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.