Neat Liquid Research Articles

Electrocatalytic reduction of oxygen by bilirubin oxidase in hydrophobic ionic liquids containing a small quantity of water

We have examined a direct electron transfer reaction of bilirubin oxidase (BOD) at a gold nanoparticle (AuNP) electrode in the mixtures of ionic liquid (IL) and water. No catalytic current induced by BOD was observed when neat ILs were used as an electrolyte solution. However, BOD-induced catalytic currents were observed when small quantities of water were added to hydrophobic ILs. The BOD-modified AuNP electrode provided a current density as high as 218μAcm−2 in 1-(2-hydroxyethyl)-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide ([C2OHmim][Tf2N]) containing 7.4wt% H2O. To investigate the effect of the added water to ILs on the enzymatic activity we compared the physicochemical properties of the IL/H2O mixtures. There was a certain relation between the water activity (aW) of the IL/H2O mixtures and the current density of BOD. The BOD was confirmed to catalyze the four-electron reduction of O2 to H2O in the [C2OHmim][Tf2N]/H2O mixture by the data with cyclic voltammetry measurements using a rotating ring-disk electrode.

Pyrrolidinium sulfate and ammonium sulfate ionic liquids as lubricant additives for steel/steel contact lubrication

In this work, we report on the BuMepyr-MeSO4 and Et3MeN-MeSO4 ionic liquids that were synthesized and used as additives in a glycerol model lubricant for steel/steel contacts. Tests were performed with three different ionic liquid concentrations, i.e. 0.625 wt%, 2.5 wt% and 8 wt%, as well as in glycerol without any ionic liquid (neat glycerol) and in neat ionic liquids (100%) at 100 °C. The wear and friction were measured and the worn surfaces were examined with scanning electron microscopy and atomic force microscopy. The results show a reduction of the wear and friction with the use of ionic liquids as additives, when compared to the neat glycerol. With an increasing ionic-liquid concentration in the glycerol, the friction was observed to decrease and the wear to increase. In this work, however, the results obtained for neat ionic liquids represent the lowest values in terms of both friction and wear.

The physicochemical properties of some imidazolium-based ionic liquids and their binary mixtures

The density, viscosity and conductivity of ionic liquids (ILs), 1-octyl-3-methylimidazolium tetrafluoroborate ([omim][BF4]), 1-octyl-3-methylimidazolium chloride ([omim][Cl]), 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim] BF4]), 1-hexyl- 3-methylimidazolium chloride ([hmim][Cl]), 1-hexyl-3-methylimidazolium hexafluorophosphate ([hmim][PF6]), and the [omim][BF4] + [omim][Cl], [hmim][BF4] + [hmim][Cl], and [hmim][PF6] + [hmim][Cl] binary mixtures were studied at different temperatures. It was demonstrated that the densities of both the neat ILs and their mixtures varied linearly with temperature. The density sensitivity of a binary mixture is between those of the two components. The excess molar volumes ( V E) of [hmim][BF4] + [hmim][Cl] and [hmim][PF6] + [hmim][Cl] mixtures are positive in the whole composition range. For [omim][BF4] + [omim][Cl], the V E is also positive in the [omim][Cl]-rich region, but is negative in the [omim][BF4]-rich region. The viscosity or conductivity of a mixture is in the intermediate of those of the two neat ILs. For all the neat ILs and the binary mixtures studied, the order of conductivity is opposite to that of the viscosity. The Vogel-Tammann-Fulcher (VTF) equations can be used to fit the viscosity and conductivity of all the neat ILs and the binary mixtures. The neat ILs and their mixtures obey the Fractional Walden Rule very well, and the values of the Walden slopes are all smaller than unit, indicating obvious ion associations in the neat ILs and the binary mixtures.

Properties of alkylbenzimidazoles for CO2 and SO2 capture and comparisons to ionic liquids

To date, few reports have been concerned with the physical properties of the liquid phases of imidazoles and benzimidazoles-potential starting materials for a great number of ionic liquids. Prior research has indicated that alkylimidazole solvents exhibit different, and potentially advantageous physical properties, when compared to corresponding imidazolium-based ionic liquids. Given that even the most fundamental physical properties of alkylimidazole solvents have only recently been reported, there is still a lack of data for other relevant imidazole derivatives, including benzimidazoles. In this work, we have synthesized a series of eight 1-n-alkylbenzimidazoles, with chain lengths ranging from ethyl to dodecyl, all of which exist as neat liquids at ambient temperature. Their densities and viscosities have been determined as functions of both temperature and molecular weight. Alkylbenzimidazoles have been found to exhibit viscosities that are more similar to imidazolium-based ILs than alkylimidazoles, owed to a large contribution to viscosity from the presence of a fused ring system. Solubilities of CO2 and SO2, two species of concern in the emission of coal-fired power generation, were determined for selected alkylbenzimidazoles to understand what effects a fused ring system might have on gas solubility. For both gases, alkylbenzimidazoles were determined to experience physical, non-chemically reactive, interactions. The solubility of CO2 in alkylbenzimidazoles is 10%–30% less than observed for corresponding ILs and alkylimidazoles. 1-butylbenzimidazole was found to readily absorb at least 0.333 gram SO2 per gram at low pressure and ambient temperature, which could be readily desorbed under an N2 flush, a behavior more similar to imidazolium-based ILs than alkylimidazoles. Thus, we find that as solvents for gas separations, benzimidazoles share characteristics with both ILs and alkylimidazoles.

Effect of evaporation and matrix interferences on the association of simulated ignitable liquid residues to the corresponding liquid standard

Identification of an ignitable liquid in fire debris evidence can be complicated due to evaporation of the liquid, matrix interferences, and thermal degradation of both the liquid and the matrix. In this research, liquids extracted from simulated fire debris were compared to the original liquid using multivariate statistical procedures. Neat and evaporated gasoline and kerosene standards were spiked onto nylon carpet, which was subsequently burned. The ignitable liquid residues were extracted using a passive headspace procedure and analyzed by gas chromatography–mass spectrometry. Pearson product moment correlation coefficients, hierarchical cluster analysis, and principal components analysis were used to compare the liquids extracted from the carpet to the corresponding neat liquid. For each procedure, association of the extracts according to liquid type was possible, albeit not necessarily to the specific evaporation level. Of the three procedures investigated, principal components analysis offered the most promise since contributions from matrix interferences were essentially eliminated.

Enhanced electrochemical response of solution-deposited n-doping polymer via cocasting with ionic liquid

A novel cocasting approach is presented for improving electroactivity of solution-cast films of conducting polymers. Solutions of the n-doping polymer poly(benzimidazobenzophenanthroline) (BBL) were co-deposited with the ionic liquid electrolyte 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)imide (EMIBTI). The resultant co-continuous mixture yielded highly porous polymer films (CC-BBL) upon removal of solvent and EMIBTI. Electrochemical quartz crystal microgravimetry revealed that the n-doping process in neat ionic liquid is anion-dominant, which is contrary to what is observed in dilute electrolyte solutions. The CC-BBL films exhibit a thirty-fold increase in initial current response and capacity relative to non-cocast BBL films. While current response and capacity of the non-cocast BBL improve with cycling, they level out after 800 cycles at 35% of those of the CC-BBL. CC-BBL shows high n-doping stability; no decrease in electroactivity is seen after 1000 cycles. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

Fluorescent Probe Studies of Polarity and Solvation within Room Temperature Ionic Liquids: A Review

Ionic liquids display an array of useful and sometimes unconventional, solvent features and have attracted considerable interest in the field of green chemistry for the potential they hold to significantly reduce environmental emissions. Some of these points have a bearing on the chemical reactivity of these systems and have also generated interest in the physical and theoretical aspects of solvation in ionic liquids. This review presents an introduction to the field of ionic liquids, followed by discussion of investigations into the solvation properties of neat ionic liquids or mixed systems including ionic liquids as a major or minor component. The ionic liquid based multicomponent systems discussed are composed of other solvents, other ionic liquids, carbon dioxide, surfactants or surfactant solutions. Although we clearly focus on fluorescence spectroscopy as a tool to illuminate ionic liquid systems, the issues discussed herein are of general relevance to discussions of polarity and solvent effects in ionic liquids. Transient solvation measurements carried out by means of time-resolved fluorescence measurements are particularly powerful for their ability to parameterize the kinetics of the solvation process in ionic liquids and are discussed as well.

ChemInform Abstract: 1,1,3,3‐Tetramethyldisilazane

AbstractReview: 30 refs.

Tribological properties of halogen-free ionic liquids

The tribological properties of halogen-free ionic liquids, tricyanomethanide [[Formula: see text]] salt, tetracyanoborate [[Formula: see text]] salts, and N-alkylimidazole-trialkylborane complexes were evaluated by laboratory tribo-testing of steel–steel contact under boundary conditions. Tricyanomethanide salt is composed of hydrogen, carbon, and nitrogen. The other two types of liquids are composed of hydrogen, boron, carbon, and nitrogen. They are free of halogens and heavier elements that are components of common ionic liquids, such as fluorine, phosphorus, and sulphur. As expected, the halogen-free ionic liquids exhibited low corrosion properties to steel. When evaluated as neat liquid, these halogen-free ionic liquids provided less tribological properties in comparison with a reference, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide. Tributylmethylphosphonium dimethylphosphate was examined as a prototype tribo-improving additive. It improved the wear-preventing properties and friction reducing properties of tetracyanoborate salts by 10–25% and 20–30% at a concentration of 10 mM (620 ppm of phosphorus), respectively. The additive performances for tricyanomethanide salt and the imidazole-trialkylborane complexes were not uniform under these conditions. Boron oxide and iron oxides were found by surface analysis of rubbed surfaces with tetracyanoborate salts.

Nanostructural Organization in Acetonitrile/Ionic Liquid Mixtures: Molecular Dynamics Simulations and Optical Kerr Effect Spectroscopy

The nanostructural organization and subpicosecond intermolecular dynamics in mixtures of acetonitrile and the ionic liquid (IL) 1-pentyl-3-methylimidazolium bis{(trifluoromethane)sulfonyl}amide ([C(5)mim][NTf(2)]) are studied as a function of concentration using molecular dynamics (MD) simulations and optical heterodyne-detected Raman-induced Kerr effect spectroscopy. The MD simulations show the IL to be nanostructurally organized into an ionic network and nonpolar domains, with CH(3)CN molecules localized in the interfacial region between the ionic network and nonpolar domains, as found previously by other researchers. The MD simulations indicate strong interactions between CH(3)CN and the hydrogen atoms on the imidazolium ring of the cation. The low-frequency (0-200 cm(-1)) intermolecular part of the reduced spectral densities (RSDs) of the mixtures narrows and shifts to lower frequency as the concentration of CH(3)CN increases. These spectral changes can be partly attributed to the increasing contribution of the low-frequency intermolecular modes of CH(3)CN to the RSD. At a given composition, the RSD of a mixture is found to be broader and higher in frequency than the corresponding ideal RSD given by the volume-fraction-weighted sum of the RSDs of the neat liquids. This difference is rationalized in terms of the competition between CH(3)CN-cation interactions and solute-induced disruption of the ionic networks.

The mechanism of proton conduction in phosphoric acid

Neat liquid phosphoric acid (H(3)PO(4)) has the highest intrinsic proton conductivity of any known substance and is a useful model for understanding proton transport in other phosphate-based systems in biology and clean energy technologies. Here, we present an ab initio molecular dynamics study that reveals, for the first time, the microscopic mechanism of this high proton conductivity. Anomalously fast proton transport in hydrogen-bonded systems involves a structural diffusion mechanism in which intramolecular proton transfer is driven by specific hydrogen bond rearrangements in the surrounding environment. Aqueous media transport excess charge defects through local hydrogen bond rearrangements that drive individual proton transfer reactions. In contrast, strong, polarizable hydrogen bonds in phosphoric acid produce coupled proton motion and a pronounced protic dielectric response of the medium, leading to the formation of extended, polarized hydrogen-bonded chains. The interplay between these chains and a frustrated hydrogen-bond network gives rise to the high proton conductivity.

Characterization of H-bonding networks in chiral alcohols using Infrared, Raman and Vibrational Circular Dichroism spectroscopies, and density functional calculations: (S)-(−)-perillyl alcohol

For the first time, a study of the H-bonding network in which the chiral alcohol (4-isopropenylcyclohex-1-en-1-yl)methanol [(S)-(−)-perillyl alcohol] is involved (in neat liquid and in CCl4 solution) is carried out. For this task, Infrared (IR), Raman and Vibrational Circular Dichroism (VCD) spectroscopies are applied together with theoretical calculations based on Density Functional Theory. Most of the experimental features are reproduced taking into account five different rotamers of the monomer, a dimer, and a cyclic trimer. For this species, IR and Raman techniques are useful to study H-bonding networks only in the 4000–3200cm−1 region. In contrast, VCD spectroscopy is useful in the 4000–900cm−1 region.

2H NMR studies of glycerol dynamics in protein matrices

We use (2)H NMR spectroscopy to investigate the rotational motion of glycerol molecules in matrices provided by the connective tissue proteins elastin and collagen. Analyzing spin-lattice relaxation, line-shape properties, and stimulated-echo decays, we determine the rates and geometries of the motion as a function of temperature and composition. It is found that embedding glycerol in an elastin matrix leads to a mild slowdown of glycerol reorientation at low temperatures and glycerol concentrations, while the effect vanishes at ambient temperatures or high solvent content. Furthermore, it is observed that the nonexponential character of the rotational correlation functions is much more prominent in the elastin matrix than in the bulk liquid. Results from spin-lattice relaxation and line shape measurements indicate that, in the mixed systems, the strong nonexponentiality is in large part due to the existence of distributions of correlation times, which are broader on the long-time flank and, hence, more symmetric than in the neat system. Stimulated-echo analysis of slow glycerol dynamics reveals that, when elastin is added, the mechanism for the reorientation crosses over from small-angle jump dynamics to large-angle jump dynamics and the geometry of the motion changes from isotropic to anisotropic. The results are discussed against the background of present and previous findings for glycerol and water dynamics in various protein matrices and compared with observations for other dynamically highly asymmetric mixtures so as to ascertain in which way the viscous freezing of a fast component in the matrix of a slow component differs from the glassy slowdown in neat supercooled liquids.

Effect of Confinement on Proton Transport Mechanisms in Block Copolymer/Ionic Liquid Membranes

Nanostructured membranes containing structural and proton-conducting domains are of great interest for a wide range of applications requiring high conductivity coupled to high thermal stability. Understanding the effect of nanodomain confinement on proton-conducting properties in such materials is essential for designing new, improved membranes. This relationship has been investigated for a lamellae-forming mixture of poly(styrene-b-2-vinyl pyridine) (PS-b-P2VP) with ionic liquid composed of imidazole and bis(trifluoromethylsulfonyl)imide, where the ionic liquid selectively resides in the P2VP domains of the block copolymer. Quasi-elastic neutron scattering and NMR diffusion measurements reveal increased prevalence of a fast proton hopping transport mechanism, which we hypothesize is due to changes in the hydrogen bond structure of the ionic liquid under confinement. This, in combination with unique ion aggregation behavior, leads to a lower activation energy for macroscopic ion transport compared with that in a mixture of ionic liquid with P2VP homopolymer. The proton transference number in both samples is significantly higher than that in the neat ionic liquid, which could be taken advantage of for applications such as proton exchange membrane fuel cells and actuators. These results portend the rational design of nanostructured membranes having improved mechanical properties and conductivity.

Simple Liquid Models with Corrected Dielectric Constants

Molecular simulations often use explicit-solvent models. Sometimes explicit-solvent models can give inaccurate values for basic liquid properties, such as the density, heat capacity, and permittivity, as well as inaccurate values for molecular transfer free energies. Such errors have motivated the development of more complex solvents, such as polarizable models. We describe an alternative here. We give new fixed-charge models of solvents for molecular simulations--water, carbon tetrachloride, chloroform, and dichloromethane. Normally, such solvent models are parametrized to agree with experimental values of the neat liquid density and enthalpy of vaporization. Here, in addition to those properties, our parameters are chosen to give the correct dielectric constant. We find that these new parametrizations also happen to give better values for other properties, such as the self-diffusion coefficient. We believe that parametrizing fixed-charge solvent models to fit experimental dielectric constants may provide better and more efficient ways to treat solvents in computer simulations.

Direct Investigation of Covalently Bound Chlorine in Organic Compounds by Solid‐State 35Cl NMR Spectroscopy and Exact Spectral Line‐Shape Simulations

35/37Cl NMR spectroscopy studies of organic systems are very rare, with only a few neat liquids having been studied.1 The lack of chlorine NMR spectroscopy data may be explained by the fact that 35Cl and 37Cl are quadrupolar (spin I=3/2) and low-frequency isotopes. The quadrupole moments of the chlorine nuclei couple with the electric field gradient (EFG) tensor at the nuclei; this phenomenon is known as the quadrupolar interaction (QI). The quadrupolar coupling constant, CQ, and the quadrupolar asymmetry parameter, ηQ, describe the magnitude and asymmetry of the QI. In solution, one of the consequences of the QI is fast relaxation, which means that the 35/37Cl NMR signals for covalently bound chlorines are very broad and are of low intensity.1 For these reasons, chemically distinct chlorine sites are very difficult to distinguish with solution NMR spectroscopy. However, in the solid state, nuclear spin relaxation is typically slower, thus enabling higher quality 35Cl NMR spectra to be collected, at least in principle. Unfortunately, the magnitude of the QI for covalently bound chlorines is very large because of the substantial, anisotropic EFG at the Cl atom, owing mainly to its electronic configuration when it forms a chlorine–carbon bond. Conventional wisdom is that such chlorine sites cannot be studied in powders by solid-state NMR spectroscopy as the central transition (CT; mI=1/2↔−1/2) can span tens of megahertz in typical commercially available magnetic fields. For this reason, only ionic chlorides2 and inorganic chlorides3 have been studied, as the EFG at these chlorides is often an order of magnitude smaller than at covalently bound chlorine atoms in organic molecules. The bonding environments for these types of chlorine atoms are substantially different from the environments in those chloride-containing molecules that have been studied previously.2, 3 A partial 35Cl NMR spectrum for hexachlorophene has been briefly mentioned in the literature.4 On the other hand, most of the interesting chlorine chemistry occurs when Cl is covalently bound to a carbon atom, where the chlorine atom often acts as a leaving group. Chlorine atoms are also important in many organic pharmaceuticals as well as in crystal design applications where they can form halogen bonds.5 Recent studies show that covalently bound chlorine is also important in biological chemistry where, for example, the tryptophan 7-halogenase was found to selectively chlorinate tryptophan moieties.6

Passive film formation in dilute ionic liquid solutions on magnesium Alloy AZ31

The ionic liquid (IL) trihexyl(tetradecyl)phosphonium bis(2,4,4-trimethylpentyl phosphinate ([P6,6,6,14][(iC8)2PO2]) has been shown to passivate magnesium alloy AZ31 at anodic potentials within the IL environment. In this study, [P6,6,6,14][(iC8)2PO2] IL was diluted in organic solvent acetonitrile to concentrations of 0.1 and 0.01M. Cyclic voltammetry (CV) at anodic potentials from open circuit potential (OCP) appeared to passivate the surface in both solutions to the same degree. Thicknesses of 80 and 29nm were determined for films formed under anodising conditions in neat IL and 0.1M IL in acetonitrile respectively. The results indicate that despite dilution in acetonitrile the electrochemical behaviour of the IL is essentially unchanged. However, the effect of dilution changes the process of film formation during anodising to 18V, resulting in different film thicknesses.

Local Structure Evolution and its Connection to Thermodynamic and Transport Properties of 1-Butyl-3-methylimidazolium Tetrafluoroborate and Water Mixtures by Molecular Dynamics Simulations

Our recently developed improved united atom force field shows a good quality to reproduce both the static and transport properties of neat ionic liquids (ILs). Combined with the TIP4P-Ew water model, the force field is used to simulate the mixture of 1-butyl-3-methylimidazolium tetrafluoroborate ([C(4)mim][BF(4)]) and water without further optimization to adjust any cross parameters. Liquid densities of the mixture are well predicted over the entire concentration range at temperatures from 298.15 to 353.15 K. Simulations also reproduce the positive values of excess volumes and excess enthalpies, as well as their increase with temperature. The simulated viscosities are in good agreement with experimental values, especially in the water-rich region. We found three distinct regions by analyzing the concentration dependent self-diffusion coefficients via Stokes-Einstein (SE) relation, indicating the mixture experiences significant microheterogeneity with the adding of water. This observation is well connected to the structure features obtained in simulations, such as radial distribution functions (RDFs), spatial distribution functions (SDFs) and water clustering analysis. At the water mole fraction (x(2)) less than 0.2, most of the water molecules are isolated in the polar cation-anion network in ionic liquids. With the increase of x(2) from 0.2 to 0.8, large water cluster forms and eventually percolates the whole system. When x(2) > 0.8, ionic liquids show a moderate degree of aggregation (with maximum around 0.9 to 0.95) before the cations and anions are fully dissolved in water.

All-optical switching based on inverse Raman scattering in liquid-core optical fibers

We report on a new platform for all-optical switching based on inverse Raman scattering in liquids. Narrowband switching, which could be suitable for wavelength-division-multiplexed applications, is demonstrated using integrated liquid-core optical fiber infiltrated with both neat liquids (CCl(4) and CS(2)) as well as an organic chromophore (β-carotene) dissolved in CCl(4). Compared to standard glass optical fibers, these liquids have much larger Raman loss coefficients, which help reduce the pump power by at least an order of magnitude. Further improvements can be expected with the development of highly soluble organic compounds possessing large Raman cross sections.

Natural polarizability and flexibility via explicit valency: The case of water

As the dominant physiological solvent, water drives the folding of biological macromolecules, influences conformational changes, determines the ionization states of surface groups, actively participates in catalytic events, and provides "wires" for long-range proton transfer. Elucidation of all these roles calls for atomistic simulations. However, currently available methods do not lend themselves to efficient simulation of proton transfer events, or even polarizability and flexibility. Here, we report that an explicit account of valency can provide a unified description for the polarizability, flexibility, and dissociability of water in one intuitive and efficient setting. We call this approach LEWIS, after the chemical theory that inspires the use of valence electron pairs. In this paper, we provide details of the method, the choice of the training set, and predictions for the neat ambient liquid, with emphasis on structure, dynamics, and polarization. LEWIS water provides a good description of bulk properties, and dipolar and quadrupolar responses.