FLNP SEMINAR 13 May 2015, 11:00, FLNP Conference hall, 3rd floor

José Teixeira (Laboratoire Léon Brillouin, Saclay, France) “Anomalous properties of low temperature water. A view at molecular scale probed by scattering experiments” Abstract Most of the so-called “anomalous” properties of liquid water are very familiar, simply because water is practically the only natural liquid present on the surface of the Earth. Despite the simplicity of the molecule, consisting of only three atoms, water is a complex liquid due to the very directional chemical bonds formed between neighbouring molecules. Despite the relatively high energy as compared to ambient temperature, the lifetime of these hydrogen bonds is very short: of the order of 1 ps. Structurally, there is a strong local order with tetrahedral symmetry: each molecule in the liquid is surrounded by about 4 neighbours forming a tetrahedron. This structure generates a large empty space allowing large amplitude vibrational motions of the atoms. For over a century, two types of models have been confronted. One type of them (mixture models) postulates the co-existence of two different phases in relative amounts depending on temperature, while another family of models (continuum models) emphasise only the possible distortion of the bonds. The more recent experiments are in favour of the latter. Within a different context, molecular dynamics simulations were performed with more than fifty ad hoc potentials (called effective potentials) that still cannot reproduce simultaneously the structure and the thermodynamic properties of the liquid. A large part of the most recent research activity focuses on supercooled water, the temperature range where anomalies are stronger. Because of polymorphism of amorphous ice and results issued from simulations with a few effective potentials, mixture models reappeared and even postulate the existence of a second critical point at low temperature and relatively high pressure, but in areas that are not accessible to experience due to nucleation of crystalline ice. The presentation will give an overview of recent experiences and problems that remain open. As an example, SAXS results for bulk supercooled water, have been recently object of debate. It will be presented also a model based on precise evaluations of the bond dynamics, which, together with the temperature dependence of the number of intact bonds, play an important role in the interpretation of thermodynamic anomalies. In this context, neutron scattering experiments using spin echo spectrometry allow access with less ambiguity to the dynamics of the bonds.