Compressibility Anomalies in Stretched Water and Their Interplay with Density Anomalies

Abstract

Water keeps puzzling scientists because of its numerous properties which behave oppositely to usual liquids: for instance, water expands upon cooling, and liquid water is denser than ice. To explain this anomalous behaviour, several theories have been proposed, with different predictions for the properties of supercooled water (liquid at conditions where ice is more stable). However, discriminating between those theories with experiments has remained elusive because of spontaneous ice nucleation. Here we measure the sound velocity in liquid water stretched to negative pressure, and derive an experimental equation of state, which reveals compressibility anomalies. We show by rigorous thermodynamic relations how these anomalies are intricately linked with the density anomaly. Some features we observe are necessary conditions for the validity of two theories of water.Liquid water exhibits numerous anomalies and different scenarios have been proposed to explain them (1). In particular, the existence of a line of density maxima along isobars has been related to putative maxima in compressibility (2) and heat capacity (3). These maxima may arise from an intriguing phase separation of water in two distinct liquids (1,4), although they can also be explained without resorting to such a phase separation (2). However, the compressibility and heat capacity maxima, whose existence is predicted by molecular dynamics simulations (3,5), have hitherto not been observed in experiments. Alternative theoretical scenarios, namely 1 arXiv:1708.00063v1 [physics.chem-ph] 31 Jul 2017 the stability limit conjecture (6) and the critical-point-free scenario (7, 8), do not require the existence of compressibility and heat capacity maxima, but rather predict a divergence of these quantities at low temperature.Another type of anomaly, namely a minimum in sound velocity along one isochore, was recently discovered at negative pressure (9,10). At negative pressure, the liquid is mechanically stretched, in a state metastable with respect to vapor. To date, the only method able to reach significantly negative pressures (beyond −100 MPa) uses 3 − 10 µm fluid inclusions (FIs) of water in a quartz crystal, and stretching is obtained by cooling liquid water at nearly constant volume (11,12). In our previous work (9, 10), we could only study two FIs along different isochores, and only one clearly showed a minimum in sound velocity. In the present work, we have measured more FIs showing a sound velocity minimum, and reached more negative pressures. We have thus established a more accurate experimental equation of state (EoS) down to −137 MPa. The new EoS strongly supports the existence of the compressibility maxima predicted by some theories of water. Furthermore, we establish new thermodynamic relations between the line of density maxima and the sound velocity anomalies. In contrast to previous works, this provides a relation between quantities that are directly observable in experiments. The corresponding lines of extrema obtained from our experimenta...

Comment on “Maxima in the thermodynamic response and correlation functions of deeply supercooled water”

“…We conclude by mentioning the first experimental report (8) of the long-sought line of maxima in k T along isobars, overlooked by Kim et al It was proposed (9) that such a line, possibly hidden by the line of homogeneous ice nucleation at positive pressure, might emerge in the experimentally accessible region at negative pressure. Early experiments in this region (9) found a minimum in sound velocity along an isochore, and a first equation of state for stretched water was obtained from interpolation of sound velocity (10).…”

“…However, because of the limited amount of data, different interpolations of similar quality yielded curves for k T at negative pressure either monotonic or with maxima (10). Recently, more sound velocity minima were measured in samples at other densities, which further constrained the possible interpolations: They all consistently lead to a line of maxima in k T along isobars around -100 MPa and 265 K (8). We emphasize that although a k T maximum is a necessary condition for the validity of both the second critical point scenario (11) and the singularity-free interpretation (12), it is not sufficient to allow a decision between them.…”

Viscosity and self-diffusion of supercooled and stretched water from molecular dynamics simulations

“…The non-ideality of the solution drives a first-order phase transition between two distinct liquids at low temperature, ending at a liquid-liquid critical point (LLCP) at T c = 228.2 K and P c = 0 MPa. We emphasize that there is currently no firmly established experimental proof of such a liquid-liquid transition and LLCP for real water, the main reason being that, in experiments, ice nucleates before reaching the putative two-phase region 13 . Nevertheless, the HSA model achieves a fit within experimental error of a comprehensive data set of thermodynamic properties (density, isothermal compressibility, thermal expansion coefficient, isobaric heat capacity, and speed of sound) in the range 200 to 310 K and 0.1 to 400 MPa.…”

“…Stretched water, or water at negative pressure, has also been studied, although less extensively (see Ref. 13 for a review). The temperature of maximum density increases from 4 • C at ambient pressure to 18 • C at −137 MPa, and a maximum in the isothermal compressibility of water along isobars has been revealed around −100 MPa and below 276 K 14 .…”

Hydrogen bond correlated percolation in a supercooled water monolayer as a hallmark of the critical region

“…The LLCP hypothesis since then has been corroborated by a series of theoretical and experimental results [39,40,13,41,42,43,44,45,46,18,19,47,48,26], although different scenarios are still under debate [49,50,10,13,51]. One of them [51,16] recently was invalidated due to conceptual errors at its origin [29,30], while another [49] was found in a colloidal model [24], but does not seem to be compatible with recent experimental data for water [36,38].…”