Maria Antonietta Ricci

Site–site pair correlation functions of water from 25 to 400 °C: Revised analysis of new and old diffraction data

Recent controversy in the literature about the structure of water away from ambient conditions is manifested by significant differences between the site–site pair correlation functions of water as derived from neutron diffraction data and the same quantities obtained in computer simulations using an effective pairwise potential for the intermolecular interactions. One possible explanation of the discrepancies between computer models of water and neutron diffraction results near the critical point is that they arise from uncertainties in the inelasticity correction, which is particularly large for light water. To test out this idea, a new method of obtaining the pair correlations is described. This new analysis method is applied both to the earlier neutron data on non-ambient water and on recently reported data for a range of densities at 573 K. For this newer data the incident neutron spectrum is derived from an ambient water moderator instead of the liquid methane (100 K) moderator of previous work, and ...

Analysis of the hydrogen-bonded structure of water from ambient to supercritical conditions

The structure of water has been analyzed at eight different thermodynamic states from ambient to supercritical conditions both by molecular dynamics (MD) and Reverse Monte Carlo (RMC) simulation. MD simulations have been carried out with two different potential models, a polarizable potential and one of the most successful nonpolarizable models, i.e., the well known Simple Point Charge potential in its revised version labeled by E (SPC/E). It has been found that, although the polarizable model can reproduce the experimental partial pair correlation functions at the high temperature states better than the nonpolarizable one, it still cannot account for all the features of the measured functions. The experimental partial pair correlation functions have been well reproduced by the RMC simulations at every state point. The resulting structures have been analyzed in detail. It has been found that the tetrahedral orientation of the hydrogen bonded neighbors is already lost at 423k, whereas the hydrogen bonds th...

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.

Layer analysis of the structure of water confined in vycor glass

A molecular dynamics simulation of the microscopic structure of water confined in a silica pore is presented. A single cavity in the silica glass has been modeled as to reproduce the main features of the pores of real Vycor glass. A layer analysis of the site–site radial distribution functions evidences the presence in the pore of two subsets of water molecules with different microscopic structure. Molecules which reside in the inner layer, close to the center of the pore, have the same structure as bulk water but at a temperature of 30 K higher. On the contrary the structure of the water molecules in the outer layer, close to the substrate, is strongly influenced by the water–substrate hydrophilic interaction and sensible distortions of the H-bond network and of the orientational correlations between neighboring molecules show up. Lowering the hydration has little effect on the structure of water in the outer layer. The consequences on experimental determinations of the structural properties of water in ...

Water confined in Vycor glass. I. A neutron diffraction study

Neutron diffraction experiments with isotopic substitution on water confined in porous Vycor glass at two hydration states are presented and analyzed in terms of the experimentally accessible site-site distribution functions. The bias on these functions as well as their limitations, due to the presence of regions where water molecules are not allowed (excluded volume effects), and to the contribution of water–Vycor interference to the measured cross sections, are discussed. In particular the relative weight of these cross correlation terms is estimated for the first time. It is shown that the traditional analysis of diffraction data of these kinds which ignore cross correlations may be erroneous. A full account of the excluded volume effects is reported in paper II [A. Soper, F. Bruni, and M. A. Ricci, J. Chem. Phys. 109, 1486 (1998), following paper].

A molecular dynamics simulation of water confined in a cylindrical SiO2 pore

A molecular dynamics simulation of water confined in a silica pore is performed in order to compare it with recent experimental results on water confined in porous Vycor glass at room temperature. A cylindrical pore of 40 A is created inside a vitreous SiO2 cell, obtained by computer simulation. The resulting cavity offers water a rough hydrophilic surface and its geometry and size are similar to those of a typical pore in porous Vycor glass. The site-site distribution functions of water inside the pore are evaluated and compared with bulk water results. We find that the modifications of the site-site distribution functions, induced by confinement, are in qualitative agreement with the recent neutron diffraction experiment, confirming that the disturbance to the microscopic structure of water mainly concerns orientational arrangement of neighboring molecules. A layer analysis of MD results indicates that, while the geometrical constraint gives an almost constant density profile up to the layers closest to...

Water confined in Vycor glass. II. Excluded volume effects on the radial distribution functions

The results of a recent neutron diffraction experiment on water confined in Vycor glass, reported in the preceding paper [F. Bruni, M. A. Ricci, and A. K. Soper, J. Chem. Phys. 109, 1478 (1998)], are analyzed by attempting to correct for the distortions brought into the site–site radial distribution functions by the presence of regions of the sample where water molecules are not allowed. These so-called excluded volume effects are evaluated through calculation of the radial distribution function of a uniform fluid in the same confinement conditions as our water samples, and are shown to have a strong effect on the relative intensity of the peaks of the site–site distribution functions. The corrected data are compared with corresponding data on bulk water where appropriate, suggesting that confinement in porous Vycor glass strongly affects the orientational arrangements of water molecules, even at room temperature. The general consequences of this analysis for diffraction studies of other confined liquids,...

Neutron diffraction studies of H2O/D2O at supercritical temperatures. A direct determination of gHH(r), gOH(r), and gOO(r)

Neutron diffraction studies are reported on H2O at temperatures of 300 and 400 °C. The method of isotopic substitution is applied to three mixtures of H2O and D2O, and the diffraction data are used to determine the three radial distribution functions gHH(r), gOH(r), and gOO(r). These results can be used to discuss changes in nearest neighbor structure between water molecules, and to assess the degree of usefulness of representative (usually pairwise) model potentials.

Water above its boiling point: Study of the temperature and density dependence of the partial pair correlation functions. I. Neutron diffraction experiment

Neutron diffraction data on water, employing the technique of hydrogen/deuterium isotope substitution, are reported at three thermodynamic states above the boiling point. The structural information is analyzed in terms of the partial radial distribution functions, OO, OH, and HH, which are extracted from the neutron data. It is found that temperature affects mainly the medium and longer range order in the liquid, while density plays a significant role in controlling the degree of hydrogen bonding. To understand the structure of water obtained from these data it appears that many‐body cooperative interactions have to be correctly accounted for.

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.