Thermodynamic response functions and Stokes-Einstein breakdown in superheated water under gigapascal pressure

Liquid water is the most intriguing liquid in nature, both because of its importance to every known form of life, and its numerous anomalous properties, largely magnified under supercooled conditions. Among the anomalous properties of water is the seeming divergence of the thermodynamic response functions and dynamic properties below the homogenous nucleation temperature (~ 232 K). Furthermore, water exhibits an increasingly decoupling of the viscosity and diffusion, upon cooling, resulting in the breakdown of the Stokes-Einstein relationship (SER). At high temperatures and pressures, however, water behaves more like a “simple” liquid. Nonetheless, experiments at 400 K and GPa pressures (Bove et al. (2011) Phys. Rev. Lett., 111:185,901) showed that although the diffusion decreases monotonically with the pressure, opposite to pressurized supercooled water, a decoupling of the viscosity and diffusion, larger than that found in supercooled water at normal pressure, is observed. Here, we studied the validity of SER and different pressure-dependent thermodynamic response functions, known to exhibit an abnormal behavior upon cooling, including the density, isothermal compressibility, and the thermal expansion coefficient along the 400 K isotherm up to 3 GPa through molecular dynamics simulations. Seven different water models were investigated. A monotonic increase of the density (~ 50%) and decrease of the isothermal compressibility (~ 90%) and thermal expansion (~ 65%) is found. Our results also show that compressed hot water has various resemblances to cool water at normal pressure, with pressure inducing the formation of a new second coordination sphere and a monotonic decrease of the diffusion and viscosity coefficients. Whereas all water models provide a good account of the viscosity, the magnitude of the violation of the SER at high pressures (> ~ 1 GPa) is significantly smaller than that found through experiments. Thus, violation of the SER in simulations is comparable to that observed for liquid supercooled water, indicating possible limitations of the water models to account for the local structure and self-diffusion of superheated water above ~ 1 GPa.