Abstract
The thermodynamic response functions of water exhibit an anomalous increase upon cooling that becomes strongly amplified in the deeply supercooled regime due to structural fluctuations between disordered and tetrahedral local structures. Here, we compare structural data from recent x-ray laser scattering measurements of water at 1 bar and temperatures down to 227 K with structural properties computed for several different water models using molecular dynamics simulations. Based on this comparison, we critically evaluate four different thermodynamic scenarios that have been invoked to explain the unusual behavior of water. The critical point-free model predicts small variations in the tetrahedrality with decreasing temperature, followed by a stepwise change at the liquid-liquid transition around 228 K at ambient pressure. This scenario is not consistent with the experimental data that instead show a smooth and accelerated variation in structure from 320 to 227 K. Both the singularity-free model and ice coarsening hypothesis give trends that indirectly indicate an increase in tetrahedral structure with temperature that is too weak to be consistent with experiment. A model that includes an apparent divergent point (ADP) at high positive pressure, however, predicts structural development consistent with our experimental measurements. The terminology ADP, instead of the commonly used liquid-liquid critical point, is more general in that it focuses on the growing fluctuations, whether or not they result in true criticality. Extrapolating this model beyond the experimental data, we estimate that an ADP in real water may lie around 1500 ± 250 bars and 190 ± 6 K.
Highlights
Since the anomalous properties of water have been shown to be related to fluctuations into local tetrahedral structures,2,3,22,23,27,54,64 we demonstrate here that structural properties can be used to test various thermodynamic models to explain the apparent divergence of the thermodynamic response functions at ambient pressure
We introduce the concept of an apparent divergent point (ADP) meaning that there is a specific point in the water phase diagram where fluctuations between two competing local structures become enhanced in a way reminiscent of criticality and where these fluctuations extend over a large P-T neighborhood
The ADP could potentially be characterized as a liquid-liquid critical point (LLCP) if critical fluctuations could develop on sufficiently large length and sufficiently long time scales in this region where ice is the stable phase
Summary
Water is one of the most important liquids and shows unusual behavior upon varying temperature and pressure. This anomalous behavior is evident already at ambient conditions and becomes even more pronounced when water is cooled below the melting point of ice (0 ◦C or 273 K), where the liquid is metastable with respect to crystallization. Of particular interest in this context is the temperature and pressure dependence of the thermodynamic response functions that are related to fluctuations in the liquid: the isothermal compressibility (κT), the specific heat capacity at constant pressure (Cp), and the thermal expansion coefficient (αp). The temperature dependence of κT and Cp has been decomposed into that of a normal liquid background and an anomalous contribution, where the latter shows an apparent power law divergence on approaching a seemingly singular temperature of about 228 K at ambient pressure. In a similar manner, the enhancement of the correlation length κT obtained from small angle x-ray scattering has been fitted to an apparent power law divergence of similar magnitude.7Numerous scenarios have been proposed to explain the rapid increase in anomalous fluctuations in liquid water upon cooling. Water is one of the most important liquids and shows unusual behavior upon varying temperature and pressure.1–3 This anomalous behavior is evident already at ambient conditions and becomes even more pronounced when water is cooled below the melting point of ice (0 ◦C or 273 K), where the liquid is metastable with respect to crystallization.. We focus on four thermodynamically consistent scenarios that have proved challenging to scrutinize experimentally because of water’s rapid crystallization kinetics in the deeply supercooled regime. Some of these hypotheses posit the existence of a metastable liquid-liquid transition (LLT) between a high-density and a low-density liquid state of water (HDL and LDL, respectively) that terminates in a liquid-liquid critical point (LLCP) at some specific temperature (TC) and pressure (PC).. Some of these hypotheses posit the existence of a metastable liquid-liquid transition (LLT) between a high-density and a low-density liquid state of water (HDL and LDL, respectively) that terminates in a liquid-liquid critical point (LLCP) at some specific temperature (TC) and pressure (PC). According to the LLCP hypothesis, there is an LLCP at positive pressure and non-zero temperature. Along the Widom line, which is the extension of the LLT phase separation line beyond the LLCP, density fluctuations would reach a maximum in the one-phase region, consistent with equal population of molecules with HDL- and LDL-like local coordination environments. Alternatively, in the critical point-free (CPF) model, the LLCP would instead occur at negative pressure. The third scenario, the singularity-free (SF) model, posits a continuous transformation without discontinuity, which would correspond to the LLCP being located at zero temperature and high positive pressure. The fourth hypothesis, which has been the subject of recent debate, posits that the enhanced fluctuations in supercooled water arise from the familiar liquid-solid transition rather than from a metastable LLT and LLCP
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
