Jeudi 24 novembre

à 11h   salle 107 del'UFR (couloir 22-23)

Lars Petterson

(FYSIKUM, AlbaNova University Center, Stockholm University, S-106 91 Stockholm)


invité par Aurélien Perera

X-ray-spectroscopies and scattering applied to water:

What can we learn from experiment and simulations?

Résumé: I will discuss recent experimental and simulation data from x-ray absorption (XAS), emission (XES) and scattering (XRD and SAXS) of liquid water and the picture of fluctuations between high-density (HDL) and low-density (LDL) liquid this has led to [1]. Such a two-liquid scenario, which is extensively discussed for deeply supercooled water, would explain many anomalous properties, e.g., density maximum, heat capacity and isothermal compressibility minima, but no direct connection has been found to simulations of ambient water. Applying the local structure index (LSI) of Shiratani and Sasai [J. Chem. Phys. 104, 7671 (1996)] to the inherent structure of TIP4P/2005 water we find a strict bimodality in terms of spatially separated HDL- and LDL-like environments in the simulations at all temperatures and pressures with distributions in agreement with the conclusions from XAS and XES, i.e. 75% HDL and 25% LDL-like at ambient conditions [2]. The SAXS signal is connected to density fluctuations in the liquid which for water decrease (as measured by the isothermal compressibility) down to 46°C but then increase as the liquid is further cooled. Comparing computed and measured SAXS from ambient down to supercooled (-21°C) temperatures shows excellent agreement at 5°C but increasingly underestimating the SAXS signal upon further cooling, indicating that structural fluctuations at cooler temperatures are underestimated in the simulations [3]. New, high-quality wide-angle x-ray diffraction (XRD) data resolve five hydration shells, i.e. out to 12 Å, also in ambient and hot water with differences in temperature dependence (amplitude and position) for the different peaks which are well-reproduced by TIP4P/2005 and thus can be analyzed [4]. I will conclude by discussing recent ab initio simulations including non-local correlation which can be interpreted as the isotropic van der Waals interaction counteracting the directional H-bonds leading to denser packing and HDL-like liquid [5].

[1] A. Nilsson and L. G. M. Pettersson, Perspective on the Structure of Liquid Water, Chem. Phys. 389, 1-34 (2011)

[2] K. T. Wikfeldt, A. Nilsson and L. G. M. Pettersson, Spatially Inhomogeneous Bimodal Inherent Structure in Simulated Liquid Water, Phys. Chem. Chem. Phys. 13, 19918-19924 (2011)

[3] K. T. Wikfeldt, C. Huang, A. Nilsson and L. G. M. Pettersson, Enhanced small-angle scattering connected to the Widom line in simulations of supercooled water, J. Chem. Phys. 134, 214506 (2011)

[4] Huang et al., “Wide-Angle X-ray Diffraction and Molecular Dynamics Study of Medium-range Order in Ambient and Hot Water “, Phys. Chem. Chem. Phys. 13, 19997 (2011)

[5] Møgelhøj et al., Ab initio van der Waals interactions in simulations of water alter structure from mainly tetrahedral to high-density-like, J. Phys. Chem. B, in press, doi:10.1021/jp2040345