Neutron scattering experiments on aqueous sodium chloride solutions and heavy water. Comparison to molecular dynamics and X-ray results (original) (raw)
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Journal of Physical Chemistry B, 2006
The microstructures of pure water and aqueous NaCl solutions over a wide range of salt concentrations (0-4 m) under ambient conditions are characterized by X-ray scattering and molecular dynamics (MD) simulations. MD simulations are performed with the rigid SPC water model as a solvent, while the ions are treated as charged Lennard-Jones particles. Simulated data show that the first peaks in the O‚‚‚O and O‚‚‚H pair correlation functions clearly decrease in height with increasing salt concentration. Simultaneously, the location of the second O‚‚‚O peak, the signature of the so-called tetrahedral structure of water, gradually disappears. Consequently, the degree of hydrogen bonding in liquid water decreases when compared to pure fluid. MD results also show that the hydration number around the cation decreases as the salt concentration increases, which is most likely because some water molecules in the first hydration shell are occasionally substituted by chlorine. In addition, the fraction of contact ion pairs increases and that of solvent-separated ion pairs decreases. Experimental data are analyzed to deduce the structure factors and the pair correlation functions of each system. X-ray results clearly show a perturbation of the association structure of the solvent and highlight the appearance of new interactions between ions and water. A model of intermolecular arrangement via MD results is then proposed to describe the local order in each system, as deduced from X-ray scattering data.
A neutron diffraction study on the structure of Cl - solutions in hydrogen-bonded molecular solvents
Journal of Physics: Condensed Matter, 1994
The method of isotopic substitution in time-of-Right neutron diffraction is applied to measure the coordination environment OF CI-in a 0.999(2) molal solution of NaQ in fully deuterated glycerol. It is shown that the glycerol molecules act as monodentate ligands to give a Cl(glycero1); complex thl comprises C1-. . .D-O hydrogen bonds, which are approximately linear and of length 2.17(3) A. The results are compared with those obtained for the solvation of C1-in solutions of its salts in water, methanol and ethylene glycol (EO). A reduction from approximately sir (water) to four (methanol and EG) to three (glycerol) is found for the C1solvation number as the solvent is changed and, for all four solvents, the Cl-. . .D-O hydrogen bond is typically linear.
Molecular clusters and correlations in water by x-ray and neutron diffraction
Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics, 1993
The combined analysis of x-rayand neutron-diffraction data of liquid water and heavy water exhibits distinct molecular tetrahedral pentamer clusters (in C2, symmetry) in water due to hydrogen bonding. The center structure is derived from the diffraction data and used to evaluate through the model intermolecular partial structures and partial correlations in water. The results are compared with the available experimental and computer-simulated data.
Biophysical Chemistry, 2006
The presence of ions in water provides a rich and varied environment in which many natural processes occur with important consequences in biology, geology and chemistry. This article will focus on the structural properties of ions in water and it will be shown how the 'difference' methods of neutron diffraction with isotopic substitution (NDIS) and anomalous X-ray diffraction (AXD) can be used to obtain direct information regarding the radial pair distribution functions of many cations and anions in solution. This information can subsequently be used to calculate coordination numbers and to determine ion-water conformation in great detail. As well as enabling comparisons to be made amongst ions in particular groups in the periodic table, such information can also be contrasted with results provided by molecular dynamics (MD) simulation techniques. To illustrate the power of these 'difference' methods, reference will be made to the alkali group of ions, all of which have been successfully investigated by the above methods, with the exception of the radioactive element francium. Additional comments will be made on how NDIS measurements are currently being combined with MD simulations to determine the structure around complex ions and molecules, many of which are common in biological systems.
The European physical journal. E, Soft matter, 2015
In complementarity with X-ray scattering and as extension of our previous publication, neutron scattering measurements are combined to DFT calculation to investigate the structural features of N-methylformamide-water mixtures (NMF-water) for three water molar fractions x w = 0.5 , 0.66 and x w = 0.75 . The recorded data at atmospheric pressure and room temperature are analyzed to yield the structure factor, the molecular form factor and the pair correlation function. Neutron investigations corroborate the X-ray ones and clearly show that liquid order in solutions is well accounted for by a tetramer. In this cluster, an NMF molecule is connected to three water molecules by one N-D (...) O and two O-D (...) O hydrogen bonds.
Physical Review B, 2006
Neutron diffraction with isotopic substitution has been used to investigate aqueous solutions of 2M NaOH in the liquid state. The data were modeled using empirical potential structure refinement which allows for the extraction of the ion-water and water-water correlations. The data show that the ion-water radial distribution functions are in accordance with those found by previous studies on NaOH solutions and follow a trend which is dependent on the concentration of the solute. In particular, the shape of the hydroxide hydration shell is found to be concentration independent, but the number of water molecules occupying this shell increases with dilution. Additionally, the water-water correlations show that there is still a measurable effect on water structure with the addition of ions at this concentration, as the second shell in the water oxygen radial distribution function is compressed relative to the first shell. The data are also used to discuss the recent claims that the published radial distribution functions of water are unreliable, showing that data taken at different neutron sources, with different diffraction geometry and systematic errors lead to the same structural information when analyzed via a realistic modeling regime.
On the Range of Water Structure Models Compatible with X-ray and Neutron Diffraction Data
The Journal of Physical Chemistry B, 2009
We use the reverse Monte Carlo (RMC) method to critically evaluate the structural information content of diffraction data on bulk water by fitting simultaneously or separately to X-ray and neutron data; the O-H and H-H, but not the O-O, pair-correlation functions (PCFs) are well-described by the neutron data alone. Enforcing at the same time different H-bonding constraints, we generate four topologically different structure models of liquid water, including a simple mixture model, that all equally well reproduce the diffraction data. Although earlier work [Leetmaa, M.; et al. J. Chem. Phys. 2008, 129, 084502] has focused on tetrahedrality in the H-bond network in liquid water, we show here that, even for the O-O-O three-body correlation, tetrahedrality is not strictly defined by the data. We analyze how well two popular MD models (TIP4P-pol2 and SPC/E) reproduce the neutron data in q-space and find differences in important aspects from the experiment. From the RMC fits, we obtain pair-correlation functions (PCFs) that are in optimal agreement with the diffraction data but still show a surprisingly strong variability both in position and height of the first intermolecular (H-bonding) O-H peak. We conclude that, although diffraction data impose important constraints on the range of possible water structures, additional data are needed to narrow the range of possible structure models.
Le Journal de Physique IV, 1993
Molecules of 5 -D e c y l -4 , 7 , 1 3 , 1 6 , 2 1 -p e n t a o x a -1 , l O -d i~i c o s a n (221D) and its sodium complex, with both a hydrophobic and a hydrophilic portion, are expected to form aggregates in water solutions. This was confirmed by surface tension measurements. The aggregation behaviour was studied by small-angle neutron scattering at two different [NaC11/[221D] molar ratios, such as to obtain, in one case, aggregates entirely made up of ionic monomers, and in the other, mixed micelles constituted by both ionic and non-ionic units. The variation of the aggregation number and number of aggregates indicated that, in the former case, smaller micelles were formed, as a consequence of repulsion among the ionic head groups.
Neutron diffraction measurements from 2.4 mole% solutions of tetrapropylammonium bromide TPrABr , C 3 H 7 4 NBr, in water, using hydrogen/deuterium isotope substitution on the alkyl groups of the cation, is used to investigate the distribution of cations in the solution. The intermolecular partial pair correlation function for the alkyl group hydrogens is obtained, which shows a broad peak with a maximum at about 11 Å, with structure in the 2.5-4 Å region, suggesting that some of the alkyl chains of neighboring ions may not be separated by water molecules. The three-dimensional structure is modeled using the reverse Monte Carlo method to fit the HH partial structure factor. The simulation results show that at least half of the cations are not separated by water from one other cation and that a few clusters of three or four cations also occur. The close contact found between the TPrA ions is discussed in relation to previous results on the structure of water in TPrABr solution at the same concentration.