Prototypical Ionic Liquid Research Articles

Redox potentials in ionic liquids: Anomalous behavior?

Redox potentials depend on the nature of the solvent/electrolyte through the solvation energies of the ionic solute species. For concentrated electrolytes, ion solvation may deviate significantly from the Born model predictions due to ion pairing and correlation effects. Recently, Ghorai and Matyushov [J. Phys. Chem. B 124, 3754-3769 (2020)] predicted, on the basis of linear response theory, an anomalous trend in the solvation energies of room temperature ionic liquids, with deviations of hundreds of kJ/mol from the Born model for certain size solutes/ions. In this work, we computationally evaluate ionic solvation energies in the prototypical ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM/BF4), to further explore this behavior and benchmark several of the approximations utilized in the solvation energy predictions. For comparison, we additionally compute solvation energies within acetonitrile and molten NaCl salt to illustrate the limiting behavior of purely dipolar and ionic solvents. We find that the overscreening effect, which results from the inherent charge oscillations of the ionic liquid, is substantially reduced in magnitude due to screening from the dipoles of the molecular ions. Therefore, for the molten NaCl salt, for which the ions do not have permanent dipoles, modulation of ionic solvation energies from the overscreening effect is most significant. The conclusion is that ionic liquids do indeed exhibit unique solvation behavior due to peak(s) in the electrical susceptibility caused by the ion shell structure; redox potential shifts for BMIM/BF4 are of more modest order ∼0.1V, but may be larger for other ionic liquids that approach molten salt behavior.

Experimental and computational studies on the ion dissociation states of 1-butyl-3-methyimidazolium tetrafluoroborate in water and alcohols

Herein, we investigated the dissociation states of ion pairs in the prototype ionic liquid (IL), 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4), when mixed with water or alcohol, a homologous substituent of water, during experimentation and computation. Specifically, the degree of dissociation of an ion pair was determined through the density, electrical conductivity and self-diffusion coefficient using the Nernst–Einstein equation, and microscopic information regarding the solution was obtained through molecular dynamics simulations. A schematic model was proposed based on the Experimental and computational results. In the region of low IL concentration, [BMIm] and BF4 existed in a loosely aggregated or free ionic state in the [BMIm]BF4/water mixture. However, [BMIm] and BF4 mainly formed ion pairs, and the ion pairs did not aggregate much with each other in the [BMIm]BF4/alcohol mixture.

Purification of Ionic Liquid Solvents in a Self-Optimizing, Continuous Microfluidic Process via Extraction of Metal Ions and Phase Separation

Industrial applications of ionic liquids (ILs)─solvents that can serve as green alternatives to volatile organic compounds─are often hampered by their high cost. Solvent recycling provides a feasible pathway to recover IL solvents to reduce lifecycle costs. Herein, we demonstrate a continuous microfluidic process to purify metal-ion-loaded IL solvents, wherein Fe(III) ions are extracted from a prototypical IL, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM-NTf2), to deionized (DI) water with subsequent membrane separation of the IL and aqueous phases. Inline analytical tools, design of experiment statistical optimization, and a self-optimizing, modified Nelder-Mead simplex algorithm facilitate locating the best parametric operating conditions to optimize both ion extraction and physical phase separation. This process was then adapted to a more challenging purification application: recovery of the IL trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethyl-pentyl)phosphinate (Cyphos 104) from the rare-earth metal Nd(III). This application demonstrated that optimized conditions obtained from a single stage could be applied across a multistage process. Together, these results demonstrate that statistical and inline optimization tools can be used to identify working parameters for different flow systems with a variety of governing fluid properties, for example, viscosity and interfacial tension.

Redox Potentials in Ionic Liquids: Anomalous Behavior?

Characterizing redox potentials in ionic liquids us essential for expanding the use of these electrolytes for electrochemical application. In particular, solvation energies of the different redox species (e.g. radical ions) determine the observed solvent-dependent trends in redox activity. In this regard, ionic liquids are fundamentally distinct from dipolar, molecular liquids in that solvation proceeds by entirely different physical mechanisms. Recently, it has been suggested that the unique ion arrangement of ionic liquids leads to dramatic deviations from Born theory in ion solvation. This conclusion was made from the theoretical assumptions of linear and homogenous response with a monopole approximation. We explore these theoretical approximations through a comparison to free energy calculations from atomistic and explicitly polarizable molecular dynamics simulations of a model ion in the prototypical ionic liquid BMIM/BF4. We find that a sharp peak in the susceptibility around 1 Å-1 is heavily influenced by the atomistic modeling and does not translate to significant free energy anomalies sans the monopole approximation. Furthermore, we show that ionic liquids largely can be approximated through linear response theory with satisfactory accuracy, although the continuum approximation does not adequately describe the system. We also explore the kinetics through the reorganization energy and observe similar trends.

Transport properties of alkali/alkaline earth cations in ionic-liquid based electrolytes

The increasing need of portable electrical resources requires to develop post-Li batteries, in which redox reactions are then based on the different alkali or earth alkaline ions. Keeping in mind the specific advantages of electrolytes based on ionic liquid (electrochemical properties, safety, stability …), this paper focuses on the specific properties of those obtained by mixing alkali or earth alkaline salts (Li+, Na+, K+, Cs+ and Mg2+) in a prototypical ionic liquid, the 1-butyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl) imide (BMImTFSI). Transport properties of these electrolytes are deciphered from viscosity, ionic conductivity and individual self-diffusion coefficients. At room temperature, change of electrolyte composition (nature of ion, concentration) induces some large variations of these transport properties. However, if these measurements are scaled towards glass transition temperatures, master curves are obtained with only slight differences between monovalent alkali and divalent earth alkaline ions. Further information is obtained from the Walden approach and evidences that these electrolytes are 'good ionic conductors'. These results are confirmed from self-diffusion coefficients which also allows to retrieve contributions of each species to ionic transport and then dissociation ratio inside these mixtures. Slight differences between alkali and earth alkali ions may be related to solvation mechanisms, as proposed from infrared spectroscopy measurements.

Interfacial Water and Microheterogeneity in Aqueous Solutions of Ionic Liquids.

In this work, aqueous solutions of two prototypical ionic liquids (ILs), [BMIM][BF4] and [BMIM][TfO], were investigated by UV Raman spectroscopy and small-angle neutron scattering (SANS) in the water-rich domain, where strong heterogeneities at mesoscopic length scales (microheterogeneity) were expected. Analyzing Raman data by a differential method, the solute-correlated (SC) spectrum was extracted from the OH stretching profiles, emphasizing specific hydration features of the anions. SC-UV Raman spectra pointed out the molecular structuring of the interfacial water in these microheterogeneous IL/water mixtures, in which IL aggregates coexist with bulk water domains. The organization of the interfacial water differs for the [BMIM][BF4] and [BMIM][TfO] solutions, being affected by specific anion–water interactions. In particular, in the case of [BMIM][BF4], which forms weaker H-bonds with water, the aggregation properties clearly depend on concentration, as reflected by local changes in the interfacial water. On the other hand, stronger water–anion hydrogen bonds and more persistent hydration layers were observed for [BMIM][TfO], which likely prevent changes in IL aggregates. The modeling of SANS profiles, extended to [BPy][BF4] and [BPy][TfO], evidences the occurrence of significant concentration fluctuations for all of the systems: this appears as a rather general phenomenon that can be ascribed to the presence of IL aggregation, mainly induced by (cation-driven) hydrophobic interactions. Nevertheless, larger concentration fluctuations were observed for [BMIM][BF4], suggesting that anion–water interactions are relevant in modulating the microheterogeneity of the mixture.

Assessing the propensity toward ionization in nanosized clusters of protic ionic liquids by Ab-initio methods

Nanosized clusters of protic ionic liquids are investigated using ab-initio DFT methods to understand their propensity to remain ionic. Protic ionic liquids are based on acid/base pairs and are ionic because of proton transfer from the acid to the base. Two prototypical ionic liquids based on triethylamine and two acids of different strengths are used here to trace the tendency toward proton transfer when the system size is gradually increased from an isolated pair of molecules to cluster of several pairs. We have found that the compounds based on the weak acid shows the existence of a well-defined minimal size that represents the threshold for the onset of ionization phenomena.

Ether tails make a large difference for the structural dynamics of imidazolium-based ionic liquids

Ionic Liquids are often characterized by multiple length scales and these tend to be well separated. As an example, for 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)imide apolar pockets or domains are separated by a characteristic distance that is different from that of positive-negative charge alternation along a charge network. Accordingly, these structural features have their own well separated signatures in the reciprocal space structure function. Each of these well separated structural features of prototypical ionic liquids relaxes on its own time scale and we have explored this in a set of recent articles. The overarching goal of the current work is to explore what happens when the structural motifs are still present, but the length scale difference between these is small. Specifically, for ionic liquids containing ether tail functionalities such as 1-(2-(2-ethoxyethoxy)ethyl)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, the separation between cationic head subspecies intercalated by anions (or anions sharing a cationic head counterion) is not that different from that of cationic heads or anions intercalated by tail regions and the time scale for relaxation of these structural motifs are also similar. For the ether-tail containing system, the slowest liquid relaxation scale associated with tail region-charge network correlations is about an order of magnitude faster than for its alkyl-tail isoelectronic analog.

Nonlinear measurements of kinetics and generalized dynamical modes. II. Application to a simulation of solvation dynamics in an ionic liquid.

Solvation dynamics in ionic liquids show features that are often associated with supercooled liquids, including "stretched" nonexponential relaxation. To better understand the mechanism behind the stretching, the nonlinear mode-correlation methods proposed in Paper I [S. R. Hodge and M. A. Berg, J. Chem. Phys. 155, 024122 (2021)] are applied to a simulation of a prototypical ionic liquid. A full Green's function is recovered. In addition, specific tests for non-Gaussian dynamics are made. No deviations from Gaussian dynamics are found. This finding is incompatible with rate heterogeneity as a cause of the nonexponential relaxation and appears to be in conflict with an earlier multidimensional analysis of the same data. Although this conflict is not resolved here, this work does demonstrate the practicality of mode-correlation analysis in the face of finite datasets and calculations.

A Pictorial View of Viscosity in Ionic Liquids and the Link to Nanostructural Heterogeneity.

Prototypical ionic liquids (ILs) are characterized by three structural motifs associated with (1) vicinal interactions, (2) the formation of positive-negative charge-alternating chains or networks, and (3) the alternation of these networks with apolar domains. In recent articles, we highlighted that the friction and mobility in these systems are nowhere close to being spatially homogeneous. This results in what one could call mechanical heterogeneity, where charge networks are intrinsically stiff and charge-depleted regions are softer, flexible, and mobile. This Letter attempts to provide a clear and visual connection between friction-associated with the dynamics of the structural motifs (in particular, the charge network)-and recent theoretical work by Yamaguchi linking the time-dependent viscosity of ILs to the decay of the charge alternation peak in the dynamic structure function. We propose that charge blurring associated with the loss of memory of where positive and negative charges are within networks is the key mechanism associated with viscosity in ILs. An IL will have low viscosity if a characteristic charge-blurring decorrelation time is low. With this in mind, engineering new low-viscosity ILs is reduced to understanding how to minimize this quantity.

Relaxation in a Prototype Ionic Liquid: Influence of Water on the Dynamics.

The influence of water on the relaxation of a prototype ionic liquid (IL) C8mimBF4 is examined in the IL-rich regime combining quasi-elastic neutron scattering (QENS) and molecular dynamics (MD) simulations. The QENS and MD simulations results for relaxation of IL and the equimolar mixture with water probed by the dynamics of the C8mim hydrogen atoms in the time range of 2 ps to 1 ns are in excellent agreement. The QENS data show that translational relaxation increases by a factor of 7 on the addition of water, while rotational relaxation involving multiple processes fitted by a KWW function with low β values is speeded up by a factor of 3 on the time scale of QENS measurements. The MD simulations show that the cation diffusion coefficient, inverse viscosity, and ionic conductivity increase on the addition of water, consistent with the very small change in ionicity. The difficulties in obtaining rotational and translational diffusion coefficients from fits to QENS experiments of pure ILs and IL-water mixtures are discussed.

Microscopic solvation environments in a prototype room-temperature ionic liquid as elucidated by resonance Raman spectroscopy of iodine and bromine

Microscopic solvation environments in a prototype ionic liquid, bmimTf2N; 1-butyl-3-methyl-imidazolium-bis(trifluoromethanesulfonyl)imide, have been studied with the use of halides, X2 and Xn− (X=I, Br; n=3,5), as molecular probes. Resonance Raman spectroscopy has been used to detect these halogen species existing in bmimTf2N as well as in reference solvents including heptane, cyclohexane, KX/H2O and benzene. In heptane and cyclohexane, only free X2 species are detected. In KX/H2O, only Xn− and, in benzene, only benzene-X2 complexes are detected. On the contrary, free X2 and Xn− are concomitantly detected in bmimTf2N, indicating that there are two distinct solvation environments in bmimTf2N, non-polar environments that solvate free X2 and polar environments that stabilize Xn−. These two distinct solvation environments are most likely to arise from microscopic structural heterogeneity of ionic liquids.

Relationship between pore size and reversible and irreversible immobilization of ionic liquid electrolytes in porous carbon under applied electric potential

Transport of electrolytes in nanoporous carbon-based electrodes largely defines the function and performance of energy storage devices. Using molecular dynamics simulation and quasielastic neutron scattering, we investigate the microscopic dynamics of a prototypical ionic liquid electrolyte, [emim][Tf2N], under applied electric potential in carbon materials with 6.7 nm and 1.5 nm pores. The simulations demonstrate the formation of dense layers of counter-ions near the charged surfaces, which is reversible when the polarity is reversed. In the experiment, the ions immobilized near the surface manifest themselves in the elastic scattering signal. The experimentally observed ion immobilization near the wall is fully reversible as a function of the applied electric potential in the 6.7 nm, but not in the 1.5 nm nanopores. In the latter case, remarkably, the first application of the electric potential leads to apparently irreversible immobilization of cations or anions, depending on the polarity, near the carbon pore walls. This unexpectedly demonstrates that in carbon electrode materials with the small pores, which are optimal for energy storage applications, the polarity of the electrical potential applied for the first time after the introduction of an ionic liquid electrolyte may define the decoration of the small pore walls with ions for prolonged periods of time and possibly for the lifetime of the electrode.

3-Dimensional Structure of a Prototypical Ionic Liquid–Solid Interface: Ionic Crystal-Like Behavior Induced by Molecule–Substrate Interactions

The molecular structure of the ionic liquid–electrode interface has been recently investigated by atomic force microscopy (AFM) methods focusing either on the vertical structure of the ion layers or on the lateral structure of the innermost layer. Here, we combine high-resolution AFM imaging with atomic force spectroscopy measurements to elucidate the structure of the interface between the ionic liquid propylammonium nitrate (PAN) and highly ordered pyrolytic graphite (HOPG). The lateral structure of the innermost layer of adsorbed molecules (i.e., the Stern layer) is resolved on the molecular scale by means of amplitude modulation atomic force microscopy (AM-AFM). A quasi (4 × 4)R0° overlayer is formed by the ionic liquid molecules on the HOPG surface. Additional dynamic mode force spectroscopy measurements reveal the existence of a layered structure of the ionic liquid normal to the surface plane and allow for a precise determination of the layer spacing. We are able to infer the three-dimensional struc...

Effect of water on the structure of a prototype ionic liquid.

The influence of water on the structure of a prototype ionic liquid (IL) 1-octyl-3-methylimidazolium tetrafluoroborate (C8mimBF4) is examined in the IL-rich regime using high-energy X-ray diffraction (HEXRD) and molecular dynamics (MD) simulations. A many-body polarizable force field APPLE&P was developed for C8mimBF4-water mixture. It predicts structure factors of pure IL and IL-water mixture in excellent agreement with the HEXRD experiments. The MD results provide detailed insights into the structural changes from the partial structure factors, 2-D projections of the simulation box and 3-D distribution functions. Water partitioning with IL and its competition with BF4(-) for complexing the imidazolium rings was examined. The added water molecules occupy a diffuse coordination shell around the imidazolium ring but are not present around the alkyl tail. The strong coordination of the fluorine atoms of the BF4(-) anions to the imidazolium ring is not significantly changed by the addition of water. A complementary packing of water and imidazolium around BF4(-) was found. These results are consistent with the very small differences in the average structure between the pure IL and the mixture.

Modern ab initio valence bond theory calculations reveal charge shift bonding in protic ionic liquids.

The nature of bonding interactions between the cation and the anion of an ionic liquid is at the heart of understanding ionic liquid properties. A particularly interesting case is a special class of ionic liquids known as protic ionic liquids. The extent of proton transfer in protic ionic liquids has been observed to vary according to the interacting species. Back proton transfer renders protic ionic liquids volatile and to be considered as inferior ionic liquids. We try to address this issue by employing modern ab initio valence bond theory calculations. The results indicate that the bonding in the cation and the anion of a prototypical ionic liquid, ethylammonium nitrate, is fundamentally different. It is neither characteristic of covalent/polar covalent bonding nor ionic bonding but rather charge shift bonding as a resonance hybrid of two competing ionic molecular electronic structure configurations. An investigation of other analogous protic ionic liquids reveals that this charge shift bonding seems to be a typical characteristic of protic ionic liquids while the ionic solid analogue compound ammonium nitrate has less charge shift bonding character as compared to protic ionic liquids. Further the extent of charge shift bonding character has been found to be congruent with the trends in many physicochemical properties such as melting point, conductivity, viscosity, and ionicity of the studied ionic liquids indicating that percentage charge shift character may serve as a key descriptor for large scale computational screening of ionic liquids with desired properties.

Study of ionic liquid as effective additive for minimum quantity lubrication during titanium machining

The objective of this study is to investigate the effectiveness of ionic liquid (IL), a low melting point salt, as a lubricant additive for minimum quantity lubrication (MQL) implementation during titanium machining. Herein 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6), a prototypical IL, has been used as an MQL cutting fluid additive. It is found that IL-based MQL can effectively reduce the tool wear by 60% when compared to dry cutting and 15% more than MQL without BMIM-PF6. It is also found that IL-based MQL results in the smallest cutting forces, largest saw-tooth/shear band spacing, and best surface finish.

Sensitive determination of Amaranth in drinks by highly dispersed CNT in graphene oxide “water” with the aid of small amounts of ionic liquid

Graphene oxide (GO) is a pH-dependent amphiphile. In this paper, it was found that carbon nanotubes (CNTs) can be highly dispersed in graphene oxide “water” with the aid of prototype ionic liquid of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]). The introduction of [BMIM][PF6] not only can minimize the defects of poor electrical conductivity of GO, but also can improve the dispersibility of CNT in water. Hence, a new composite of CNT/GO-[BMIM][PF6] with high dispersibility and strong conductivity was presented for the first time and employed in the sensitive determination of Amaranth in soft drinks. The detection limit achieved (0.1nM) is much lower than the guideline values in soft drinks. The ease of preparation, low background current, high sensitivity and stability can create novel avenues and applications for fabricating robust sensors for determination of other azo dyes in foods.

Highly dispersed carbon nanotube in new ionic liquid-graphene oxides aqueous dispersions for ultrasensitive dopamine detection

Graphite oxide (GO) is a pH-dependent amphiphile with hydrophilic edges and a more hydrophobic basal plane. It was found that when the hydrophobic prototype ionic liquid (IL) of 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) was added into GO water, it experienced changes from coagulation to being redispersed in water phase by constantly turning its pH to alkali condition. Then new aqueous dispersions of GO-[BMIM][PF6] “water” formed after facile sonicate. The introduction of [BMIM][PF6] not only minimize the defects of poor electrical conductivity of GO and more interesting is that the carbon nanotube (CNT) can be highly dispersed in this GO-[BMIM][PF6] water through a simple and facile ultrasonic method at room temperature, forming a new composite of GO-[BMIM][PF6]-CNT with high dispersibility. This novel composite enhanced its electrochemical signal obviously in the measurement of dopamine (DA) in biological systems and exhibited a wider linear response ranging from 8.0×10−12–1.5×10−5M. The detection limit of 3.0×10−12M at a signal to-noise ratio of 3 was two orders of magnitude lower than that reported previously.

Ionic conductivity enhancement of sputtered gold nanoparticle-in-ionic liquid electrolytes

Ionic liquids (ILs) are being widely investigated as advanced electrolytes within electric double-layer capacitors (EDLCs) due to their inherent ionic conductivity, wide electrochemical windows, essentially zero volatility, and high temperature stability. Despite being composed entirely of ions, the ionic conductivity of a typical IL is significantly hindered by its high viscosity, rendering it akin to normal electrolytes. In this light, in order to increase the applicability of IL electrolytes, it is of the utmost priority to discover approaches for improving the electrochemical properties of ILs without adversely affecting their other beneficial attributes. In this work, we make important strides toward this goal by employing low energy sputtering to generate novel electrolytes comprising gold nanoparticle dispersions within the prototypical IL 1-ethyl-3-methylimidazolium ethyl sulfate, [emim][EtSO4]. This study also afforded the unique opportunity to investigate nanoscale growth mechanisms occurring within the IL. Cyclic voltammetry and electrochemical impedance spectroscopy analyses revealed that when the IL contained a substantial fraction of sub-nanometer-sized particles, the double-layer capacitance was increased by ∼190%, concomitant with a bulk electrolyte resistance decrease of ∼70% with respect to a gold-free control. An exponential rise in resistance accompanied by a proportional decrease in capacitance accompanies nanoparticle growth until a critical size is reached—typically within 10 h at room temperature—beyond which the final capacitance is typically ∼60% higher than the control with an electrolyte resistance similar to the control. Overall, our results reveal an anomalous capacitance increase and low internal resistance for nanoparticle-in-IL dispersions, suggesting intriguing potential as electrolytes for next-generation EDLCs, fuel cells, and sensors.