Mixtures of octanol and an ionic liquid: Structure and transport.

Ionic liquids (ILs) with long alkyl substituents are amphiphilic, which leads to a bicontinuous liquid structure. The strongly interacting anionic and cationic head groups form a long range charge network, with the hydrocarbon tails forming a nonpolar domain. Such nonpolar domains have been shown to dissolve a variety of neutral organic solvents. In mixtures of ILs with solvents the neutral organic molecules residing in the nonpolar domains experience different environments and friction from the charged cations and anions. Thus, the neutral molecules diffuse much faster than predicted by hydrodynamic scaling using the average viscosity of the mixture. In this work, we report studies on the structure and transport properties of mixtures of 1-octanol with the IL trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)imide (P6,6,6,14 +/NTf2 -). The majority of the atom fraction in the P6,6,6,14 + cation comprises four hydrocarbon substituents. The unique amphiphilic nature of ILs with the P6,6,6,14 + cation makes 1-octanol fully miscible with the IL at ambient temperatures. X-ray scattering experiments show that the IL structure persists in the mixtures for 1-octanol mole fractions as large as xoct = 0.90. The self-diffusion coefficients of the three molecular species in the mixtures were measured by NMR experiments. The self-diffusion of the P6,6,6,14 + cation is well described by the Stokes-Einstein equation, while the diffusivity of the NTf2 - anion is slightly lower than the hydrodynamic prediction. The measured diffusivities of octanol in these mixtures are 1.3-4 times higher than the hydrodynamic predictions.

Structural analysis of ionic liquids with symmetric and asymmetric fluorinated anions.

Ionic liquids (ILs) with relatively low viscosities and broad windows of electrochemical stability are often constructed by pairing asymmetric cations with bisfluorosulfonylimide (FSI-) or bistriflimide (NTf2 -) anions. In this work, we systematically studied the structures of ILs with these anions and related perfluorobis-sulfonylimide anions with asymmetry and/or longer chains: (fluorosulfonyl)(trifluoromethylsulfonyl)imide (BSI0,1 -), bis(pentafluoroethylsulfonyl)imide (BETI-), and (trifluoromethylsulfonyl) (nonafluorobutylsulfonyl)imide (BSI1,4 -) using high energy X-ray scattering and molecular dynamics simulation methods. 1-alkyl-3-methylimidazolium cations with shorter (ethyl, Im2,1 +) and longer (octyl, Im1,8 +) hydrocarbon chains were selected to examine how the sizes of nonpolar hydrocarbon and fluorous chains affect IL structures and properties. In comparison with these, we also computationally explored the structure of ionic liquids with anions having longer fluorinated tails.

Structural analysis of zwitterionic liquids vs. homologous ionic liquids.

Zwitterionic liquids (Zw-ILs) have been developed that are homologous to monovalent ionic liquids (ILs) and show great promise for controlled dissolution of cellulosic biomass. Using both high energy X-ray scattering and atomistic molecular simulations, this article compares the bulk liquid structural properties for novel Zw-ILs with their homologous ILs. It is shown that the significant localization of the charges on Zw-ILs leads to charge ordering similar to that observed for conventional ionic liquids with monovalent anions and cations. A low-intensity first sharp diffraction peak in the liquid structure factor S(q) is observed for both the Zw-IL and the IL. This is unexpected since both the Zw-IL and IL have a 2-(2-methoxyethoxy)ethyl (diether) functional group on the cationic imidazolium ring and ether functional groups are known to suppress this peak. Detailed analyses show that this intermediate range order in the liquid structure arises for slightly different reasons in the Zw-IL vs. the IL. For the Zw-IL, the ether tails in the liquid are shown to aggregate into nanoscale domains.

Structure and dynamics of propylammonium nitrate-acetonitrile mixtures: An intricate multi-scale system probed with experimental and theoretical techniques.

In this article, we report the study of structural and dynamical properties for a series of acetonitrile/propylammonium nitrate mixtures as a function of their composition. These systems display an unusual increase in intensity in their X-ray diffraction patterns in the low-q regime, and their 1H-NMR diffusion-ordered NMR spectroscopy (DOSY) spectra display unusual diffusivities. However, the magnitude of both phenomena for mixtures of propylammonium nitrate is smaller than those observed for ethylammonium nitrate mixtures with the same cosolvent, suggesting that the cation alkyl tail plays an important role in these observations. The experimental X-ray scattering data are compared with the results of molecular dynamics simulations, including both ab initio studies used to interpret short-range interactions and classical simulations to describe longer range interactions. The higher level calculations highlight the presence of a strong hydrogen bond network within the ionic liquid, only slightly perturbed even at high acetonitrile concentration. These strong interactions lead to the symmetry breaking of the NO3- vibrations, with a splitting of about 88 cm-1 in the ν3 antisymmetric stretch. The classical force field simulations use a greater number of ion pairs, but are not capable of fully describing the longest range interactions, although they do successfully account for the observed concentration trend, and the analysis of the models confirms the nano-inhomogeneity of these kinds of samples.

Photoinduced Bimolecular Electron Transfer in Ionic Liquids.

The present work seeks to better understand the role of solute diffusion and solvation dynamics on bimolecular electron transfer in ionic liquids (ILs). Steady-state and time-resolved measurements of the reductive fluorescence quenching of five fluorophores ("F") by six quenchers ("Q"; electron donors) are reported in acetonitrile and two ionic liquids, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide and trihexyl(tetradecyl)phosphonium bis(trifluoromethylsulfonyl)amide. Data were collected on 66 different F-Q-solvent systems, which span a 2.0 eV range in driving force and viscosities that vary 1000-fold, enabling stringent tests of bimolecular electron transfer models. A Stern-Volmer analysis yielded much larger diffusion-limited rates than simple kinetic theory predictions in the ILs and the absence of a Marcus turnover. Use of an approximate solution to the spherical diffusion-reaction equation enabled testing of several models for the reaction rate distance dependence. The Smoluchowski and Collins-Kimball models, which assume reaction at a single distance, are able to fit the data collected in acetonitrile solutions reasonably well, but not the data in the IL solvents. An extended sink model, incorporating a finite reaction zone, was able to fit all data satisfactorily with only three adjustable parameters. Diffusion coefficients extracted from these fits were much larger for the neutral versus anionic quenchers and close to predicted values. Molecular dynamics simulations and density-functional methods were then used to explore solvation structures and electronic couplings. The electronic coupling between contact F-Q pairs was found to vary strongly with the relative location and orientation of the reactants. Information from these simulations was used to constrain a model based on classical Marcus theory, which provided physically reasonable fits with only two adjustable parameters, but required systematic reduction of the driving forces in order to suppress a rate turnover at large driving force. The latter requirement indicates that reaction rates in ionic liquids are limited by some factor not properly accounted for in bimolecular electron transfer models based on a spherical diffusion-reaction approach. Small-amplitude motions within contact F-Q pairs, which gate the electronic coupling, are suggested to be the limiting dynamics.

Intriguing transport dynamics of ethylammonium nitrate-acetonitrile binary mixtures arising from nano-inhomogeneity.

Binary mixtures of ethylammonium nitrate and acetonitrile show interesting properties that originate from the structural and dynamical nano-heterogeneity present in ionic liquids. These effects are most pronounced when the ionic liquid is the minority compound. In this study the transport properties of such mixtures are studied, including viscosity, self-diffusion and conductivity. The results strongly support the presence of structural inhomogeneity and show an interesting composition-dependent behaviour in the mixtures.

Ionic liquid ultrathin films at the surface of Cu(100) and Au(111).

Monolayer to multilayer ultrathin films of the ionic liquid (IL) 1-methyl-3-octylimidazolium bis(trifluoromethylsulfonyl)amide have been prepared on Au(111) and Cu(100) surfaces using physical vapor deposition. The ion-surface interactions are studied using a combination of scanning tunnel microscopy, as well as ultraviolet and x-ray photoemission spectroscopies. It is found that the IL does not decompose at the surface of the metals, and that the IL interaction with the Cu(100) surface is much stronger than with the Au(111) surface. As a consequence, STM imaging at room temperature results in more stable imaging at the monolayer coverage on Cu(100) than on Au(111), and work function measurements indicate a large interface dipole upon deposition of a monolayer of IL on Cu. Additional IL depositions on the two surfaces result in two distinct behaviors for the IL core levels: a gradual energy shift of the core levels on Au and a set of two well defined monolayer and multilayer core level components found at fixed energies on Cu, due to the formation of a tightly bound monolayer. Finally, it is proposed that the particularly strong cation-Cu interaction leads to stabilization of the anion and prevents its decomposition at the surface of Cu(100).

Structure and dynamics of ionic liquids: Trimethylsilylpropyl-substituted cations and bis(sulfonyl)amide anions.

Ionic liquids with cationic organosilicon groups have been shown to have a number of useful properties, including reduced viscosities relative to the homologous cations with hydrocarbon substituents on the cations. We report structural and dynamical properties of four ionic liquids having a trimethylsilylpropyl functional group, including 1-methyl-3-trimethylsilylpropylimidazolium (Si-C3-mim+) cation paired with three anions: bis(fluorosulfonyl)imide (FSI-), bis(trifluoromethanesulfonyl)imide (NTf2-), and bis(pentafluoroethanesulfonyl)imide (BETI-), as well as the analogous N-methyl-N-trimethylsilylpropylpyrrolidinium (Si-C3-pyrr+) cation paired with NTf2-. This choice of ionic liquids permits us to systematically study how increasing the size and hydrophobicity of the anions affects the structural and transport properties of the liquid. Structure factors for the ionic liquids were measured using high energy X-ray diffraction and calculated from molecular dynamics simulations. The liquid structure factors reveal first sharp diffraction peaks (FSDPs) for each of the four ionic liquids studied. Interestingly, the domain size for Si-C3-mim+/NTf2- indicated by the maxima for these peaks is larger than for the more polar ionic liquid with a similar chain length, 1-pentamethyldisiloxymethyl-3-methyl-imidazolium bis(trifluoromethanesulfonyl)imide (SiOSi-mim+/NTf2-). For the series of Si-C3-mim+ ionic liquids, as the size of the anion increases, the position of FSDP indicates that the intermediate range order domains decrease in size, contrary to expectation. Diffusivities for the anions and cations are compared for a series of both hydrocarbon-substituted and silicon-substituted cations. All of the anions show the same scaling with temperature, size, and viscosity, while the cations show two distinct trends-one for hydrocarbon-substituted cations and another for organosilicon-substituted cations, with the latter displaying increased friction.

Structure of cyano-anion ionic liquids: X-ray scattering and simulations.

Ionic liquids with cyano anions have long been used because of their unique combination of low-melting temperatures, reduced viscosities, and increased conductivities. Recently we have shown that cyano anions in ionic liquids are particularly interesting for their potential use as electron donors to excited state photo-acceptors [B. Wu et al., J. Phys. Chem. B 119, 14790-14799 (2015)]. Here we report on bulk structural and quantum mechanical results for a series of ionic liquids based on the 1-ethyl-3-methylimidazolium cation, paired with the following five cyano anions: SeCN(-), SCN(-), N(CN)2 (-), C(CN)3 (-), and B(CN)4 (-). By combining molecular dynamics simulations, high-energy X-ray scattering measurements, and periodic boundary condition DFT calculations, we are able to obtain a comprehensive description of the liquid landscape as well as the nature of the HOMO-LUMO states for these ionic liquids in the condensed phase. Features in the structure functions for these ionic liquids are somewhat different than the commonly observed adjacency, charge-charge, and polarity peaks, especially for the bulkiest B(CN)4 (-) anion. While the other four cyano-anion ionic liquids present an anionic HOMO, the one for Im2,1 (+)/B(CN)4 (-) is cationic.

Communication: Unusual structure and transport in ionic liquid-hexane mixtures.

Ionic liquids having a sufficiently amphiphilic cation can dissolve large volume fractions of alkanes, leading to mixtures with intriguing properties on molecular length scales. The trihexyl(tetradecyl)phosphonium cation paired with the bis(trifluoromethylsulfonyl)amide anion provides an ionic liquid that can dissolve large mole fractions of hexane. We present experimental results on mixtures of n-C6D14 with this ionic liquid. High-energy X-ray scattering studies reveal a persistence of the characteristic features of ionic liquid structure even for 80% dilution with n-C6D14. Nuclear magnetic resonance self-diffusion results reveal decidedly non-hydrodynamic behavior where the self-diffusion of the neutral, non-polar n-C6D14 is on average a factor of 21 times faster than for the cation. Exploitation of the unique structural and transport properties of these mixtures may lead to new opportunities for designer solvents for enhanced chemical reactivity and interface science.

Interactions between water and 1-butyl-1-methylpyrrolidinium ionic liquids.

We report experimental results on the diffusivity of water in two ionic liquids obtained using the pulsed-gradient spin-echo NMR method. Both ionic liquids have the same cation, 1-butyl-1-methylpyrrolidinium, but different trifluoromethyl-containing anions. One has a strongly hydrophobic anion, bis(trifluoromethylsulfonyl)amide, while the second has a hydrophilic anion, trifluoromethylsulfonate. Transport of water in these ionic liquids is much faster than would be predicted from hydrodynamic laws, indicating that the neutral water molecules experience a very different friction than the anions and cations at the molecular level. Temperature-dependent viscosities, conductivities, and densities are reported as a function of water concentration to further analyze the properties of the ionic liquid-water mixtures. These results on the properties of water in ionic liquids should be of interest to researchers in diverse areas ranging from separations, solubilizing biomass and energy technologies.

Structural Origins of Viscosity in Imidazolium and Pyrrolidinium Ionic Liquids Coupled with the NTf2- Anion.

Ionic liquid viscosity is one of the most important properties to consider for practical applications. Yet, the connection between local structure and viscosity remains an open question. This article explores the structural origin of differences in the viscosity and viscoelastic relaxation across several ionic liquids, including cations with alkyl, ether, and thioether tails, of the imidazolium and pyrrolidinium families coupled with the NTf2- anion. In all cases, for the systems studied here, we find that pyrrolidinium-based ions are "harder" than their imidazolium-based counterparts. We make a connection between the chemical concept of hardness vs softness and specific structural and structural dynamic quantities that can be derived from scattering experiments and simulations.

Ionic liquids: structure and photochemical reactions.

Ionic liquids are subjects of intense current interest within the physical chemistry community. A great deal of progress has been made in just the past five years toward identifying the factors that cause these salts to have low melting points and other useful properties. Supramolecular structure and organization have emerged as important and complicated topics that may be key to understanding how chemical reactions and other processes are affected by ionic liquids. New questions are posed, and an active debate is ongoing regarding the nature of nanoscale ordering in ionic liquids. The topic of reactivity in ionic liquids is still relatively unexplored; however, the results that have been obtained indicate that distributed kinetics and dynamical heterogeneity may sometimes, but not always, be influencing factors.

Vacuum Interfacial Structure and X-ray Reflectivity of Imidazolium-Based Ionic Liquids with Perfluorinated Anions from a Theory and Simulations Perspective.

We report studies of the vacuum interfacial structure of a series of 1-methyl-3-alkylimidazolium bis(perfluoroalkanesulfonyl)imide ionic liquids (ILs) and predict and explain their Fresnel-normalized X-ray reflectivity. To better interpret the results, we use a theory we recently developed dubbed "the peaks and antipeaks analysis of reflectivity" which splits the overall signal into that of different pair subcomponents. Whereas the overall reflectivity signal is not very informative, the peak and trough intensities for the pair subcomponents provide rich information for analysis. When species containing cationic alkyl or anionic fluoroalkyl tails are present at the interface, a tail layer is found next to a vacuum, and this tail layer can be composed of both alkyl and fluoroalkyl moieties. To maintain the positive-negative alternation of charged groups, alkyl and fluoroalkyl tails must necessarily be nearby and cannot segregate. Charged groups are found in the subsequent layer just below the interface and arranged to achieve lateral charge neutrality. In general, fluctuations at and away from the interface are based on polarity (i.e., heads and tails) and not on charge; when there are no significant alkyl or fluoroalkyl moieties in the IL, atomic density fluctuations away from the interface are small and appear to exist for the purpose of achieving lateral charge balance. For all the systems reported here, the persistence length of density fluctuations does not go beyond ∼7 nm.

Communication: X-ray scattering from ionic liquids with pyrrolidinium cations.

We report the structure functions obtained from x-ray scattering experiments on a series of four homologous ionic liquids. The ionic liquids are 1-alkyl-1-methylpyrrolidinium cations paired with the bis(trifluoromethylsulfonyl)amide anion, with alkyl chain lengths of n = 4, 6, 8, and 10. The structure functions display two intense diffraction peaks for values of the scattering vector q in the range from 0.6 to 1.5 Å(-1) for all samples. Both diffraction peaks shift to lower values of q for increasing temperature. First sharp diffraction peaks are observed in the structure functions for q < 0.5 Å(-1) for liquids with n = 6, 8, and 10.

Temperature-dependent structure of methyltributylammonium bis(trifluoromethylsulfonyl)amide: X ray scattering and simulations.

We report the combined results of computational and x ray scattering studies of amorphous methyltributylammonium bis(trifluoromethylsulfonyl)amide as a function of temperature. These studies included the temperature range for the normal isotropic liquid, a deeply supercooled liquid and the glass. The low q peaks in the range from 0.3 to 1.5 A(-1) in the structure function of this liquid can be properly accounted for by correlations between first and second nearest neighbors. The lowest q peak can be assigned to real space correlations between ions of the same charge, while the second peak arises mostly from nearest neighbors of opposite charge. Peaks at larger q values are mostly intramolecular in nature. While our simulated structure functions provide an excellent match to our experimental results and our experimental findings agree with previous studies reported for this liquid, the prior interpretation of the experimental data in terms of an interdigitated smectic A phase is not supported by our simulations. In this work, we introduce a set of general theoretical partitions of real and reciprocal space correlations that allow for unambiguous analysis of all intra- and interionic contributions to the structure function and coherent scattering intensity. We find that the intermolecular contributions to the x ray scattering intensity are dominated by the anions and cross terms between cations and anions for this ionic liquid.

Ionic liquids and solids with paramagnetic anions.

Four paramagnetic ionic compounds have been prepared and their magnetic, structural and thermal properties have been investigated. The four compounds are methylbutylpyrrolidinium tetrachloroferrate(III) ([Pyrr(14)](+)/[FeCl(4)](-)), methyltributylammonium tetrachloroferrate(III) ([N(1444)](+)/[FeCl(4)](-)), butylmethylimidazolium tetrachloroferrate(III) ([bmim](+)/[FeCl(4)](-)) and tetrabutylammonium bromotrichloroferrate(III) ([N(4444)](+)/[FeBrCl(3)](-)). Temperature-dependent studies of their magnetic behaviors show that all four compounds are paramagnetic at ambient temperatures. Glass transitions are observed for only two of the four compounds, [Pyrr(14)](+)/[FeCl(4)](-) and [bmim](+)/[FeCl(4)](-). Crystal structures for [Pyrr(14)](+)/[FeCl(4)](-) and [N(1444)](+)/[FeCl(4)](-) are compared with the previously reported [N(4444)](+)/[FeBrCl(3)](-).

Spotlight on ionic liquids.

Ionic liquids are an emerging class of materials with a diverse and extraordinary set of properties. Understanding the origins of these properties and how they can be controlled by design to serve valuable practical applications presents a wide array of challenges and opportunities to the chemical physics and physical chemistry community. We highlight here some of the significant progress already made and future research directions in this exciting area.

Microscopic Structural and Dynamic Features in Triphilic Room Temperature Ionic Liquids.

Here we report a thorough investigation of the microscopic and mesoscopic structural organization in a series of triphilic fluorinated room temperature ionic liquids, namely [1-alkyl,3-methylimidazolium][(trifluoromethanesulfonyl)(nonafluorobutylsulfonyl)imide], with alkyl = ethyl, butyl, octyl ([Cnmim][IM14], n = 2, 4, 8), based on the synergic exploitation of X-ray and Neutron Scattering and Molecular Dynamics simulations. This study reveals the strong complementarity between X-ray/neutron scattering in detecting the complex segregated morphology in these systems at mesoscopic spatial scales. The use of MD simulations delivering a very good agreement with experimental data allows us to gain a robust understanding of the segregated morphology. The structural scenario is completed with determination of dynamic properties accessing the diffusive behavior and a relaxation map is provided for [C2mim][IM14] and [C8mim][IM14], highlighting their natures as fragile glass formers.