Liquid Structure of Ionic Liquids with [NTf2]- Anions, Derived from Neutron Scattering.

The liquid structure of three common ionic liquids (ILs) was investigated by neutron scattering for the first time. The ILs were based on the bis(trifluoromethanesulfonyl)imide anion, abbreviated in the literature as [NTf2]- or [TFSI]-, and on the following cations: 1-ethyl-3-methylimidazolium, [C2mim]+; 1-decyl-3-methylimidazolium, [C10mim]+; and trihexyl(tetradecyl)phosphonium, [P666,14]+. Comparative analysis of the three ILs confirmed increased size of nonpolar nanodomains with increasing bulk of alkyl chains. It also sheds light on the cation-anion interactions, providing experimental insight into strength, directionality, and angle of hydrogen bonds between protons on the imidazolium ring, as well as H-C-P protons in [P666,14]+, to oxygen and nitrogen atoms in the [NTf2]-. The new Dissolve data analysis package enabled, for the first time, the analysis of neutron scattering data of ILs with long alkyl chains, in particular, of [P666,14][NTf2]. Results generated with Dissolve were validated by comparing outputs from three different models, starting from three different sets of cation charges, for each of the three ILs, which gave convergent outcomes. Finally, a modified method for the synthesis of perdeuterated [P666,14][NTf2] has been reported, with the aim of reporting a complete set of synthetic and data processing approaches, laying robust foundations that enable the study of the phosphonium ILs family by neutron scattering.

Visualization of Self-Assembly and Hydration of a ß-Hairpin through Integrated Small and Wide-Angle Neutron Scattering.

Fundamental understanding of the structure and assembly of nanoscale building blocks is crucial for the development of novel biomaterials with defined architectures and function. However, accessing self-consistent structural information across multiple length scales is challenging. This limits opportunities to exploit atomic scale interactions to achieve emergent macroscale properties. In this work we present an integrative small- and wide-angle neutron scattering approach coupled with computational modeling to reveal the multiscale structure of hierarchically self-assembled ß hairpins in aqueous solution across 4 orders of magnitude in length scale from 0.1 Å to 300 nm. Our results demonstrate the power of this self-consistent cross-length scale approach and allows us to model both the large-scale self-assembly and small-scale hairpin hydration of the model ß hairpin CLN025. Using this combination of techniques, we map the hydrophobic/hydrophilic character of this model self-assembled biomolecular surface with atomic resolution. These results have important implications for the multiscale investigation of aqueous peptides and proteins, for the prediction of ligand binding and molecular associations for drug design, and for understanding the self-assembly of peptides and proteins for functional biomaterials.

Elucidating the hydrotropism behaviour of aqueous caffeine and sodium benzoate solution through NMR and neutron total scattering analysis.

Hydrotropism is a convenient way to increase the solubility of drugs by up to several orders of magnitude, and even though it has been researched for decades with both experimental and simulation methods, its mechanism is still unknown. Here, we use caffeine/sodium benzoate (CAF-SB) as model system to explore the behaviour of caffeine solubility enhancement in water through NMR spectroscopy and neutron total scattering. 1H NMR shows strong interaction between caffeine and sodium benzoate in water. Neutron total scattering combined with empirical potential structure refinement, a systematic method to study the solution structure, reveals π-stacking between caffeine and the benzoate anion as well as Coulombic interactions with the sodium cation. The strongest hydrogen bond interaction in the system is between benzoate and water, which help dissolve CAF-SB complex and increase the solubility of CAF in water. Besides, the stronger interaction between CAF and water and the distortion of water structure are further mechanisms of the CAF solubility enhancement. It is likely that the variety of mechanisms for hydrotropism shown in this system can be found for other hydrotropes, and NMR spectroscopy and neutron total scattering can be used as complementary techniques to generate a holistic picture of hydrotropic solutions.

The structure of liquid thiophene from total neutron scattering.

The structure of pure liquid thiophene is revealed by using a combination of total neutron scattering experiments with isotopic substitution and molecular simulations via the next generation empirical potential refinement software, Dissolve. In the liquid, thiophene presents three principle local structural motifs within the first solvation shell, in plane and out of the plane of the thiophene ring. Firstly, above/below the ring plane thiophenes present a single H towards the π cloud, due to a combination of electrostatic and dispersion interactions. Secondly, around the ring plane, perpendicular thiophene molecules find 5 preferred sites driven by bifurcated C-H⋯S interactions, showing that hydrogen-sulfur bonding prevails over the charge asymmetry created by the heteroatom. Finally, parallel thiophenes sit above and below the ring, excluded from directly above the ring center and above the sulfur.

Liquids with High Compressibility.

Compressibility is a fundamental property of all materials. For fluids, that is, gases and liquids, compressibility forms the basis of technologies such as pneumatics and hydraulics and determines basic phenomena such as the propagation of sound and shock waves. In contrast to gases, liquids are almost incompressible. If the compressibility of liquids could be increased and controlled, new applications in hydraulics and shock absorption could result. Here, it is shown that dispersing hydrophobic porous particles into water gives aqueous suspensions with much greater compressibilities than any normal liquids such as water (specifically, up to 20 times greater over certain pressure ranges). The increased compressibility results from water molecules being forced into the hydrophobic pores of the particles under applied pressure. The degree of compression can be controlled by varying the amount of porous particles added. Also, the pressure range of compression can be reduced by adding methanol or increased by adding salt. In all cases, the liquids expand back to their original volume when the applied pressure is released. The approach shown here is simple and economical and could potentially be scaled up to give large amounts of highly compressible liquids.

Role of Hydrogen Bond Defects for Cluster Formation and Distribution in Ionic Liquids by Means of Neutron Diffraction and Molecular Dynamics Simulations.

Defects fundamentally govern the properties of all real materials. Correlating molecular defects to macroscopic quantities remains a challenge, particularly in the liquid phase. Herein, we report the influence of hydrogen bonds (HB) acting as defects in mixtures of non-hydroxyl-functionalized ionic liquids (ILs) with an increasing concentration of hydroxyl-functionalized ILs. We observed two types of HB defects: The conventional HBs between cation and anion (c-a), and the elusive HBs between cations (c-c) despite the repulsive Coulomb forces. We use neutron diffraction with isotopic substitution in combination with molecular dynamics simulations for measuring the geometry, strength, and distribution of mobile OH defects in the IL mixtures. In principle, this procedure allows relating the number and stability of defects to macroscopic properties such as diffusion, viscosity, and conductivity, which are of utmost importance for the performance of electrolytes in batteries and other electrical devices.

Insights into the CO2 Capture Characteristics within the Hierarchical Pores of Carbon Nanospheres Using Small-Angle Neutron Scattering.

Understanding adsorption processes at the molecular level has transformed the discovery of engineered materials for maximizing gas storage capacity and kinetics in adsorption-based carbon capture applications. In this work, we studied the molecular mechanism of gas (CO2, H2, methane, and ethane) adsorption inside an interconnected porous network of carbon. This was achieved by synthesizing novel macro-meso-microporous carbon (M3C) nanospheres with interconnected pore structures. The M3Cs showed a CO2 capture capacity of 5.3 mmol/g at atmospheric CO2 pressure, with excellent kinetics. This was due to fast CO2 adsorption within the interconnected hierarchical macro-meso-microporous M3C. In situ small-angle neutron scattering (SANS) under various CO2 pressures indicated that the macro- and mesopores of M3C enable fast diffusion of CO2 molecules inside the micropores, where adsorbed CO2 molecules densify into a liquid-like state. This strong densification of CO2 molecules causes fast CO2 diffusion in the macro- and mesopores of M3C, restarting the adsorption cycle for fresh CO2 molecules until all pores are completely filled. Notably, M3C also showed good capture capacities for hydrogen and various hydrocarbons, with excellent selectivity toward ethane over methane.

Quantification and Distribution of Three Types of Hydrogen Bonds in Mixtures of an Ionic Liquid with the Hydrogen-Bond-Accepting Molecular Solvent DMSO Explored by Neutron Diffraction and Molecular Dynamics Simulations.

The concept of hydrogen bonding is celebrating its 100th birthday. Hydrogen bonds (H-bonds) play a key role in the structure and function of biological molecules, the strength of materials, and molecular binding. Herein, we study H-bonding in mixtures of a hydroxyl-functionalized ionic liquid with the neutral, H-bond-accepting molecular liquid dimethylsulfoxide (DMSO) using neutron diffraction experiments and molecular dynamics simulations. We report the geometry, strength, and distribution of three different types of H-bond OH···O, formed between the hydroxyl group of the cation and either the oxygen atom of another cation, the counteranion, or the neutral molecule. Such a variety of different strengths and distributions of H-bonds in one single mixture could hold the promise of providing solvents with potential applications in H-bond-related chemistry, for example, to alter the natural selectivity patterns of catalytic reactions or the conformation of catalysts.

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.

How Temperature Rise Can Induce Phase Separation in Aqueous Biphasic Solutions.

Ionic-liquid-based acidic aqueous biphasic solutions (AcABSs) recently offered a breakthrough in the field of metal recycling. The particular mixture of tributyltetradecylphosphonium chloride ([P4,4,4,14]Cl), acid, and water presents the unusual characteristic of a lower solution critical temperature (LCST), leading to phase separation upon a temperature rise of typically a few tens of degrees. We address here the microscopic mechanisms driving the phase separation. Using small-angle neutron scattering, we characterized the spherical micelle formation in a binary ionic liquid/water solution and the micelle aggregation upon the addition of acid due to the screening of electrostatic repulsion. The increase in both the acid concentration and the temperature eventually leads to micelle flocculation and phase separation. This last step is achieved through chloride ion adsorption at the surface of the micelle. This exothermic adsorption compensates for the entropic cost, leading to a counterintuitive behavior, and may be generalized to a number of molecular systems with an LCST.

The ability of trimethylamine N-oxide to resist pressure induced perturbations to water structure.

Trimethylamine N-oxide (TMAO) protects organisms from the damaging effects of high pressure. At the molecular level both TMAO and pressure perturb water structure but it is not understood how they act in combination. Here, we use neutron scattering coupled with computational modelling to provide atomistic insight into the structure of water under pressure at 4 kbar in the presence and absence of TMAO. The data reveal that TMAO resists pressure-induced perturbation to water structure, particularly in retaining a clear second solvation shell, enhanced hydrogen bonding between water molecules and strong TMAO - water hydrogen bonds. We calculate an 'osmolyte protection' ratio at which pressure and TMAO-induced energy changes effectively cancel out. Remarkably this ratio translates across scales to the organism level, matching the observed concentration dependence of TMAO in the muscle tissue of organisms as a function of depth. Osmolyte protection may therefore offer a molecular mechanism for the macroscale survival of life in extreme environments.

The Double-Faced Nature of Hydrogen Bonding in Hydroxy-Functionalized Ionic Liquids Shown by Neutron Diffraction and Molecular Dynamics Simulations.

We characterize the double-faced nature of hydrogen bonding in hydroxy-functionalized ionic liquids by means of neutron diffraction with isotopic substitution (NDIS), molecular dynamics (MD) simulations, and quantum chemical calculations. NDIS data are fit using the empirical potential structure refinement technique (EPSR) to elucidate the nearest neighbor H⋅⋅⋅O and O⋅⋅⋅O pair distribution functions for hydrogen bonds between ions of opposite charge and the same charge. Despite the presence of repulsive Coulomb forces, the cation-cation interaction is stronger than the cation-anion interaction. We compare the hydrogen-bond geometries of both "doubly charged hydrogen bonds" with those reported for molecular liquids, such as water and alcohols. In combination, the NDIS measurements and MD simulations reveal the subtle balance between the two types of hydrogen bonds: The small transition enthalpy suggests that the elusive like-charge attraction is almost competitive with conventional ion-pair formation.

An integrated total neutron scattering - NMR approach for the study of heterogeneous catalysis.

Nuclear magnetic resonance (NMR) and total neutron scattering techniques are established methods for the characterisation of liquid phases in confined pore spaces during chemical reactions. Herein, we describe the first combined total neutron scattering - NMR setup as a probe for the catalytic heterogeneous reduction of benzene-d6 with D2 in 3 wt% Pt/MCM-41.

Confinement Effects on the Benzene Orientational Structure.

Liquids under confinement exhibit different properties compared with their corresponding bulk phases, for example, miscibility, phase transitions, and diffusion. The underlying cause is the local ordering of molecules, which is usually only studied using pure simulation methods. Herein, we derive experimentally the structure of benzene confined in MCM-41 using total neutron scattering measurements. The study reveals a layering of molecules across a pore, and four concentric cylindrical shells can be distinguished for a pore with the radius of 18 Å. The nanoscale confinement of the liquid has a major effect on the spatial and orientational correlations observed between the molecules, when compared with the structure of the bulk liquid. These differences are most marked for molecules in parallel configurations, and this suggests differences in chemical reactivity between the confined and bulk liquids.

Formation of Methane Hydrate in the Presence of Natural and Synthetic Nanoparticles.

Natural gas hydrates occur widely on the ocean-bed and in permafrost regions, and have potential as an untapped energy resource. Their formation and growth, however, poses major problems for the energy sector due to their tendency to block oil and gas pipelines, whereas their melting is viewed as a potential contributor to climate change. Although recent advances have been made in understanding bulk methane hydrate formation, the effect of impurity particles, which are always present under conditions relevant to industry and the environment, remains an open question. Here we present results from neutron scattering experiments and molecular dynamics simulations that show that the formation of methane hydrate is insensitive to the addition of a wide range of impurity particles. Our analysis shows that this is due to the different chemical natures of methane and water, with methane generally excluded from the volume surrounding the nanoparticles. This has important consequences for our understanding of the mechanism of hydrate nucleation and the design of new inhibitor molecules.

The Structure of Ethylbenzene, Styrene and Phenylacetylene Determined by Total Neutron Scattering.

Organic solvents such as phenylacetylene, styrene and ethylbenzene are widely used in industrial processes, especially in the production of rubber or thermoplastics. Despite their important applications detailed knowledge about their structure is limited. In this paper the structures of these three aromatic solvents were investigated using neutron diffraction. The results show that many of their structural characteristics are similar, although the structure of phenylacetylene is more ordered and has a smaller solvation sphere than either ethylbenzene or styrene. Two regions within the first coordination sphere, in which the surrounding molecules show different preferable orientations with respect to the central molecule, were found for each liquid. Additionally, the localisation of the aliphatic chains reveals that they tend to favour closer interactions with each other than to the aromatic rings of the adjacent molecules.

Neutron Scattering Measurements of Carbon Dioxide Adsorption in Pores within the Marcellus Shale: Implications for Sequestration.

Shale is an increasingly viable source of natural gas and a potential candidate for geologic CO2 sequestration. Understanding the gas adsorption behavior on shale is necessary for the design of optimal gas recovery and sequestration projects. In the present study neutron diffraction and small-angle neutron scattering measurements of adsorbed CO2 in Marcellus Shale samples were conducted on the Near and InterMediate Range Order Diffractometer (NIMROD) at the ISIS Pulsed Neutron and Muon Source, STFC Rutherford Appleton Laboratory along an adsorption isotherm of 22 °C and pressures of 25 and 40 bar. Additional measurements were conducted at approximately 22 and 60 °C at the same pressures on the General-Purpose Small-Angle Neutron Scattering (GP-SANS) instrument at Oak Ridge National Laboratory. The structures investigated (pores) for CO2 adsorption range in size from Å level to ∼50 nm. The results indicate that, using the conditions investigated densification or condensation effects occurred in all accessible pores. The data suggest that at 22 °C the CO2 has liquid-like properties when confined in pores of around 1 nm radius at pressures as low as 25 bar. Many of the 2.5 nm pores, 70% of 2 nm pores, most of the <1 nm pores, and all pores <0.25 nm, are inaccessible or closed to CO2, suggesting that despite the vast numbers of micropores in shale, the micropores will be unavailable for storage for geologic CO2 sequestration.

Structure of Aqueous Trehalose Solution by Neutron Diffraction and Structural Modeling.

The molecular structure of an aqueous solution of the disaccharide trehalose (C12H22O11) has been studied by neutron diffraction and empirical potential structure refinement modeling. Six different isotope compositions with 33 wt % trehalose (corresponding to 38 water molecules per trehalose molecule) were measured to ensure that water-water, trehalose-water, and trehalose-trehalose correlations were accurately determined. In fact, this is the first neutron diffraction study of an aqueous trehalose solution in which also the nonexchangeable hydrogen atoms in trehalose are deuterated. With this approach, it was possible to determine that (1) there is a substantial hydrogen bonding between trehalose and water (∼11 hydrogen bonds per trehalose molecule), which is in contrast to previous neutron diffraction studies, and (2) there is no tendency of clustering of trehalose, in contrast to what is generally observed by molecular dynamics simulations and experimentally found for other disaccharides. Thus, the results give the structural picture that trehalose prefers to interact with water and participate in a hydrogen-bonded network. This strong network character of the solution might be one of the key reasons for its extraordinary stabilization effect on biological materials.

Solvation Structure of Uracil in Ionic Liquids.

The local solvation environment of uracil dissolved in the ionic liquid 1-ethyl-3-methylimidazolium acetate has been studied using neutron diffraction techniques. At solvent:solute (ionic liquid:uracil) ratios of 3:1 and 2:1, little perturbation of the ion-ion correlations compared to those of the neat ionic liquid are observed. We find that solvation of the uracil is driven predominantly by the acetate anion of the solvent. While short distance correlations exist between uracil and the imidazolium cation, the geometry of these contacts suggest that they cannot be considered as hydrogen bonds, in contrast to other studies by Araújo et al. (J. M. Araújo, A. B. Pereiro, J. N. Canongia-Lopes, L. P. Rebelo, I. M. Marrucho, J. Phys. Chem. B 2013, 117, 4109-4120). Nevertheless, this combination of interactions of the solute with both the cation and anion components of the solvents helps explain the high solubility of the nucleobase in this media. In addition, favourable uracil-uracil contacts are observed, of similar magnitude to those between cation and uracil, and are also likely to aid dissolution.