An extensive series of neutron diffraction experiments and molecular dynamics simulations has shown that mixtures of methanol and water exhibit extended structures in solution despite the components being fully miscible in all proportions. Of particular interest is a concentration region (methanol mole fraction between 0.27 and 0.54) where both methanol and water appear to form separate, percolating networks. This is the concentration range where many transport properties and thermodynamic excess functions reach extremal values. The observed concentration dependence of several of these material properties of the solution may therefore have a structural origin.

Hydrogen/deuterium isotopic substitution neutron diffraction techniques have been used to measure the solute−solute intermolecular structural correlations in 0.06, 0.11, and 0.16 mole fraction tertiary butanol−water solutions. Empirical potential structure refinement (EPSR) procedures have been used to extract detailed information relating to the intermolecular structure in these systems. A trend from hydrophobic to hydrophilic character of the solute−solute correlations as a function of solute concentration is observed. Of particular note is the domination of nonpolar to nonpolar solute contacts at 0.06 mole fraction concentration compared with a more complex mixture of nonpolar and polar solute−solute intermolecular contact configurations at the higher alcohol concentration.

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.

Recently it has been shown how the measured partial structure factors for diatomic molecular liquids might be used to generate a detailed view of the local intermolecular orientational correlation function, via the standard spherical harmonic expansion. The present work generalizes this analysis to the case of molecules of arbitrary shape. An analysis of the measured atom–atom partial structure factors for liquid water is presented, and the corresponding maps of the orientational correlation function between water molecules are derived. It is seen that this method of presenting diffraction data is rich in detail about the nature of the short-range interactions between water molecules in the liquid state. In particular, it is found that while there is a pronounced directionality in the organization of neighboring molecules around any given molecule at the origin, corresponding to the hydrogen bonding in the liquid, there are nonetheless a range of local orientations compatible with the neutron data. This argues against the notion of water forming short lived, ‘‘icelike’’ clusters at ambient temperature and pressure, as has been speculated in the past. The general implications for this kind of analysis for other molecular liquids, solutions, and mixtures is discussed.

Using isotope substitution neutron scattering data, we present a detailed structural analysis of the short and intermediate range structures of the five known forms of amorphous ice. Two of the lower density forms--amorphous solid water and hyperquenched glassy water--have a structure very similar to each other and to low density amorphous ice, a structure which closely resembles a disordered, tetrahedrally coordinated, fully hydrogen bonded network. High density and very high density amorphous ices retain this tetrahedral organization at short range, but show significant differences beyond about 3.1 A from a typical water oxygen. The first diffraction peak in all structures is seen to be solely a function of the intermolecular organization. The short range connectivity in the two higher density forms is more homogeneous, while the hydrogen site disorder in these forms is greater. The low Q behavior of the structure factors indicates no significant density or concentration fluctuations over the length scale probed. We conclude that these three latter forms of ice are structurally distinct. Finally, the x-ray structure factors for all five amorphous systems are calculated for comparison with other studies.