Equation of state for water and its line of density maxima down to -120 MPa.

As water is involved in countless natural and industrial processes, its thermodynamic properties have been measured in a wide temperature and pressure range. Data on supercooled water are also available down to -73 °C and up to 400 MPa. In contrast, data at negative pressures are extremely scarce. Here we provide an experimental equation of state for water down to -120 MPa. In particular, we obtain the line of density maxima (LDM) of water down to a pressure six times more negative than previously available. As temperature increases from 4 up to 18 °C, the pressure PLDM(T) along the LDM decreases monotonically from 0 down to -120 MPa, while the slope dPLDM/dT becomes more negative. The experimental results are compared with molecular dynamic simulations of TIP4P/2005 water and a two-state model. We also discuss the possibility to observe extrema in compressibility and heat capacity at negative pressures, features that have remained elusive at positive pressures.

Brillouin spectroscopy of fluid inclusions proposed as a paleothermometer for subsurface rocks.

As widespread, continuous instrumental Earth surface air temperature records are available only for the last hundred fifty years, indirect reconstructions of past temperatures are obtained by analyzing "proxies". Fluid inclusions (FIs) present in virtually all rock minerals including exogenous rocks are routinely used to constrain formation temperature of crystals. The method relies on the presence of a vapour bubble in the FI. However, measurements are sometimes biased by surface tension effects. They are even impossible when the bubble is absent (monophasic FI) for kinetic or thermodynamic reasons. These limitations are common for surface or subsurface rocks. Here we use FIs in hydrothermal or geodic quartz crystals to demonstrate the potential of Brillouin spectroscopy in determining the formation temperature of monophasic FIs without the need for a bubble. Hence, this novel method offers a promising way to overcome the above limitations.

Anomalies in bulk supercooled water at negative pressure.

Water anomalies still defy explanation. In the supercooled liquid, many quantities, for example heat capacity and isothermal compressibility κT, show a large increase. The question arises if these quantities diverge, or if they go through a maximum. The answer is key to our understanding of water anomalies. However, it has remained elusive in experiments because crystallization always occurred before any extremum is reached. Here we report measurements of the sound velocity of water in a scarcely explored region of the phase diagram, where water is both supercooled and at negative pressure. We find several anomalies: maxima in the adiabatic compressibility and nonmonotonic density dependence of the sound velocity, in contrast with a standard extrapolation of the equation of state. This is reminiscent of the behavior of supercritical fluids. To support this interpretation, we have performed simulations with the 2005 revision of the transferable interaction potential with four points. Simulations and experiments are in near-quantitative agreement, suggesting the existence of a line of maxima in κT (LMκT). This LMκT could either be the thermodynamic consequence of the line of density maxima of water [Sastry S, Debenedetti PG, Sciortino F, Stanley HE (1996) Phys Rev E 53:6144-6154], or emanate from a critical point terminating a liquid-liquid transition [Sciortino F, Poole PH, Essmann U, Stanley HE (1997) Phys Rev E 55:727-737]. At positive pressure, the LMκT has escaped observation because it lies in the "no man's land" beyond the homogeneous crystallization line. We propose that the LMκT emerges from the no man's land at negative pressure.