Neutron diffraction data with hydrogen isotope substitution on aqueous solutions of NaCl and KCl at concentrations ranging from high dilution to near-saturation are analyzed using the Empirical Potential Structure Refinement technique. Information on both the ion hydration shells and the microscopic structure of the solvent is extracted. Apart from obvious effects due to the different radii of the three ions investigated, it is found that water molecules in the hydration shell of K+ are orientationally more disordered than those hydrating a Na+ ion and are inclined to orient their dipole moments tangentially to the hydration sphere. Cl- ions form instead hydrogen-bonded bridges with water molecules and are readily accommodated into the H-bond network of water. The results are used to show that concepts such as structure maker/breaker, largely based on thermodynamic data, are not helpful in understanding how these ions interact with water at the molecular level.

The ion induced modification to the tetrahedral structure of water is a topic of much current interest. We address this question by interpreting neutron diffraction data from monovalent ionic solutions of NaCl and KCl using a computer assisted structural modeling technique. We investigate the effect that these ions have on the water-water O-O, O-H and H-H radial distribution functions as a function of ionic concentration. It is found that the O-H and H-H functions are only marginally affected by ionic composition, signaling that hydrogen bonding between water molecules remains largely intact, even at the highest concentrations. On the other hand the O-O functions are strongly modified by the ions. In particular the position of the second peak in g(OO)(r), is found to move inwards with increasing salt concentration, in a manner closely analogous to what happens in pure water under pressure. Furthermore by recalculating g(OO)(r) after excluding all the water molecules in the first hydration shell of each ion, we show that this structural perturbation exists outside the first hydration shell of the ions.

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.

The issue of the existence of a minimum of density at 210 K in water confined in MCM41 is tackled by neutron diffraction with H/D isotopic substitution over a wide range of momentum transfer, which allows refining in a single experiment both the meso-and the atomic-scale structure. We observe clear density fluctuations of confined water, with temperature-dependent length scale, due to changes of the balance between cohesive and adhesive interactions. These results cast some doubt about previous interpretation of SANS experiments on water confined in the same substrate as evidence for the existence of a point of minimum density at 210 K. This interpretation is based on the assumption that water homogeneously occupies the available volume within the confining substrate. We demonstrate that this is not the case in water confined in MCM-41-S-15, and provide an alternative explanation of small-angle neutron scattering results.

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.