R. Mancinelli

Aqueous solutions of divalent chlorides: Ions hydration shell and water structure

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.

Multiple relaxation processes versus the fragile-to-strong transition in confined water

Broadband dielectric spectroscopy data on water confined in three different environments, namely at the surface of a globular protein or inside the small pores of two silica substrates, in the temperature range 140 K ≤ T ≤ 300 K, are presented and discussed in comparison with previous results from different techniques. It is found that all samples show a fast relaxation process, independently of the hydration level and confinement size. This relaxation is well known in the literature and its cross-over from Arrhenius to non-Arrhenius temperature behavior is the object of vivid debate, given its claimed relation to the existence of a second critical point of water. We find such a cross-over at a temperature of ~180 K, and assign the relaxation process to the layer of molecules adjacent and strongly interacting with the substrate surface. This is the water layer known to have the highest density and slowest translational dynamics compared to the average: its apparent cross-over may be due to the freezing of some degree of freedom and survival of very localized motions alone, to the onset of finite size effects, or to the presence of a calorimetric glass transition of the hydration shell at ~170 K. Another relaxation process is visible in water confined in the silica matrices: this is slower than the previous one and has distinct temperature behaviors, depending on the size of the confining volume and consequent ice nucleation.

Influence of concentration and anion size on hydration of H+ ions and water structure.

Neutron diffraction experiments with hydrogen isotope substitution on aqueous solutions of HCl and HBr have been performed at concentrations ranging from 1:17 to 1:83 solute per water molecules, at ambient conditions. Data are analyzed using the empirical potential structure refinement technique in order to extract information on both the ion hydration shells and the microscopic structure of the solvent. It is found that the influence of these solutes on the water structure is less concentration dependent than that of salts or hydroxides. Moreover protons readily form a strong H-bond with a water molecule upon solvation, at all proportions. The majority of them is also bonded via a longer bond to another water molecule, giving a prepeak in the g(OwOw). At high solute concentration, the second water molecule may be substituted by the counterion. In particular at solute concentrations of the order of 1:17 or higher, all protons have an anion within a distance of 4.5 A.

Study of percolation and clustering in supercritical water-CO2 mixtures

The microscopic structure of supercritical water-CO(2) mixture is investigated by neutron diffraction experiments exploiting the isotopic HD substitution. The investigated water reach mixtures are in the liquidlike region of the phase diagram, according to the behavior of the radial distribution functions, yet a reduction of the average number of hydrogen bonds, compared to equivalent states of pure water, is found. As a consequence, the average dimension of water clusters is reduced and the system stays below the percolation threshold. These results, along with the shift of the main peaks of the site-site radial distribution functions, suggest that the excess volume in these supercritical mixtures is likely associated with the CO(2) solvation shell.

Microscopic structure of water in a water/oil emulsion

We have determined the microscopic structure of water within a water/oil emulsion, by combining neutron diffraction data, exploiting the isotopic H/D substitution, and a fully atomistic Monte Carlo simulation of a portion of a water droplet, containing the water/oil interface. The dependence of the data on the simulation box size and the reliability of the water-water radial distribution functions are discussed. Although water in the emulsion forms shorter and stronger hydrogen bonds compared to pure bulk water, its overall microscopic structure looks more disordered.

Studies of water at inorganic solid surfaces

After a brief review of calorimetric evidences from experiments performed on confined water, we describe the results of a neutron diffraction experiment performed on water confined in MCM41-S-15. This, inequivocally shows that, when the size of the confining volume is small enough, water occupies the available volume in a seriously inhomogeneous manner and that the coexistence of different phases, microcrystals and void regions cannot be excluded. Upon cooling the sample a more uniform filling of the pores can be obtained, and the microscopic structure of water changes accordingly, recovering a more tetrahedral simmetry. The consequences of these results on the interpretation of other independent experiments are also considered.