HDR de Livia Bove - Mercredi 1er juillet 2015 à 14 h

IMPMC, Université P. et M. Curie, 4, Place Jussieu, 75005 Paris

Salle de conférence, 4e étage, Tour 22-23, Salle 1

"Structure and dynamics of water and salty water under extreme conditions"

  P-T phase diagram of LiCl-ice. Li (yellow/orange), Cl (green/blue), Oxygen (red), Hydrogen (white)

Abstract

Water is an inexhaustible source of awe for scientists as, despite its rather simple electronic and molecular structure, it manifests a range of anomalies, with respect to other simple molecules, that account for its crucial role in nature. Its uniqueness derives from the conjunction of the molecule’s open geometry, which produces an almost tetrahedral coordination, and the presence of weak, and fast reorganizing, intermolecular hydrogen bonds (HB). The tetrahedral arrangement of water molecules represents a key point for understanding water properties and ice richness of isomorphs. Other tetrahedral coordinated systems, such as SiO2 and Si, also form a wide range of crystal structures, however, water builds them through hydrogen bonding, with an equal number of hydrogen- bond donors and acceptors, and this is somehow unique. The ease with which hydrogen bonds break and form in water (with a characteristic time of the order of 1 ps) together with the strong temperature dependence of the number of hydrogen bonds per water molecule account for most anomalies of water transport properties with temperature. 
In this work I will review our recent results on the study of water and salty water structure and dynamics under extreme conditions, where extreme is meant for both the deeply undercooled regime and the high-pressure regime. A particular effort has been made to provide new experimental tools, based on Paris-Edinburg pressure cells and neutron scattering techniques, to directly have access to hydrogen dynamics under these conditions.
Employing this new technology we could, as an example, measure the translational diffusion coefficient and the re-orientational time of water molecules at high temperature up to ice VII crystallization, and along its melting curve, i.e. on a region of water p-T phase diagram never explored before. We thus observed unexpected phenomena, such as the complete decoupling of the translational and rotational dynamics under high pressure, which indicates the residual presence of hydrogen bonds in the high-density liquid. Fundamental phenomena as the concerted proton tunneling of hydrogens in ordinary ice were also observed, and related to the proton-disordered nature of the system.
Salty water has been also investigated in the attempt to address the thermodynamic and structural properties of water in the deep undercooled regime, the stability of which is promoted by the addition of ions in water. The presence of a polyamorphism phenomenon similar to the one shown by pure water has been observed, thought no hints of a liquid-liquid transition are found in the undercooled liquid. The study of the high-pressure phase diagram of salty water has also brought to the unexpected discovery that ice, contrarily to what observed at ambient pressure, can homogeneously include into its high-pressure structures large amounts of salts. The inclusion of ions leads to relevant effects on the density, the lattice dynamics, or the conductivity properties, and promotes new states. This result could impact the physics of ice bodies, where the interior modelling generally assumes physical properties of ice phases made of pure water, despite the likely presence of salts. Beyond the geophysical relevance, the existence of salty ices poses fundamental questions on the water network rearrangement and dynamics in the presence of the electric field generated by the ionic impurity, as well as on the effect of ions on nuclear quantum effects (Figure), which will be discussed.

Composition du jury

• Mme  Marie Claire BELLISSENT-FUNEL, CNRS-CEA LLB, rapporteur
• M. Frederic CAUPIN, Université Lyon I,  rapporteur
• M. Fabio FINOCCHI, CNRS-UPMC,  president
• M. Werner KUHS, Goettingen University
• M. Paul  MCMILLAN, UCL, rapporteur
• Mme Maria Antonietta RICCI, Università Roma III
• M. Jose TEIXEIRA, CNRS-CEA LLB