Dielectric susceptibility and neutron scattering of liquid water: A unified description

Resumen del trabajo presentado al 8th International Discussion Meeting on Relaxations in Complex Systems: "New results, Directions and Opportunities", celebrado en Wisla (Polonia) del 23 al 28 de julio de 2017.Water dynamics has paramount importance in many areas of research and industrial applications. One of the main techniques used from the early times to investigate water dynamics is dielectric spectroscopy (DS). Thanks to the development of the tera-herz (THz) techniques it was recently possible to fill the gap between dipolar relaxation and intermolecular streatching vibrations at ≈5THz and to have a full picture of the dielectric permittivity of liquid water in a broad frequency range. However, the microscopic understanding of the dielectric spectrum of water and the nature of the molecular motions involved in the different dielectric processes are still very unclear. This is mainly due to the fact that DS is a 'macroscopic' technique, which follows the fluctuations of the total dipole moment of the system without spatial resolution. Interestingly, the relevant frequency range for the dielectric response of liquid water can also be covered by neutron scattering (NS), a tehnique delivering microscopic information with spatial/time resolution. In a recent work, we have considered DS data together with coherent and incoherent NS results of water at room temperature. The NS data were analyzed in terms of the corresponding susceptibility –a less conventional way that allows distinguishing better the different processes involved as well as the comparison with spectroscopy DS data. We have also carried out molecular dynamics (MD) simulations in order to unveil the microscopic origin of the different processes involved. We achieve a unified description of both NS and DS susceptibilities, which allows clarifying the nature of the different relaxation processes, the molecular motions involved in the DS spectra and their impact on the structural relaxation. These results also open a new way of approaching dynamics of water under different conditions (supercooled, confined, etc.) and that of other H-bonded liquids.Peer reviewe