Structure of supercritical water

This review paper gives the more up to date information on the structure of supercritical water, at different thermodynamic states, for densities ranging from 0.2 to 0.7 g.cm⁻³, studied by neutron scattering. The experimental partial pair correlation functions g_OH(r), G_HH(r) and g_OO(r) are compared with the results of molecular dynamics simulations using the (SPCE) model potential for water. The results confirm that hydrogen bonding is still present in dense supercritical water and are in good agreement with data obtained by NMR and Raman spectroscopies.     

Neutron diffraction studies of the structure of water at ambient temperatures,revisited [a review of past developments and current problems]

Neutron diffraction techniques used in the study of the structural characteristics of liquid water under ambient conditions are reviewed, showing the way that developments in the experimental and analytic procedures have led to improvements in the extracted results. The three partial pair correlation functions (OH, HH and OO) can be isolated from a set of diffraction measurements with varying hydrogen/deuterium isotopic composition. Although isotopic substitution is used routinely in many other investigations there are particular difficulties with hydrogenous systems due to the high incoherent scattering from hydrogen and the possibility that the H/D substitution is not a true isomorphic replacement. These features are discussed in some detail to analyse the systematic errors and the precision of the final results. An additional aspect of the comparison of data taken by different groups concerns the use of either reactor or pulsed neutron methods. Studies using the Orphee reactor with an over-determined set of five datasets are considered in detail, using analytic correction procedures based on the Powles formalism. Various consistency checks are applied to the data such that smoothing or iteration routines are not required. Two independent sets of the OD and DD pair correlation functions are obtained and are compared with the latest results from Soper et al. The small discrepancies between the various datasets are discussed in terms of propagating systematic errors and also possible variations from H/D equivalence. The relevance of the new results for the interpretation and modelling of ambient water structure are presented and the review ends with a brief comment on likely future developments that incorporate additional experimental information.     

Diffusional and vibrational dynamics of ZnCl2 aqueous solutions by inelastic neutron scattering

We have studied saturated solutions of ZnCl₂ (and NiCl₂) in D₂O by time-of-flight quasi-elastic and inelastic neutron scattering spectroscopy. Spectra were taken at room temperature as a function of transferred momentum. We find that ionic diffusion in these solutions is best described by a crystal vacancy diffusion model. We also find evidence of collective excitations in the region of ～ 20 meV.     

Dynamics from picoseconds to nanoseconds of trehalose in aqueous solutions as seen by quasielastic neutron scattering

We present a study of the dynamical behavior of trehalose, a cryoprotecting agent, in concentrated aqueous solutions. Dynamics in a wide time range from picoseconds to nanoseconds has been observed using both neutron time of flight and neutron spin-echo techniques. Fast dynamics has been described using a simple diffusion model, while dynamical processes at longer times show a more complex behavior, described by a stretched exponential decay. Obtained relaxation times show a good agreement with data from viscosity measurements on aqueous trehalose solutions by Magazu et al. [Branca, Magazu, Maisano et al., J. Phys.: Condens. Matter 11, 3823 (1999)]. Experimental data provide us with some insight into the cryoprotecting properties and processes of trehalose. We conclude that an increase of the solvent viscosity in embedded biological material due to the production or the presence of trehalose might prevent biomolecules from damage.     

Local order in aqueous NaCl solutions and pure water: X-ray scattering and molecular dynamics simulations study

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.     

Structural studies of water in hydrophilic and hydrophobic mesoporous silicas: an x-ray and neutron diffraction study at 297 K

Water confined in a sol-gel network has been characterized by x-ray and neutron diffraction for two samples of mesoporous silica: one with a hydrophilic character (a nonmodified one) and another with a hydrophobic character (a modified one with a methylated internal pore surface). The pore size has been previously characterized [J. Jelassi et al., Phys. Chem. Chem. Phys. 134, 1039 (2010)] to have a mean pore diameter of approximately 55 A?. The diffraction measurements presented in this paper have been made at room temperature [293 K] for a filling factor of 0.45, giving a mean thickness of 8-9 A? for the water layer. The results show that the local order of the confined water molecules in the intermediate region of 3-6 A? is significantly different from that of the bulk water and also for the two different environments. For the hydrophilic sample, the siloxyl groups at the surface modify the water structure through the effects of interfacial hydrogen-bonding, which influences the orientational configuration of local water molecules and creates a modified spatial arrangement in the pore. In the case of the hydrophobic sample, there is no specific interaction with the pore wall, which is primarily van der Waals type, and the water molecules at the interface are differently oriented to create a hydrogen-bonded network linked more directly to the rest of the water volume. In the present circumstances, the thickness of the water layer has a relatively small dimension so that the interpretation of the measured diffraction pattern is not as straightforward as for the bulk liquids, and it is necessary to consider the effects of diffraction-broadening from a distributed sample volume and also the contribution from cross-terms that remain after conducting a "wet-minus-dry" analysis procedure. These analytic difficulties are discussed in the context of the present measurements and compared with the work of other groups engaged in the study of water confined in different environments. The present results, again, emphasize the complexity influencing the properties of water in a confined geometry and the strong influence of surface interactions on its behavior.     

Interlayer water molecules in vanadium pentoxide hydrate. IX. Anisotropic translational diffusion leading to anisotropic ac conductivity

The anisotropy of the dynamic properties of interlayer water molecules along the a and b axes of vanadium pentoxide hydrate, orthorhombic V2O5.nH2O, was studied using quasielastic neutron scattering (QENS) in relation to the anisotropy of the ac conductivity. The QENS spectra were analyzed using a stretched exponential function and a Lorentzian function. Both methods showed that the double-layer water molecules along the b axis are more mobile than those along the a axis. The difference in mobility between the two axes is more pronounced using a Lorentzian function analysis. These facts suggest that the diffusion coefficient of water molecules along the b axis is larger than that along the a axis, which is closely related to the ac conductivity originating from proton hopping. The anisotropy of the dynamic motion of water molecules can be attributed to the shorter b-axis length (b=3.60 A), with respect to the longer and less regular repetition of the atomic arrangements along the a axis (42.34 A).     

Studies of cavitation and ice nucleation in 'doubly-metastable' water: time-lapse photography and neutron diffraction

High-speed photographic studies and neutron diffraction measurements have been made of water under tension in a Berthelot tube. Liquid water was cooled below the normal ice-nucleation temperature and was in a doubly-metastable state prior to a collapse of the liquid state. This transition was accompanied by an exothermic heat release corresponding with the rapid production of a solid phase nucleated by cavitation. Photographic techniques have been used to observe the phase transition over short time scales in which a solidification front is observed to propagate through the sample. Significantly, other images at a shorter time interval reveal the prior formation of cavitation bubbles at the beginning of the process. The ice-nucleation process is explained in terms of a mechanism involving hydrodynamically-induced changes in tension in supercooled water in the near vicinity of an expanding cavitation bubble. Previous explanations have attributed the nucleation of the solid phase to the production of high positive pressures. Corresponding results are presented which show the initial neutron diffraction pattern after ice-nucleation. The observed pattern does not exhibit the usual crystalline pattern of hexagonal ice [I(h)] that is formed under ambient conditions, but indicates the presence of other ice forms. The composite features can be attributed to a mixture of amorphous ice, ice-I(h)/I(c) and the high-pressure form, ice-III, and the diffraction pattern continues to evolve over a time period of about an hour.