Wide-angle X-ray diffraction and molecular dynamics study of medium-range order in ambient and hot water

Abstract

We have developed x-ray diffraction measurements with high energy-resolution and accuracy to study water structure at three different temperatures (7, 25 and 66 C) under normal pressure. Using a spherically curved Ge crystal an energy resolution better than 15 eV has been achieved which eliminates influence from Compton scattering. The high quality of the data allows a precise oxygen-oxygen pair correlation function (PCF) to be directly derived from the Fourier transform of the experimental data resolving shell structure out to ~12 Å, i.e. 5 hydration shells. Large-scale molecular dynamics (MD) simulations using the TIP4P/2005 force-field reproduce excellently the experimental shell-structure in the range 4-12 Å although less agreement is seen for the first peak in the PCF. The Local Structure Index [J. Chem. Phys. 104, 7671 (1996)] identifies a tetrahedral minority giving the intermediate-range oscillations in the PCF and a disordered majority providing a more featureless background in this range. The current study supports the proposal that the structure of liquid water, even at high temperatures, can be described in terms of a two-state fluctuation model involving local structures related to the high-density and low-density forms of liquid water postulated in the liquid-liquid phase transition hypothesis.

Microscopic structure of water at elevated pressures and temperatures

“…Using the local structure index (LSI) (7,51) to identify high (I > 0.03) and low (I < 0.03) structured local coordinations results in an amount of high-LSI molecules of ∼50% at ambient conditions and a rapid decrease of this number with increasing p, T. However, a spatial statistical analysis of these subpopulations also shows no spatial correlations beyond r ) 5 Å even for the high-LSI molecules (SI Text), which indicates that molecules with different local environments are present at all simulated temperatures and pressures but are not spatially correlated beyond their characteristic distances (i.e., the second coordination shell). This is in line with the findings of English et al (4).…”

“…4,11,14) or as an inhomogeneous mixture of two types of structural states with a hydrogen-bond (H-bond) distorted structure surrounding less dense, tetrahedrallylike coordinated patches (e.g., refs. 7,10,12). In this article, we present significant information obtained by an in situ study of changes of the water structure at high pressure and temperature conditions up to the supercritical state.…”

“…However, there is still no consensus on how to describe the microscopic structure of this elusive liquid (2)(3)(4). The most prominent methods of study are the determination of site-site correlation functions using neutron or X-ray diffraction (5)(6)(7)(8), the analysis of density fluctuations by small angle X-ray and neutron scattering (9)(10)(11), and the investigation of the oxygen's local environment by spectroscopic methods (2,3,10,12,13) providing controversial interpretations of the local atomic structure of water. This has resulted in an intense discussion of whether the microscopic structure of ambient water can be described as a liquid with homogeneous density (e.g., refs.…”

Fluctuations in ambient water

“…Here indeed we observe a minimum in κ T that is not too far from the experiment. This is the reason why we have chosen to use this particular MD model for our simulations in the ambient and deeply supercooled regimes [38,49]. However, in this context it is important to note that this model is not perfect.…”

The individual and collective effects of exact exchange and dispersion interactions on the ab initio structure of liquid water

“…1(a), the S OO (Q) taken from various X-ray scattering experiments [82][83][84] with increasing values of Q max are very similar across the range of Q values accessible in a given scattering experiment; despite this fact, each of these X-ray scattering experiments yields significantly different g OO (r), especially with respect to the intensity and position of the first peak.…”