Water - Structure and origin of its anomalous properties
Molecular Physics seminar
Monday 10 March 2014
Lars Pettersson (SU)
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 [1,2]. Such a two-liquid scenario 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 . 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 . Going into “No man’s land” to do experimental measurements has recently become possible through the Linac Coherent Light Source (LCLS) x-ray free-electron laser at SLAC by exploiting evaporative cooling in vacuum of micrometer-sized water droplets from which a full diffraction pattern of individual droplets is obtained through the 100 fs fully coherent x-ray pulses delivered by the LCLS showing a continuous development towards an LDL liquid down to at least 227 K.
 A. Nilsson and L. G. M. Pettersson, Perspective on the Structure of Liquid Water, Chem. Phys. 389, 1-34 (2011).
 A. Nilsson, C. Huang and L.G.M .Pettersson, Fluctuations in Ambient Water, J. Mol. Liq. 176, 2-16 (2012).
 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).
 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).