Structure Of Protic Ionic Liquids Research Articles

Exploring Solvation Properties of Protic Ionic Liquids by Employing Solvatochromic Dyes and Molecular Dynamics Simulation Analysis

Solvation properties are key for understanding the interactions between solvents and solutes, making them critical for optimizing chemical synthesis and biochemical applications. Designable solvents for targeted optimization of these end-uses could, therefore, play a big role in the future of the relevant industries. The tailorable nature of protic ionic liquids (PILs) as designable solvents makes them ideal candidates. By alteration of their constituent structural groups, their solvation properties can be tuned as required. The solvation properties are determined by the polar and non-polar interactions of the PIL, but they remain relatively unknown for PILs as compared to aprotic ILs and their characterization is non-trivial. Here, we use solvatochromic dyes as probe molecules to investigate the solvation properties of nine previously uncharacterized alkyl- and dialkylammonium PILs. These properties include the Kamlet–Aboud–Taft (KAT) parameters: π* (dipolarity/polarizability), α (H-bond acidity) and β (H-bond basicity), along with the ET(30) scale (electrophilicity/polarizability). We then used molecular dynamics simulations to calculate the radial distribution functions (RDF) of 21 PILs, which were correlated to their solvation properties and liquid nanostructure. It was identified that the hydroxyl groups on the PIL cation increase α, π* and ET(30), and correspondingly increase the cation–anion distance in their RDF plots. The hydroxyl group, therefore, reduces the strength of the ionic interaction but increases the polarizability of the ions. An increase in the alkyl chain length on the cation led to a decrease in the distances between cations, while also increasing the β value. The effect of the anion on the PIL solvation properties was found to be variable, with the nitrate anion greatly increasing π*, α and anion–anion distances. The research presented herein advances the understanding of PIL structure–property relationships while also showcasing the complimentary use of molecular dynamics simulations and solvatochromic analysis together.

Influence of anion structure on volumetric properties of dilute binary systems containing carboxylate functioned trihexylammonium ionic liquids in toluene / dodecane

We are actively involved in synthesis of hydrophobic ionic liquids, their volumetric behaviour in various solvents of different polarity and their application in metal ions extraction from water. In continuation of previous work, we are presenting new protic ionic liquids (PILs) containing Trihexylammonium cation and carboxylate anions namely Trihexylammonium propionate [THA][Pro] and Trihexylammonium pentanoate [THA] [Pent]. The simultaneous measurements of densities, ρ along with speeds of sound, u for binary systems of [THA][Pro] and [THA][Pent] in toluene and in dodecane were collected. The measurements were carried out in dilute concentrations range of PILs, at 0.1 MPa and different temperatures from 293.15 K to 328.15 K. The experimental values of densities, ρ and speeds of sound, u, were utilized to obtain calculated values of apparent molar properties: Vϕ (apparent molar volume) and Ks,ϕ (apparent molar isentropic compressibility). It is observed that the apparent molar properties of investigated systems are quite sensitive to variation in temperature, solvent and anion’s structure of PILs. The values of Vϕ and Ks,ϕ increases with each increment in studied temperature and when toluene was replaced by dodecane as a solvent in the system. The each addition of –CH2 group in anion chain of PILs leading to increase Vϕ whereas decrease with concentration of PILs in the binary system. The limiting values of apparent molar volume, Vϕ∞ and apparent molar isentropic compressibility, Kϕ∞ were also reported. The strength of interactions between PIL-PIL and PIL-solvent in studied binary systems was explained by Sk and Sv parameters. The PILs + dodecane systems exhibit greater interaction between PILs and solvent than binary systems of PIL + toluene.

The structure of protic ionic liquids based on sulfuric acid, doped with excess of sulfuric acid or with water.

Neutron scattering with isotopic substitution was used to study the structure of concentrated sulfuric acid, and two protic ionic liquids (PILs): a Brønsted-acidic PIL, synthesised using pyridine and excess of sulfuric acid, [Hpy][HSO4]·H2SO4, and a hydrated PIL, in which an equimolar mixture of sulfuric acid and pyridine has been doped with water, [Hpy][HSO4]·2H2O. Brønsted acidic PILs are excellent solvents/catalysts for esterifications, driving reaction to completion by phase-separating water and ester products. Water-doped PILs are efficient solvents/antisolvents in biomass fractionation. This study was carried out to provide an insight into the relationship between the performance of PILs in the two respective processes and their liquid structure. It was found that a persistent sulfate/sulfuric acid/water network structure was retained through the transition from sulfuric acid to PILs, even in the presence of 2 moles (∼17 wt%) of water. Hydrogen sulfate PILs have the propensity to incorporate water into hydrogen-bonded anionic chains, with strong and directional hydrogen bonds, which essentially form a new water-in-salt solvent system, with its own distinct structure and physico-chemical properties. It is the properties of this hydrated PIL that can be credited both for the good performance in esterification and beneficial solvent/antisolvent behaviour in biomass fractionation.

Crystal structure of protic ionic liquids and their hydrates

Crystal structure of protic ionic liquids and their hydrates

Assessing the Structure of Protic Ionic Liquids Based on Triethylammonium and Organic Acid Anions.

We present a computational analysis of the short-range structure of three protic ionic liquids based on strong organic acids: trifluoracetate, methanesulfonate, and triflate of triethylammonium. Accurate ab initio computations carried out on the gas-phase dimers show that the protonation of triethylamine is spontaneous. We have identified the anion-cation binding motif that is due to the presence of a strong hydrogen bond and to electrostatic interactions. The strength of the hydrogen bond and the magnitude of the binding energy decrease in the order trifluoroacetate ≳ methanesulfonate > triflate. The corresponding simulations of the bulk phases, obtained using a semiempirical evaluation of the interatomic forces, reveal that on short timescales, the state of the three liquids remains highly ionized and that the gas-phase cation-/anion-binding motif is preserved while no other peculiar structural features seem to emerge.

Structural Design of Ionic Liquids for Optimizing Aromatic Dissolution.

Certain protic ionic liquids (PILs) are potentially low-cost, high-efficiency solvents for the extraction and processing of aromatic compounds. To understand the key design features of PILs that determine solubility selectivity at the atomic level, neutron diffraction was used to compare the bulk structure of two PILs with and without an aromatic solute, guaiacol (2-methoxyphenol). Guaiacol is a common lignin residue in biomass processing, and a model compound for anisole- or phenol-based food additives and drug precursors. Although the presence of amphiphilic nanostructure is important to facilitate the dissolution of solute nonpolar moieties, the local geometry and competitive interactions between the polar groups of the cation, anion, and solute are found to also strongly influence solvation. Based on these factors, a framework is presented for the design of PIL structure to minimize competition and to enhance driving forces for the dissolution of small aromatic species.

Dielectric response and transport properties of alkylammonium formate ionic liquids.

Dielectric relaxation spectra of three members of the alkylammonium formate family of protic ionic liquids (PILs), namely, ethylammonium formate (EAF), n-butylammonium formate (BuAF), and n-pentylammonium formate (PeAF), as well as the pseudo-PIL triethylamine + formic acid (molar ratio 1:2; TEAF) have been studied over a wide frequency (50 MHz to 89 GHz) and temperature range (5-65 °C), complemented by measurements of their density, viscosity, and conductivity. It turned out that the dominating relaxation of EAF, BuAF, and PeAF arises from both cation and anion reorientations which are synchronized in their dynamics due to hydrogen bonding. Amplitudes and relaxation times of this mode reflect the-compared to nitrate-different nature of H bonding between the formate anion and ethylammonium cation, as well as increasing segregation of the PIL structure into polar and non-polar domains. The TEAF data suggest that its dominating relaxation is due to the rotation of the complex triethylamine⋅(formic acid)2 in which no significant proton transfer to an ion pair occurred. Weak dissociation of this complex into ions was postulated to account for the high conductivity of TEAF.

Nanostructure of propylammonium nitrate in the presence of poly(ethylene oxide) and halide salts.

Nanoscale structure of protic ionic liquids is critical to their utility as molecular electrochemical solvents since it determines the capacity to dissolve salts and polymers such as poly(ethylene oxide) (PEO). Here we use quantum chemical molecular dynamics simulations to investigate the impact of dissolved halide anions on the nanostructure of an archetypal nanostructured protic ionic liquid, propylammonium nitrate (PAN), and how this impacts the solvation of a model PEO polymer. At the molecular level, PAN is nanostructured, consisting of charged/polar and uncharged/nonpolar domains. The charged domain consists of the cation/anion charge groups, and is formed by their electrostatic interaction. This domain solvophobically excludes the propyl chains on the cation, which form a distinct, self-assembled nonpolar domain within the liquid. Our simulations demonstrate that the addition of Cl- and Br- anions to PAN disrupts the structure within the PAN charged domain due to competition between nitrate and halide anions for the ammonium charge centre. This disruption increases with halide concentration (up to 10 mol. %). However, at these concentrations, halide addition has little effect on the structure of the PAN nonpolar domain. Addition of PEO to pure PAN also disrupts the structure within the charged domain of the liquid due to hydrogen bonding between the charge groups and the terminal PEO hydroxyl groups. There is little other association between the PEO structure and the surrounding ionic liquid solvent, with strong PEO self-interaction yielding a compact, coiled polymer morphology. Halide addition results in greater association between the ionic liquid charge centres and the ethylene oxide components of the PEO structure, resulting in reduced conformational flexibility, compared to that observed in pure PAN. Similarly, PEO self-interactions increase in the presence of Cl- and Br- anions, compared to PAN, indicating that the addition of halide salts to PAN decreases its utility as a molecular solvent for polymers such as PEO.

Protic Ionic Liquids Used as Metal-Forming Green Lubricants for Aluminum: Effect of Anion Chain Length

Among the applications for protic ionic liquids (PILs), lubrication is one of the newest and the most promising. In this work, ammonium-based protic ionic liquids were tested as lubricant fluids for aluminum-steel contacts. PILs were synthesized with 2-hydroxyethylamine (2HEA) and a carboxylic acid (formic and pentanoic), aiming to understand the effect of two different anion chain lengths on the lubricant behavior. The synthesized PILs were characterized by RMN, FTIR and TGA. Wear tests, conducted using a ball-on-plate configuration, showed that the increase of the anion carbon chain length in the PIL structure reduced significantly the coefficient of friction value. Besides, after the wear tests, the PILs structural integrity was not affected. In the same way, bending under tension (BUT) tests evidenced that the performance for stamping conditions of the PIL with the longest anion carbon chain was similar to that of the commercial lubricant. Since, both formed a uniform tribofilm, developed the same lubrication regime and the drawing forces values were close and constant. Hence, the ionic liquid obtained with 2HEA and pentanoic acid (2HEAPe) is as suitable as the commercial lubricant for metal forming processes.

(Invited) Polymer Electrolytes Based on Ionic Liquids for PEMFC and Lithium Batteries

Ionic liquids (IL) are considered as hi-tech new media with emerging applications as solvents for organic reactions or as electrolytes. The organization degree in these complex fluids is actually very challenging since it dictates microscopic as well as macroscopic properties. However in both electrochemical devises, i.e. the Proton Exchange Membrane Fuel Cells (PEMFC) and Lithium Polymer Batteries (LBP), polymer electrolytes with high conductivities and good mechanical properties are obtained by blending the IL with an appropriate polymer. We have shown recently that the membrane properties are guided by the IL structure, i.e. by its interactions with the host polymer1-5. We propose in this work a deep evaluation of the impact of IL structure on the membranes morphology and functional properties. The work is divided in two parts, discussing membranes for PEMFC and LPB. In the PEMFC, one class of ionic liquids with interesting properties, i.e. Protic Ionic Liquids (PIL), is extensively used. Our study is focused more specifically on a series of PIL obtained by reacting alkylamine with various acids6. In this work we show that well-chosen PIL incorporated in the membrane can act as smart nanoscopic probes able to reveal interesting information on the polymer organization. As example, the morphology of Nafion® membranes doped with PIL based on triethylammonium (TEA) and perfluorosulfonates with varying perfluoroalkyl chain length (C1, C4, C8) was investigated by small-angle neutron scattering (SANS). The membranes were carefully designed by solution casting method. The evolution of the SANS spectra of C1-TEA containing membranes is characteristic to the membranes swollen with polar species e.g. water as revealed by the shift of the ionomer peak toward small angles. In contrast, the introduction of C4-TEA and C8-TEA ionic liquids into Nafion® matrix, even in pretty large quantities, does not result in structural modifications at the nanoscale. Depending on the PIL structure and concentration, the resulting doped membranes exhibit very different functional properties (conductivity, water uptake, gas permeability coefficients, etc.). With respect with the safety, LPB solvent free polymer electrolyte films are very attractive in particular those made from poly(oxyethylene) (POE) host polymer. POE has outstanding ability to solvate lithium cation. It suffers however of various handicaps, namely (i) a high crystallinity that penalizes the electrolyte conductivity below the melting temperature (ii) a mechanical strength that vanishes following the polymer electrolyte melting and (iii) a stability in oxidation limited to roughly 3.9 V vs Li/Li+. In addition to the high performances such as the thermo mechanical, chemical, electrochemical properties, a special attention has to be paid to ion transport. The use of a battery based on solvent-free polymer electrolyte is partly conditioned by its melting temperature as the conductivity occurs mainly at the amorphous state. The architecture modification of POE is one of the routes to decrease the crystallinity but does not allow overcoming the bottleneck of a low cationic transference number, T+, in POE electrolytes. The use of IL such as tetraalkyl ammoniums is one of the routes to get sufficient conductivities at low temperatures but, once again, T+ remains low. Furthermore these ionic liquids, which benefit to the conductivity, do not participate in the electrode reactions. We have therefore directed our researches towards lithium-conducting ionic liquids. Hence, this work proposes to address these issues by designing new ionic liquid lithium salts containing the oxyethylene moiety (mPEG) and the perfluorosulfonate/ sulfonimide anion. Consequently, a large number of new ionic liquids were synthetized through original methods. The impact of (i) the number of oxyethylene units (ii) the different chemical structures and (iii) the anion on their intrinsic performances and those of the related polymer electrolytes was investigated. High cation transference numbers were obtained in host polymers based on linear POE or cross-linked POE, and its values increase with the lengthening of the mPEG. The adding of mPEG chain on the anion leads generally to greater conductivity of polymer electrolytes that increases with increasing mPEG lengths. 1) R. Sood, C. Iojoiu, E. Espuche, et al. J. Memb. Sci., 2015, 495, 445-456. 2) R. Sood, C. Iojoiu, E. Espuche, et al. , J. Phys. Chem. 2014, 18 ( 26), 14157-14168. 3) R. Sood, C. Iojoiu, E. Espuche, et al J. Phys. Chem. C, 2012, 116 , 46, 24413-24423 4) V. di Noto, E. Negro, JC. Iojoiu, J. Am. Chem. Soc., 2010, 132, 2183-2193 5) M. Martinez, C. Iojoiu, P. Judeinstein, et al., J. Power Sources, 2010, 195(18), 5829-5839 6) P. Judeinstein, C. Iojoiu, J.-Y. Sanchez et al. J. Phys. Chem. B, 2008, 112, 12, 3680-3683.

Inter- and Intramolecular Interactions in Imidazolium Protic Ionic Liquids

The interactions of alkyl substituted imidazolium bis(trifluoromethanolsulfonyl)imide protic ionic liquids (PILs) HCnImNTf2 (n = 0-12) were studied using vibrational spectroscopy (FT-IR/ATR and FT-Raman) and differential scanning calorimetry (DSC). The effect of alkyl substituent length (n = 0-12) and temperature on the relative magnitude of the different interactions is elucidated. For short carbon alkyl chains (n < 3), the PIL structure is affected from intramolecular interaction caused from the induction effect (+I) due to the chain substituent of the imidazolium ring, while for PILs with n > 3 the van der Waals forces between the chains and π-π interaction between neighboring imidazolium rings become important. The tendency of reducing the melting point and increasing glass transition values with the lengthening of the alkyl chain was also noticed as a result the increasing contribution of the van der Waals forces to the overall interactions. Finally, we also show that the conformational isomerism of the anion (expressed by ΔHeq) is a good indicator of the relative magnitude of the interactions. When Coulombic interactions are predominant, the trans conformer is the most probable, while when other type of interactions (HB, vdW, etc.) become important the cis conformer is favored.

Phase Transition of a Quaternary Ammonium Gemini Surfactant Induced by Minor Structural Changes of Protic Ionic Liquids

The aggregation behaviors of a Gemini surfactant [C12H25(CH3)2N(+)(CH2)2N(+)(CH3)2C12H25]Br2(-) (12-2-12) in two protic ionic liquids (PILs), propylammonium nitrate (PAN) and butylammonium nitrate (BAN), were investigated by means of several experimental techniques including small and wide-angle X-ray scattering, the polarized optical microscopy and the rheological measurement. Compared to those in ethylammonium nitrate (EAN), the minor structural changes with only one or two methylene units (-CH2-) increase in cationic chain length of PIL, result in a dramatic phase transition of formed aggregates. The critical micellization concentration was increased in PAN, while no micelle formation was detected in BAN. A normal hexagonal phase was observed in the 12-2-12/PAN system, while the normal hexagonal, bicontinuous cubic, and lamellar phases were mapped in the 12-2-12/BAN system. Such aggregation behavior changes can be ascribed to the weaker solvophobic interactions of 12-2-12 in PAN and BAN. The unique molecular structure of 12-2-12 is also an important factor to highlight such a dramatic phase transition due to the PIL structure change.

Thermochemistry of 1,1,3,3-tetramethylguanidine and 1,1,3,3-tetramethylguanidinium nitrate

Considerable efforts have recently been made to investigate how the structure of protic ionic liquids determines some of their most important properties for technological applications (e.g. low vapour pressure, conductivity). In contrast, the assessment of cohesive energies (which are also linked to those properties) based on thermodynamic results is still practically unexplored. This problem is addressed here for 1,1,3,3-tetramethylguanidinium nitrate, [Htmg][NO3], through a combination of experimental and computational chemistry results at the reference temperature of 298.15K. The standard molar enthalpies of formation, ΔfHmo([Htmg][NO3],cr)=-(311.8±2.3)kJ·mol-1, and fusion, ΔfusHmo([Htmg][NO3])=16.2±3.8kJ·mol-1, of solid 1,1,3,3-tetramethylguanidinium nitrate (form I polymorph) were obtained by combustion and differential scanning calorimetry, respectively. From these results ΔfHmo([Htmg][NO3],l)=-(295.6±4.4)kJ·mol-1 could be derived. Also determined were the standard molar enthalpies of formation, ΔfHmo(tmg, l)=7.7±2.8kJ·mol-1, and vaporisation, ΔvapHmo(tmg,l)=50.0±1.2kJ·mol-1, of 1,1,3,3-tetramethylguanidine, by reaction solution and Calvet-drop microcalorimetry, respectively, leading to ΔfHmo(tmg,g)=57.7±3.0kJ·mol-1. This result is in excellent agreement with ΔfHmo(tmg,g)=58.4±4.0kJ·mol-1, obtained from the ab initio calculation of the standard molar enthalpy of atomization of tmg using the W1-F12 procedure. From the above results, the cohesive energy of [Htmg][NO3] (l) could be evaluated. Finally, the nature of a solid to solid phase transition observed for [Htmg][NO3] at 221.4±1.2K using DSC was investigated by single crystal X-ray diffraction. The structural results indicate that complete proton transfer from HNO3 to tmg only occurs above the phase transition temperature.

Unravelling the Structure of Protic Ionic Liquids with Theoretical and Experimental Methods: Ethyl-, Propyl- and Butylammonium Nitrate Explored by Raman Spectroscopy and DFT Calculations

We present an analysis of gas-phase structures of small clusters of n-alkylammonium nitrates (ethyl, propyl, and butyl) together with vibrational Raman spectroscopy of their respective liquid phases. The assignment and interpretation of the resonant frequencies have been performed by comparison with high-quality ab initio (DFT) computations. The theoretical spectra are in excellent agreement with the measured ones and allow the interpretation and assignment of almost all the spectral features. A careful analysis of the vibrational frequencies and of the electronic structure of the compounds has provided additional information on various structural features and on the rather complex hydrogen bonding network that exists in such compounds. A geometric structure of the short-range local arrangement in the bulk phases is also proposed.

Composition of the outermost layer and concentration depth profiles of ammonium nitrate ionic liquid surfaces

Differences in the surface structure of protic ionic liquids (ILs) with three different cations and a common anion; ethyl-, propyl- and 2-hydroxyethyl- (or ethanol-) ammonium nitrate (EAN, PAN and EtAN, respectively) have been observed by neutral impact collision ion scattering spectroscopy (NICISS) and metastable induced electron spectroscopy/ultraviolet photoelectron spectroscopy (MIES/UPS). NICISS is used to determine the concentration depth profiles of the elements in each IL and it reveals an enrichment of cation alkyl chains of PAN and EtAN in the outermost layer compared to EAN, and a corresponding depletion of nitrate from the outermost layer of the EtAN surface. MIES probes the molecular orbitals of only the species in the outermost layer of a sample and confirms that, while both the anion and the cation are present to some degree at the surface of all three ILs, the cation is enriched to a greater extent at the surface of PAN and EtAN compared to EAN.

Nanostructured Protic Ionic Liquids Retain Nanoscale Features in Aqueous Solution While Precursor Brønsted Acids and Bases Exhibit Different Behavior

Small- and wide-angle X-ray scattering (SWAXS) has been used to investigate the effect that water has on the nanoscale structure of protic ionic liquids (PILs) along with their precursor Brønsted acids and bases. The series of PILs consisted of primary, secondary, and tertiary alkylammonium cations in conjunction with formate, nitrate, or glycolate anions. Significant differences were observed for these systems. The nanoscale aggregates present in neat protic ionic liquids were shown to be stable in size on dilution to high concentrations of water, indicating that the water is localized in the ionic region and has little effect on the nonpolar domains. The Brønsted acid-water solutions did not display nanostructure at any water concentration. Primary amine Brønsted bases formed aggregates in water, which generally displayed characteristics of poorly structured microemulsions or a form of bicontinuous phase. Exceptions were butyl- and pentylamine with high water concentrations, for which the SWAXS patterns fitted well to the Teubner-Strey model for microemulsions. Brønsted base amines containing multiple alkyl chains or hydroxyl groups did not display nanostructure at any water concentration. IR spectroscopy was used to investigate the nature of water in the various solutions. For low PIL concentrations, the water was predominately present as bulk water for PIL molar fractions less than 0.4-0.5. At high PIL concentrations, in addition to the bulk water, there was a significant proportion of perturbed water, which is water influenced in some way by the cations and anions. The molecular state of the water in the studied amines was predominately present as bulk water, with smaller contributions from perturbed water than was seen in the PILs.

Do all the protic ionic liquids exist as molecular aggregates in the gas phase?

According to an EI-MS study of 1,1,3,3-tetramethylguanidium-based protic ionic liquids (PILs), it has been concluded that not all PILs exist as molecular aggregates in the gas phase. The detection of both ions of m/z 115.0 and m/z 116.0 for the 1,1,3,3-tetramethylguanidinium trifluoromethylsulfonate (TMGS) protic ionic liquid indicates that both the molecular and ionic aggregates co-exist in the gas phase, which is to say that the TMGS may also evaporate via the ionic aggregates just like aprotic ionic liquids. Furthermore, investigation on triethylamine-based and 1-methylimidazole-based PILs confirmed that the gas phase structure of PILs depends on both the acidity and basicity of the corresponding acid and base.

Formation of Amphiphile Self-Assembly Phases in Protic Ionic Liquids

A range of protic ionic liquids (PILs) have been identified as being capable of supporting the self-assembly of the nonionic surfactants myverol 18-99 K (predominantly monoolein) and phytantriol. PIL-surfactant penetration scans have provided a high throughput technique to determine which lyotropic liquid crystalline phases were formed in the 40 PIL-surfactant systems investigated. Lamellar, inverse hexagonal, and bicontinuous cubic phases that are stable in excess PIL have been observed in surfactant-PIL systems. The studied PILs possess a wide range of solvent properties, including surface tension and viscosity. The nature of the formed amphiphile self-assembly phases is discussed in terms of the PIL structure and solvent properties.