Silvia Imberti

The Nature of Hydrogen Bonding in Protic Ionic Liquids

The size, direction, strength, and distribution of hydrogen bonds in several protic ionic liquids (PILs) has been elucidated using neutron diffraction and computer simulation. There is significant variation in PIL hydrogen bond interactions ranging from short and linear to long and bi-/trifurcated. The nature of the PILs hydrogen bonds reflects its macroscopic properties.

How Water Dissolves in Protic Ionic Liquids

In recent years, ionic liquids (ILs) have emerged as useful chemical solvents for an enormous number of processes and technologies. 2] Their ions have more complex chemical structures than inorganic salts; by incorporating large, sterically mismatched anions and cations, ILs melt at low temperatures because, compared to typical inorganic salts, Coulombic attractions are weakened and lattice-packing arrangements frustrated. ILs are regarded as “designer solvents”, as molecular control over liquid properties is possible depending on how the ions are functionalized. Hydrogen bonding can play a key role in IL chemistry. Whereas most inorganic salts cannot form hydrogen bonds and are dominated by electrostatic interactions between ions, many ILs have extensive H-bonding capacity. For example, H-bond donor and acceptor sites are created during synthesis of protic ionic liquids (PILs). This enables some PILs to develop dense Hbond networks and thus mirrors a number of remarkable structural and solvent properties of water. Finally, ILs have the capacity to self-assemble, forming well-defined nanostructures in the bulk phase as well as at interfaces. IL nanostructure arises because at least one of the ions (frequently the cation) is amphiphilic, with distinct charged and uncharged moieties. This drives segregation of ionic and nonionic groups in ILs, reminiscent of self-assembly in aqueous surfactant mesophases. 12] Here we elucidate the bulk solvent structure of mixtures of a PIL, ethylammonium nitrate (EAN), and water (Figure 1). EAN is one of the oldest known and most extensively studied PILs. As EAN is completely miscible with water, this raises questions such as: how do EAN and water mix? Are the forces that lead to self-assembly in pure EAN sufficient to maintain a solvophobic nanostructure? What is the nature of ion solvation in such mixtures? If nanostructure persists in aqueous mixtures and key solvent properties are retained, this will increase PIL utility by offering an additional mechanism for tuning liquid behavior and lowering the overall cost of the solvent medium. While primitive (continuum solvent) models of dilute aqueous electrolyte solutions are generally successful, understanding ion–water interactions and concentrated solutions has proved challenging, and is complicated in part by the absence of a satisfactory model for liquid water. 19] The structure in aqueous electrolyte solutions is understood in terms of Hofmeister and hydrophobic effects, which can only be probed using sophisticated experimental and computational techniques. Solvated ions induce a different local structure of water molecules in the first, and even the second or third solvation shells, to accommodate the dissolved species. This leads to ions being classified as either “structure making” or “structure breaking” through the creation of “solute cavities”. Recent, growing interest in IL/water mixtures has been motivated, at least in part, by the desire to understand the dramatic changes in IL solvent properties observed upon water contamination. Water is probably the most common impurity in ILs; even nominally hydrophobic ILs absorb significant quantities of water when exposed to the atmosphere. Many computational models have been developed that examined changes in IL solvent structure by dissolved water, often over the full concentration range. At low water concentration, the models predict that the IL nanostructure is relatively unperturbed, but at high water content the system resembles aqueous solutions of ionic surfactants. However, these studies have overwhelmingly investigated aprotic ILs; largely absent are corresponding studies of PILs and experimental verification of the findings. Only one paper has directly investigated the structure of PIL/water mixtures. Smalland wide-angle X-ray scattering (SWAXS) was used to investigate the effect of water on a range of PILs. For neat PILs like EAN, the SWAXS spectra were consistent with nanoscale structure, and were essentially invariant with increasing water content. A micelle-like model was proposed for the solution structure, with water located in the bulk polar domains and associated with the charge groups on the ions. Figure 2 shows the neutron diffraction data and EPSR fits to the three EAN/water mixtures in a molar ratio of 1:6 for different neutron contrasts ([D3]EAN + D2O, [D8]EAN + Figure 1. Molecular structure and atom types of the ethylammonium (EA) cation, nitrate (NO3 ) anion, and water molecule. Different C, N, O, and H atoms are distinguished using subscripts.

Ions in water: The microscopic structure of a concentrated HCl solution

A neutron diffraction experiment with isotopic H/D substitution on a concentrated HCl/H2O solution is presented. The full set of partial structure factors is extracted, by combining the diffraction data with a Monte Carlo simulation. This allows us to investigate both the changes of the water structure in the presence of ions and their solvation shell, overcoming the limitations of standard diffraction experiments. It is found that the interaction with the solutes affects the tetrahedral network of hydrogen bonded water molecules, in a manner similar to the application of an external pressure to pure water, although HCl seems less effective than other solutes, such as NaOH, at the same concentration. Consistent with experimental and theoretical data, the number of water molecules in the solution is not sufficient to completely dissociate the acid molecule. As a consequence, both dissociated H+ and Cl- ions and undissociated HCl molecules coexist in the sample, and this mixture is correctly reproduced in the simulation box. In particular, the hydrated H+ ions, forming a H3O+ complex, participate in three strong and short hydrogen bonds, while a well-defined hydration shell is found around the chlorine ion. These results are not consistent with the findings of early diffraction experiments on the same system and could only be obtained by combining high quality experimental data with a proper computer simulation.

Ions in water: The microscopic structure of concentrated hydroxide solutions

Neutron-diffraction data on aqueous solutions of hydroxides, at solute concentrations ranging from 1 solute per 12 water molecules to 1 solute per 3 water molecules, are analyzed by means of a Monte Carlo simulation (empirical potential structure refinement), in order to determine the hydration shell of the OH- in the presence of the smaller alkali metal ions. It is demonstrated that the symmetry argument between H+ and OH- cannot be used, at least in the liquid phase at such high concentrations, for determining the hydroxide hydration shell. Water molecules in the hydration shell of K+ orient their dipole moment at about 45 degrees from the K+-water oxygen director, instead of radially as in the case of the Li+ and Na+ hydration shells. The K+-water oxygen radial distribution function shows a shallower first minimum compared to the other cation-water oxygen functions. The influence of the solutes on the water-water radial distribution functions is shown to have an effect on the water structure equivalent to an increase in the pressure of the water, depending on both ion concentration and ionic radius. The changes of the water structure in the presence of charged solutes and the differences among the hydration shells of the different cations are used to present a qualitative explanation of the observed cation mobility.

Solvation of hydroxyl ions in water

The solvation shell of the hydroxyl ion in water is explored experimentally for the first time by using a combination of neutron diffraction with hydrogen isotope substitution and Monte Carlo simulation within the empirical potential structure refinement framework. The data are compatible with the presence of nonplanar hydrogen bonded (H9O5)− complexes. The presence of a fifth water molecule in the hydration shell of the hydroxyl ion, weakly hydrogen-bonded to the hydrogen site is also revealed. (H7O4)− complexes, which have been suggested by ab initio simulations to promote proton transfer, are not detectable in the present data, implying that, if they are present at all, they can only be formed transiently.

Aqueous solutions of divalent chlorides: Ions hydration shell and water structure

By combining neutron diffraction and Monte Carlo simulations, we have determined the microscopic structure of the hydration ions shell in aqueous solutions of MgCl(2) and CaCl(2), along with the radial distribution functions of the solvent. In particular the hydration shell of the cations, show cation specific symmetry, due to the strong and directional interaction of ions and water oxygens. The ions and their hydration shells likely form molecular moieties and bring clear signatures in the water-water radial distribution functions. Apart from these signatures, the influence of divalent salts on the microscopic structure of water is similar to that of previously investigated monovalent solutes, and it is visible as a shift of the second peak of the oxygen-oxygen radial distribution function, caused by distortion of the hydrogen bond network of water.

Multiscale Approach to the Structural Study of Water Confined in MCM41

We present a protocol for simultaneous structural characterization of a confined fluid and the confining substrate, along with the extraction of site-site pair correlation functions of the liquid of interest. This is based on neutron diffraction experiments, exploiting where feasible the isotopic substitution technique, analyzed through numerical coarse graining calculations and atomistic simulations. All of the subtleties of the experimental procedure, the needed ancillary measurements, and the recipe for tailoring the numerical codes to the real experiment and sample are described in the case of water confined in MCM41-S-15. In particular the excluded volume effects and the relevance of liquid-substrate cross-correlation terms in the neutron cross section are quantitatively discussed. The results obtained for the microscopic structure of water evidence a non-homogeneous distribution of molecules within the pore, with the presence of water-substrate hydrogen bonds, and a strong distortion of the water-water radial distribution functions with respect to those of bulk water extending at least up to three hydration layers.

Neutron diffractometer INES for quantitative phase analysis of archaeological objects.

With the Italian Neutron Experimental Station (INES) a new general purpose neutron powder diffractometer is available at ISIS, characterized by a high resolution at low d-spacings, and particularly suited for the quantitative phase analysis of a wide range of archaeological materials. Time-of-flight neutron diffraction is notable for being a non-destructive technique, allowing a reliable determination of the phase compositions of multiphase artefacts, with or without superficial corrosion layers. A selection of archaeometric studies carried out during the first year of the INES user programme is presented here to demonstrate the capabilities of the instrument.

Structural study of low concentration LiCl aqueous solutions in the liquid, supercooled, and hyperquenched glassy states

Neutron diffraction experiments on a solution of LiCl in water (R = 40) at ambient conditions and in the supercooled and hyperquenched states are reported and analyzed within the empirical potential structure refinement framework. Evidence for the modifications of the microscopic structure of the solvent in the presence of such a small amount of salt is found at all investigated thermodynamic states. On the other hand, it is evident that the structure of the hyperquenched salty sample is similar to that of pure low density amorphous water, although all the peaks of the radial distribution functions are broader in the present case. Changes upon supercooling or hyperquenching of the ions hydration shells and contacts are of limited size and evidence for segregation phenomena at these states does not clearly show up, although the presence of water separated contacts between ion of the same sign is intriguing.

Formic and acetic acid aggregation in the liquid state.

The microscopic structure of neat formic and acetic acid have been measured by neutron diffraction with H/D substitution on SANDALS at the ISIS neutron spallation source. These data, together with complementary x-ray data, have been modeled via the empirical potential structure refinement (EPSR) method, which integrates information obtained from the diffraction data in a Monte Carlo simulation in order to provide a three-dimensional model of the system under study compatible with the measured structure factors. Two models have been generated for each acid, in order to test their consistency, with positive results. The final structure obtained is that of two liquids that are very similar to each other, with high connectivity although rather disordered. They present a hierarchy of probability for hydrogen bond formation, where weaker bonds involving the carbonyl hydrogen for formic acid or the methyl hydrogen for acetic acid are more abundant than the stronger bonds involving the hydroxyl hydrogen. Cooperative effects are found to be fundamental for the description of aggregation of formic and acetic acid, but the structure in the liquid presents a greater variety of bonds than in the solid state.