metal cations as the basic magnitude controlling it.

On the physical properties of mixtures of nitrate salts and protic ionic liquids

We report a systematic study of the effect of salt concentration and its cation valence on several equilibrium and transport properties of mixtures of the model protic ionic liquid ethylammonium nitrate with monovalent (LiNO3), divalent (Mg(NO3)2 and Ca(NO3)2), and trivalent (Al(NO3)3) salts. These properties, determined by appropriate experimental techniques, include density, sound velocity, refractive index, surface tension, conductivity and viscosity. Single-particle dynamics and radial distribution functions were also analyzed by means of molecular dynamics simulations. The temperature dependence of the conductivity was studied in the Vogel-Fulcher-Tammann framework, and we obtained effective activation energies, fragility indexes, and Vogel temperatures. In addition, we performed a high-temperature Arrhenius analysis and we calculated the activation energies of both conductivity and viscosity. Finally, the exponents of the fractional Walden rule for the different mixtures were obtained and the ionicities and fragilities of the systems were analyzed, proving that all the mixtures are subionic and fragile. The distortion of the network of hydrogen bonds characteristic of protic ionic liquids and the formation of long-lived anionic aggregates composed of the cations of the added salt and the nitrate anions in their first solvation shell were found to have a deep impact on the analyzed properties. The role of the surface charge density of the salt cations on the structure and transport properties of the solutions is detailedly analyzed and related to solvation of salt species in the polar nanoregions of the ionic liquid (nanostructured solvation).

Dynamics of ethylammonium nitrate near PTFE surface

Self-diffusion of ions in the protic ionic liquid ethylammonium nitrate (EAN) was studied by 1H NMR pulsed field gradient techniques between 294 and 393 K in the presence of a PTFE insert in a 5-mm NMR tube. At all temperatures, the bulk diffusion of ions (measured by 1H and 15N NMR) can be described by a unique diffusion coefficient. The presence of solid hydrophobic surfaces of PTFE induces regions of EAN in their vicinity, where diffusion of ions, both cations and anions, is reduced compared to the bulk values. An additional line-shape analysis in 1H NMR spectra showed that local mobility of ethylammonium cations in the surface layers near PTFE is also reduced.

An ultrafast vibrational study of dynamical heterogeneity in the protic ionic liquid ethyl-ammonium nitrate. I. Room temperature dynamics.

Using ultrafast two-dimensional infrared spectroscopy (2D-IR), a vibrational probe (thiocyanate, SCN-) was used to investigate the hydrogen bonding network of the protic ionic liquid ethyl-ammonium nitrate (EAN) in comparison to H2O. The 2D-IR experiments were performed in both parallel (⟨ZZZZ⟩) and perpendicular (⟨ZZXX⟩) polarizations at room temperature. In EAN, the non-Gaussian lineshape in the FTIR spectrum of SCN- suggests two sub-ensembles. Vibrational relaxation rates extracted from the 2D-IR spectra provide evidence of the dynamical differences between the two sub-ensembles. We support the interpretation of two sub-ensembles with response function simulations of two overlapping bands with different vibrational relaxation rates and, otherwise, similar dynamics. The measured rates for spectral diffusion depend on polarization, indicating reorientation-induced spectral diffusion (RISD). A model of restricted molecular rotation (wobbling in a cone) fully describes the observed spectral diffusion in EAN. In H2O, both RISD and structural spectral diffusion contribute with similar timescales. This complete characterization of the dynamics at room temperature provides the basis for the temperature-dependent measurements in Paper II of this series.

Rotational correlation times, diffusion coefficients and quadrupolar peaks of the protic ionic liquid ethylammonium nitrate by means of 1H fast field cycling NMR relaxometry

The protic ionic liquid (PIL) ethylammonium nitrate (EAN) is characterized by hydrogen bonding between the NH bonds of the cations and the oxygen atoms of the anions. The three-dimensional H-bond network of EAN very much resembles that of water. Although this first ionic liquid is known for more than 100 years, there is still lack of information about the rotational and translational dynamics, and how both are related to each other. For that purpose, we measured the 1H nuclear magnetic relaxation dispersion curves (NMRD) of EAN by means of Fast Field Cycling (FFC) NMR relaxometry. We were able to analyze the measured 1H spin-lattice relaxation rates R1 covering a large temperature range between 248 and 333 K simultaneously. Applying the well-known Bloembergen-Purcell-Pound (BPP) approach, we decomposed the total relaxation rates into intramolecular and intermolecular contributions, describing the rotational and translational dynamics of the ethylammonium cation. Here, we report rotational correlation times, τR, and translational diffusion coefficients, DT, both derived from the total 1H spin-lattice relaxation rates as a function of temperature and frequency. Self-diffusion coefficients, DT, were also determined from the total relaxation rates in the low frequency range, wherein only the intermolecular relaxation contribution is frequency dependent. Then, DT results from the slope of a linear fit of the spin-lattice relaxation rate R1 as a function of the square root of frequency, ν. The diffusion coefficients obtained from both methods are in good agreement with self-diffusion coefficients measured by pulsed-field-gradient (PFG) NMR. The rotational correlation times, τR, derived from the intramolecular 1H relaxation contribution were compared to correlation times obtained from high-field NMR deuteron relaxation experiments, dielectric spectroscopy (DS) and femto-second-infrared (fs-IR) spectroscopy. For EAN, we also observed a local enhancement of the 1H spin-lattice relaxation rates R1 at high frequencies. This so-called quadrupole relaxation enhancement (QRE) results from the interference of the Zeeman transition energy of the 1H spins and the energy difference of two levels of the quadrupole 14N nuclei. QRE is usually known for solids, but rarely for the liquid state as observed here for EAN. Overall, we show that FFC relaxometry provides access to rotational correlation times, translational diffusion coefficients and quadrupole relaxation enhancement at the same time.

The interaction of ammonia with the protic ionic liquid ethylammonium nitrate: A simulation study

In this paper, we present a molecular dynamics study of ammonia solutions in a protic ionic liquid with a structurally similar cation, ethylammonium nitrate. Using two different models of the ammonia molecule, a conventional 4-site model and a 6-site model optimizing the dipolar and quadrupolar electron distributions, we evaluate their effect on the microstructure and single-particle dynamics of these mixtures, as well as on their behaviour at the electrochemical interface. Quite significant differences are found on the solvation of ammonia in these dense ionic solvents, the 6-atom model predicting a stronger affinity of solvent molecules for the cations of the solvent as well as a caging effect on NH3 solvation shells, not predicted by the 4-site model. Finally, we discuss the structure of the liquid layers near charged graphene walls. The structure in the layer next to the wall varies with the charge on the wall and varies with the model used for ammonia.

High-pressure crystal polymorph of the protic ionic liquid: Ethylammonium nitrate

A high pressure (HP) crystal polymorph was obtained for the protic ionic liquid ethylammonium nitrate (EAN). Crystal structures of the HP phases were determined by X-ray diffraction. The formation of either a single crystal or polycrystal depended on the temperature and pressure pathway. In a previous work, a low temperature (LT) crystal polymorph was identified, and multiple crystallization pathways were observed by simultaneous X-ray diffraction and differential scanning calorimetry measurements. The non-reversible HP-crystal polymorph was different from the LT-one. Despite the simple molecular structure of EAN, HP-crystal polymorph is observed in addition to LT-crystal polymorph and multiple LT-crystallization pathways.

Formation of Surface Protic Ionic Liquid Nanodroplets for Nanofabrication

AbstractFemtoliter droplets at solid interfaces attract significant interest as the basic units in many physical and biological processes. One challenge in the application of these tiny droplets is their fast evaporation rate in air due to their large surface‐to‐volume ratio. Ionic liquids with low volatility present an opportunity to overcome this challenge. The advanced properties of ionic liquids (ILs) have enabled them to be widely applied in chemical reactions, biopolymers, molecular self‐assembly and separations. At interfaces, the wettability of ILs is pursued in next‐generation lubricants and battery technology. Previously, IL droplets at solid surfaces have been prepared by nanodispensing, micropipette, and solvent evaporation. Here, the solvent exchange protocol is extended to yield protic ionic liquid droplets at the interface with controlled size, distribution, location, and stability in both liquid and air surroundings. During growth, the droplets demonstrate an interesting dewetting dynamic. This behavior has not been observed for molecular liquids during solvent exchange and suggests interesting interfacial dynamics of the ionic liquid. One proof‐of‐concept application of using surface nanodroplets of the protic ionic liquid ethylammonium nitrate for compartmentalized reactions and templating SiO2 nanostructures is demonstrated. This work broadens and intertwines the opportunities of ILs and nanodroplets.

Liquid range of ionic liquid – Metal salt mixtures for electrochemical applications

The effect of the nitrate salt addition on the liquid range of the protic ionic liquid ethylammonium nitrate (EAN) was studied in this work. The salts used were monovalent, LiNO3; divalent, Mg(NO3)2 and Ca(NO3)2, and trivalent, Al(NO3)3; inorganic salts containing metals relevant for electrochemical applications. Solid-liquid transitions, which define the lower limit of the liquid range of the fluids, were determined using differential scanning calorimetry (DSC), whereas the upper limit, which is given by degradation or boiling temperatures, was determined by means of thermogravimetric analysis (TGA). Our results show that salt addition induces melting-point depressions, but no significant boiling point elevations were detected. The so-called nanostructured solvation mechanism is used in order to rationalize the thermodynamics of the mixtures in terms of the occurrence of highly incompressible, low mobility solvation complexes [M(NO3)n]z+−n in the polar cavities of the IL giving rise to pseudo-lattice nanocrystals that are stable in the liquid phase.

NMR investigation of the structure and single-particle dynamics of inorganic salt solutions in a protic ionic liquid

With the aim to analyze the microstructure of the polar nanoregions of mixtures of ionic liquids and inorganic salts and the single particle dynamics of metal cations in these densely ionic environments, solutions of inorganic nitrate salts (lithium, magnesium, calcium and aluminium nitrates) in the protic ionic liquid ethylammonium nitrate have been characterized by means of nuclear magnetic resonance. The measured spectra reveal that, despite being dried under vacuum, all the studied mixtures contained variable amounts of water, but, while in those with the salt cations of lower ionic potentials (Li+, Ca2+) only bulk free water that can be almost completely removed from the mixture is found, mixtures with the cations of higher ionic potentials (Mg2+, Al3+) contain only hydration water structurally anchored in the primary solvation shells of the metal cations forming coordination complexes inside the polar nanoregions of the densely ionic solvent and, thus, non-removable under vacuum. This hydration water is not properly detected by common Karl Fisher methods and these solutions are sometimes mistakenly considered anhydrous. Moreover, our NMR measurements confirm the nanostructured solvation paradigm, since they show that the influence of the addition of salt on the chemical shifts of the signals associated to hydrogen atoms in the apolar regions of the protic ionic liquid is negligible. Single-particle dynamics of cations of low and high ionic potentials was also studied, and our observations for the diffusion coefficients of lithium and aluminium cations support the conclusion that Li+ diffusion takes place inside anhydrous [Li(NO3)4]3− complexes, while kinetic complexes with Al3+ gather around [Al(H2O)6]3+) aqua ions.

A methanol and protic ionic liquid Ugi multicomponent reaction path to cytotoxic α-phenylacetamido amides

The Ugi four component reaction of an aldehyde, amine, isocyanide and an ethanoic acid was effected smoothly in protic ionic liquids ethylammonium nitrate (EAN) and propylammonium nitrate (PAN) to afford analogues of α-phenylacetamido amides in good to excellent isolated yields. The corresponding reactions in [BMIM][PF6] and the protic ionic liquid ethanolammonium nitrate (ETAN) failed. Microwave irradiation in EAN facilitated rapid access to three focused libraries, based on the parent isocyanide: cyclohexyl isocyanide, benzyl isocyanide and ethyl isocyanoacetate. Analysis of the structure activity relationship data suggested the presence of a bulky moiety originating from the isocyanide (cyclohexyl and benzyl) enhanced cytotoxicity. Removal of the acetylenic H-atom from the ethanoic acid moiety was detrimental to cytotoxicity. The most active analogues produced, N-(2-cyclohexylamino)-1-(4-methoxyphenyl)-2-oxoethyl-N-(3,5-dimethoxyphenyl)propiolamide, returned average GI50 values of ≤1 μM across the cancer cell lines evaluated. Combined, these data suggest that analogues of this nature are interesting potential anti-cancer development leads.

Stability and activity of lysozyme in stoichiometric and non-stoichiometric protic ionic liquid (PIL)-water systems.

There has been a substantial increase in enzyme applications within the biochemical and pharmaceutical industries, for example, as industrial biocatalysts. However, enzymes have narrow marginal stability which makes them prone to become inactive and/or denature with a slight change in the solvent environment. Typically industrial applications require harsher solvent environments than enzyme native environments, and hence there is a need to understand solvent-protein interactions in order to develop strategies to maintain, or enhance, the enzymatic activity under industrially relevant solvent conditions. Previously we have shown that protic ionic liquids (PILs) with water can have a stabilising effect on lysozyme, with a large variation dependent on which PIL ions are present, and the water concentration [E. C. Wijaya et al., Phys. Chem. Chem. Phys. 18(37), 25926-25936 (2016)]. Here we extend on this work using non-stoichiometric aqueous PIL solvents to investigate, and isolate, the role of pH and ionicity on enzymes. We have used the PILs ethylammonium nitrate (EAN) and ethanolammonium formate (EOAF) since our previous work has identified these as good solvents for lysozyme. Solvent libraries were made from these two PILs with an additional precursor acid or base to modify the acidity/basicity of the neutral stoichiometric PIL, and with water added, to have solutions with 4-17 mol. % of the PIL ions in water. Molar ratios of base:acid were varied between 1:1.05 and 2:1 for EAN and 1:1.25 and 2:1 for EOAF, which enabled from highly basic to highly acidic solutions to be obtained. This was to modify the acidity/basicity of the neutral stoichiometric PILs, without the addition of buffers. The structure and stability of hen egg white lysozyme (HEWL) were explored under these solvent conditions using synchrotron small angle X-ray scattering (SAXS), Fourier transform infrared (FTIR), and activity assays. The radius of gyration and Kratky plots obtained from the SAXS data showed little change with varying ionicity or acid:base ratio. FTIR showed that α-helix was maintained in all, except for the most acidic solvent conditions. The activity data show that HEWL was active between pH 0 and 11 for the EA:N-water system and pH 4.4 and 11 for the EOA:F-water system. This work indicates that ionic liquids have the potential to enable enzymes to maintain activity across a broader range of solvent conditions.

Surface Ordering in Binary Mixtures of Protic Ionic Liquids.

The surface composition of binary mixtures of the protic ionic liquids ethylammonium nitrate and propylammonium nitrate has been investigated using surface tension measurements and the perfectly surface sensitive method metastable induced electron spectroscopy. Given that the latter technique is sensitive only to the outermost layer, it allows for the determination of the surface fraction occupied by a given species. The piecewise linear relationship between surface fraction and surface tension found in this study can be described by a phase separation within the surface layer.

Properties of hydrogen bonds in the protic ionic liquid ethylammonium nitrate

Comparative molecular dynamics simulations of a hexamer cluster of the protic ionic liquid ethylammonium nitrate are performed using density functional theory and density functional-based tight-binding (DFTB) methods. The focus is on assessing the performance of the DFTB approach to describe the dynamics and infrared spectroscopic signatures of hydrogen bonding between the ions. Average geometries and geometric correlations are found to be rather similar. The same holds true for the far-infrared spectral region. Differences are more pronounced for the NH- and CH-stretching bands, where DFTB predicts a broader intensity distribution. DFTB completely fails to describe the fingerprint range shaped by nitrate anion vibrations. Finally, charge fluctuations within the H bonds are characterized yielding moderate dependencies on geometry. On the basis of these results, DFTB is recommended for the simulation of H bond properties of this type of ionic liquid.

How Does CeIII Nitrate Dissolve in a Protic Ionic Liquid? A Combined Molecular Dynamics and EXAFS Study.

A diluted solution of Ce(NO3 )3 in the protic ionic liquid (IL) ethylammonium nitrate (EAN) was investigated using molecular dynamics (MD) simulations and extended X-ray absorption fine structure (EXAFS) spectroscopy. For the first time polarizable effects were included in the MD force field to describe a heavy metal ion in a protic IL, but, unlike water, they were found to be unessential. The CeIII ion first solvation shell is formed by nitrate ions arranged in an icosahedral structure, and an equilibrium between monodentate and bidentate ligands is present in the solution. By combining distance and angular distribution functions it was possible to unambiguously identify this peculiar coordination geometry around the ions dissolved in solution. The metal ions are solvated within the polar domains of the EAN nanostructure and the dissolved salt induces almost no reorganization of the pre-existing structure of EAN upon solubilization.

Dielectric relaxation of nitromethane and its mixtures with ethylammonium nitrate: Evidence for strong ion association induced by hydrogen bonding

Binary mixtures of the aprotic protophobic solvent nitromethane (NM) and the protic room-temperature ionic liquid ethylammonium nitrate (EAN) were studied with dielectric relaxation spectroscopy in the two single-phase regions, 0 < xEAN ≤ 0.013 and 0.4 ≤ xEAN < 1, of the binary mixtures at 25 °C. All spectra were well described by a superposition of two relaxation processes whose origins were of composite nature. At low xEAN the solutions behave as a strongly associated electrolyte, displaying moderate ion solvation by nitromethane at the infinite-dilution limit. At high xEAN far from the miscibility gap, the observed dynamics represents the typically observed behaviour of a lubricated ionic liquid smoothly reaching the properties of pure EAN. Additionally, neat NM was investigated in the temperature range of 5 to 65 °C to explore its potential as a calibration standard in dielectric spectroscopy. The obtained 0.05 to 89 GHz spectra were well fitted by a single Debye equation. The dynamics of this medium-permittivity dipolar liquid is governed by rotational diffusion of NM dipoles close to slip boundary conditions with only weak dipole-dipole correlations. Unfortunately, difficulties in obtaining samples of reproducible purity discredit NM as a calibration standard for dielectric measurements.

A pH Scale for the Protic Ionic Liquid Ethylammonium Nitrate.

To quantify the properties of protic ionic liquids (PILs) as acid-base reaction media, potentiometric titrations were carried out in a neat PIL, ethylammonium nitrate (EAN). A linear relationship was found between the 14 pKa values of 12 compounds in EAN and in water. In other words, the pKa value in EAN was found to be roughly one unit greater than that in water regardless of the charge and hydrophobicity of the compounds. It is possible that this could be explained by the stronger acidity of HNO3 in EAN than that of H3 O(+) in water and not by the difference in the solvation state of the ions. The pH value in EAN ranges from -1 to 9 on the pH scale based on the pH value in water.

A pH Scale for the Protic Ionic Liquid Ethylammonium Nitrate

AbstractTo quantify the properties of protic ionic liquids (PILs) as acid–base reaction media, potentiometric titrations were carried out in a neat PIL, ethylammonium nitrate (EAN). A linear relationship was found between the 14 pKa values of 12 compounds in EAN and in water. In other words, the pKa value in EAN was found to be roughly one unit greater than that in water regardless of the charge and hydrophobicity of the compounds. It is possible that this could be explained by the stronger acidity of HNO3 in EAN than that of H3O+ in water and not by the difference in the solvation state of the ions. The pH value in EAN ranges from −1 to 9 on the pH scale based on the pH value in water.

Molecular dynamics simulations of temperature-dependent structures and dynamics of ethylammonium nitrate protic ionic liquid: The role of hydrogen bond

Molecular dynamics simulations have been employed to systematically investigate the structure and dynamics properties, hydrogen bond (HB) dynamics of protic ionic liquid (IL) ethylammonium nitrate (EAN) in the temperature range between 300K and 400K. The simulation results demonstrate clearly that the temperature almost has little influence on the structures of EAN IL, whereas the translational and the rotational motions of both cations and anions become much faster at higher temperatures. Furthermore, both anions and cations are found to display an obvious sub-diffusive behavior. These changes can be attributed to the temperature-dependent HB strength between the cations and the anions, where the strength of HBs decreases significantly with increasing temperature. Accordingly, the ion-pair association/dissociation dynamics decreases considerably with increasing temperature. Therefore, our simulations reveal at a molecular level that the HBs interactions play an essential role in determining the dynamics properties of protic ILs.