Thermodynamic Anomalies in Stretched Water.

Via molecular dynamics simulations of the TIP4P/2005 water model, we study liquid water's anomalous behavior at large negative pressure produced through isochoric cooling. We find that isochores without a pressure minimum can display "reentrant" behavior whereby a system that cavitates upon cooling can then rehomogenize upon further cooling. This behavior is a consequence of the underlying density maximum along the spinodal, but its actual manifestation in simulations is strongly influenced by finite size effects. These observations suggest that water under strong hydrophilic confinement may display richer phase behavior than hitherto assumed. This also suggests that propensity toward cavitation does not always correlate with greater tension, contrary to the prevailing assumption for interpreting water stretching experiments. We also show that a maximum spinodal density in water results in a locus of maximum compressibility and a minimum speed of sound that are independent from any influence of a liquid-liquid critical point (LLCP). However, we demonstrate that structural signatures of a Widom line, which likely emanates from an LLCP at elevated pressure, extend to large negative pressure, but such signatures are only observed upon sampling water's underlying potential energy landscape, rather than the thermalized metastable liquid.