Small-angle scattering and the structure of ambient liquid water

Abstract

Structural polyamorphism has been promoted as a means for understanding the anomalous thermodynamics and dynamics of water in the experimentally inaccessible supercooled region. In the metastable liquid region, theory has hypothesized the existence of a liquid-liquid critical point from which a dividing line separates two water species of high and low density. A recent smallangle X-ray scattering study has claimed that the two structural species postulated in the supercooled state are seen to exist in bulk water at ambient conditions. We analyze new small-angle X-ray scattering data on ambient liquid water taken at third generation synchrotron sources, and large 32,000 water molecule simulations using the TIP4P-Ew model of water, to show that the small-angle region measures standard number density fluctuations consistent with water's isothermal compressibility temperature trends. Our study shows that there is no support or need for heterogeneities in water structure at room temperature to explain the small-angle scattering data, as it is consistent with a unimodal density of the tetrahedral liquid at ambient conditions. anomalous scattering | density distribution | isothermal compressibility | structural polyamorphism W ater appears to be a unique liquid relative to other fluids in exhibiting several anomalous features of structure, thermodynamics, and dynamics. Both experiment and molecular dynamics simulation have shown that water in the metastable supercooled region exhibits response functions and transport properties that appear to diverge near −45°C (1). Below its glass transition temperature, experimental evidence shows that there are polyamorphic states of water, in particular the formation of low-density amorphous and high-density amorphous glasses (2, 3). This polyamorphism has been promoted as a means for understanding the anomalous thermodynamics and dynamics of water, such as the large increase in its isothermal compressibility with decreasing temperature, when extrapolated into the experimentally inaccessible supercooled liquid region (3, 4). Polyamorphism underlies one of the main assumptions of the second critical point hypothesis (5), i.e., the postulated existence of a liquid-liquid critical point from which a dividing line separates two fluctuating species of high and low-density liquids (HDL and LDL).However, recent studies have shown that while there may be low-temperature criticality in the ST2 water model, there is no need to invoke an explanation for a second critical point in water based on structural polyamorphism (6). Furthermore, it is also possible to explain the existing anomalies without invoking an additional thermodynamic singularity, in the so-called singularityfree interpretation (4, 7). Sastry and coworkers have shown that a finite increase in isothermal compressibility upon lowering the temperature of a liquid that expands upon cooling (like water) is a thermodynamic necessity, and was illustrated for a water-like lattice model that has no singularities (7). This diversity in ...

Microscopic properties of liquid water from combined ab initio molecular dynamics and energy decomposition studies

“…It has been suggested that a large fraction of molecules form only two strong HBs: one acceptor and one donor bond 2,152-156 . However, the"rings and chains" structure of liquid water 2 , as well as the inhomogeneous two-state model 152,157 implied by such an interpretation, have been challenged on many fronts 143,147,[158][159][160][161][162][163][164][165][166][167] and are a matter of an ongoing debate 80,143,147,[155][156][157][158][159][160][161][162][163][164][166][167][168][169][170][171][172] .…”

Crystal nucleation as the ordering of multiple order parameters

“…Also glassy water has been found to exist in different states, called low-density (LDA), high-density (HDA) and very high-density (VHDA) [186] amorphous ices, which can interconvert with each other by the application of pressure. But also at ordinary conditions, many physical quantities exhibit behaviours suggestive of the presence of different states, such as infrared and Raman spectra [187,188], X-ray absorption spectroscopy, X-ray emission spectroscopy and X-ray small angle scattering [189,190] (but these results are still debated [191,192]), and simulations [193,194]. These phenomena directly point to the ability of the water molecule to sustain different local environments [195,196].…”

High density amorphous ice at room temperature

“…Neutron diffraction experiments and classical MD simulations (12,30) provide evidence for the existence of two different liquids, LDW and HDW. This finding was later explained in terms of density fluctuation in liquid rather than a first-order phase transition in liquid (31). Under rapid "jet" cooling, water transforms into LDA, an amorphous solid, rather than ice Ih at the cooling rate of 10 6 K∕s and at T < 140 K. Under low-temperature compression (below 150 K) LDA or ice Ih transforms to HDA, another amorphous solid (4-6).…”