Solvated neutron diffraction study

The important limitation of neutron diffraction experiments is that the necessary isotopes are often not available so that the difference technique described here cannot be applied. Thus, neutron diffraction studies have been carried out to study solvation of Li and K" " cations but not Na" ". In the latter case, X-ray techniques, which do not provide information about hydrogen bonding, are used. 

In COlL-1, all of the main aspects of the liquid-state structure of room-temperature ionic liquids were already laid out in contributions by several groups, although sometimes still in an incipient way. Hardacre and co-workers reported neutron diffraction studies of short-chain dialkylimidazolium ionic liquids, perfectly illustrating the charge ordering and n-interactions [17]. They also used different spectroscopic and simulation methods to study how the solvation of aromatic and polar molecules (benzene and ethanenitrile, respectively) in the ionic liquids modifies the structure of the media [46] and the balance between the different terms in the interactions coulombic, van der Waals (dispersive), hydrogen bonds and multipolar. 

By the time COlL-2 took place in 2007, the nanostructured nature of the ionic liquids had been postulated using molecular simulation [50] and evidenced by indirect experimental data [54, 85] or by direct X-ray or neutron diffraction studies [56]. This microscopic vision of these fluids changed the way their physico-chemical properties could be explained. The concept of ionicity was supported by this microscopic vision, and indirect experimental evidence came from viscosity and conductivity measurements, as presented by Watanabe et al. [54, 86]. This molecular approach pointed towards alternative ways to probe the structure of ionic liquids, not by considering only the structure of the conponent ions but also by using external probes (e.g. neutral molecular species). Solubility experiments with selected solute molecules proved to be the most obvious experimental route different molecular solutes, according to their polarity or tendency to form associative interactions, would not only interact selectively with certain parts of the individual ions but might also be solvated in distinct local environments in the ionic liquid. 

Beryllium(II) is the smallest metal ion, r = 27 pm (2), and as a consequence forms predominantly tetrahedral complexes. Solution NMR (nuclear magnetic resonance) (59-61) and x-ray diffraction studies (62) show [Be(H20)4]2+ to be the solvated species in water. In the solid state, x-ray diffraction studies show [Be(H20)4]2+ to be tetrahedral (63), as do neutron diffraction (64), infrared, and Raman scattering spectroscopic studies (65). Beryllium(II) is the only tetrahedral metal ion for which a significant quantity of both solvent-exchange and ligand-substitution data are available, and accordingly it occupies a... 

A primary hydration number of 6 for Fe + in aqueous (or D2O) solution has been indicated by neutron diffraction with isotopic substitution (NDIS), XRD, 16,1017 EXAFS, and for Fe " " by NDIS and EXAFS. Fe—O bond distances in aqueous solution have been determined, since 1984, for Fe(H20)/+ by EXAFS and neutron diffraction, for ternary Fe " "-aqua-anion species by XRD (in sulfate and in chloride media, and in bromide media ), for Fe(H20)g by neutron diffraction, and for ternary Fe -aqua-anion species. The NDIS studies hint at the second solvation shell in D2O solution high energy-resolution incoherent quasi-elastic neutron scattering (IQENS) can give some idea of the half-lives of water-protons in the secondary hydration shell of ions such as Fe aq. This is believed to be less than 5 X I0 s, whereas t>5x10 s for the binding time of protons in the primary hydration shell. X-Ray absorption spectroscopy (XAS—EXAFS and XANES) has been used... 

During the last two decades, studies on ion solvation and electrolyte solutions have made remarkable progress by the interplay of experiments and theories. Experimentally, X-ray and neutron diffraction methods and sophisticated EXAFS, IR, Raman, NMR and dielectric relaxation spectroscopies have been used successfully to obtain structural and/or dynamic information about ion-solvent and ion-ion interactions. Theoretically, microscopic or molecular approaches to the study of ion solvation and electrolyte solutions were made by Monte Carlo and molecular dynamics calculations/simulations, as well as by improved statistical mechanics treatments. Some topics that are essential to this book, are included in this chapter. For more details of recent progress, see Ref. [1]. 

X-ray and neutron diffraction methods and EXAFS spectroscopy are very useful in getting structural information of solvated ions. These methods, combined with molecular dynamics and Monte Carlo simulations, have been used extensively to study the structures of hydrated ions in water. Detailed results can be found in the review by Ohtaki and Radnai [17]. The structural study of solvated ions in lion-aqueous solvents has not been as extensive, partly because the low solubility of electrolytes in 11011-aqueous solvents limits the use of X-ray and neutron diffraction methods that need electrolyte of -1 M. However, this situation has been improved by EXAFS (applicable at -0.1 M), at least for ions of the elements with large atomic numbers, and the amount of data on ion-coordinating atom distances and solvation numbers for ions in non-aqueous solvents are growing [15 a, 18]. For example, according to the X-ray diffraction method, the lithium ion in for-mamide (FA) has, on average, 5.4 FA molecules as nearest neighbors with an... 

The structure of Li and K ammonia solution has been recently studied by neutron diffraction experiments [36]. The results show, for saturated lithium-ammonia solutions, that the cation is tetrahedrally solvated by ammonia molecules. On the other hand, from the data of the microscopic structure of potassium-ammonia solutions, the potassium is found to be octahedrally coordinated with ammonia molecules. The Li+ is a structure making ion and K+ is a structure breaking ion in alkali metal-ammonia solutions [37, 38]. 

Kundrot and Richards (1987, 1988) described the solvation shell in the hen egg white lysozyme crystal, in connection with a study of the compressibility of protein and solvent. Mason et al. (1984) carried out a neutron diffraction analysis of triclinic lysozyme at 1.4 A resolution, with 239 water molecules included in the refinement. 

The protein-solvent interface was studied in an explicit solvent environment of 3182 water molecules by MD simulations performed on metmyoglobin [31].Both the structure and dynamics of the hydrated surface of myoglobin are similar to that obtained by experimental methods calculating three-dimensional density distributions, temperature factors and occupancy weights of the solvent molecules. On the basis of trajectories they identified multiple solvation layers around the protein surface including more than 500 hydration sites. Properties of theoretically calculated hydration clusters were compared to that obtained from neutron and X-ray data. This study indicates that the simulation unified the hydration picture provided by X-ray and neutron diffraction experiments. 

Deviations from predicted behaviour are here interpreted in terms of solvation, but other factors such as ion association may also be involved. Ion association leads to deviations in the opposite direction and so compensating effects of solvation and ion association may come into play. The deviations may also be absorbing inadequacies of the Debye-Hiickel model and theory, and so no great reliance can be placed on the actual numerical value of the values emerging. This major method has now been superseded by X-ray diffraction, neutron diffraction, NMR and computer simulation methods. The importance of activity measurements may lie more in the way in which they can point to fundamental difficulties in the theoretical studies on activity coefficients and conductance. The estimates of ion size and hydration studies could well provide a basis for another interpretation of conductance and activity data, or to modify the theoretical equations for mean activity coefficients and molar conductivities. 

The secondary solvation shell about an ion can be studied by neutron diffraction and incoherent neutron scattering.When applied to 5 m LiCl, these methods, indicate that Li" " does not have a secondary solvation shell. The same result would be expected for larger inorganic monovalent cations. The absence of a secondary solvation shell around monovalent ions is not surprising given the relatively small values of T, and T. quoted... 

A great deal of effort has been placed in determining the solvation structure of ions, i.e, the solvent structure in their vicinity, from a variety of spectroscopic techniques such as NMR, XAFS [258-260], Mossbauer, IR, and Raman [261] scattering techniques such as X-rays, electron and neutron diffraction [261,262] electrochemical techniques [2,36] and simulation methods [261]. The rationale behind these studies hinges upon the idea that a realistic description of the thermophysical properties of electrolyte solutions must take into account the ion-induced local distortion of the solvent properties, i.., it should go beyond the so-called continuum or primitive models. The challenge resides in our ability to probe the properties of the solvent in the vicinity of ions, and then, to make explicit contact with meaningful solvation-related macroscopic properties. 

The solvation of cation and electrical double layer structure near clay surface was studied by neutron diffraction methods (156-158). The intaplay between molecular simulations and neutron diffraction techniques also has been also applied to this clay mineral-water-cation interface system. Park and Sposito (112) simulated the total radial distribution function (TRDF) of interlayer water from Na-, Li-, and K-montmorillonite hydrates as a physical quantity from molecular simulations. They obtained TRDF values from Monte Carlo simulations and directly compared with previously obtained H/ D isotopic difference neutron diffraction results (9,10). 

Because of the important differences between the two diffraction experiments, the strategies used in carrying out these studies are not the same. X-ray experiments are certainly more common since the equipment used is easily obtained. Neutron experiments are carried out at a nuclear reactor site or at an accelerator with the appropriate facilities. In the following sections, some results from diffraction experiments are presented with emphasis on the structural information which has been obtained regarding ion solvation in electrolyte solutions. 

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