A comprehensive scenario of the thermodynamic anomalies of water using the TIP4P/2005 model

Abstract

The striking behavior of water has deserved it to be referred to as an "anomalous" liquid. The water anomalies are greatly amplified in metastable (supercooled and/or stretched) regions. This makes difficult a complete experimental description since, beyond certain limits, the metastable phase necessarily transforms into the stable one. Theoretical interpretation of the water anomalies could then be based on simulation results of well validated water models. But the analysis of the simulations has not yet reached a consensus. In particular, one of the most popular theoretical scenarios-involving the existence of a liquid-liquid critical point (LLCP)-is disputed by several authors. In this work, we propose to use a number of exact thermodynamic relations which may shed light on this issue. Interestingly, these relations may be tested in a region of the phase diagram which is outside the LLCP thus avoiding the problems associated to the coexistence region. The central property connected to other water anomalies is the locus of temperatures at which the density along isobars attain a maximum (TMD line) or a minimum (TmD). We have performed computer simulations to evaluate the TMD and TmD for a successful water model, namely, TIP4P/2005. We have also evaluated the vapor-liquid (VL) spinodal in the region of large negative pressures. The shape of these curves and their connection to the extrema of some response functions, in particular the isothermal compressibility and heat capacity at constant pressure, provides very useful information which may help to elucidate the validity of the theoretical proposals. In this way, we are able to present for the first time a comprehensive scenario of the thermodynamic water anomalies for TIP4P/2005 and their relation to the vapor-liquid spinodal. The overall picture shows a remarkable similarity with the corresponding one for the ST2 water model, for which the existence of a LLCP has been demonstrated in recent years. It also provides a hint as to where the long-sought for extrema in response functions might become accessible to experiments.

Two-structure thermodynamics for the TIP4P/2005 model of water covering supercooled and deeply stretched regions

“…42. We carried out a least-squares fit to these data, and assigned to S 1 and S 2 exactly the values derived from this least-squares fit.…”

“…The second set of fitted data corresponds to Ref. 42. These simulations from the Madrid group were performed along isobars and isotherms, covering pressures from 170 to 300 MPa and temperatures from 195 to 320 K. Data along isobars and isotherms are displayed in Fig.…”

“…1,37,38 However, the debates on the behavior of the stability limits in doubly metastable water are far from over, especially in view of a "critical-point-free" scenario, the possibility of continuation of the first-order LLPT down to the absolute stability of the liquid state. 9,35,39,41 In the present work, we have applied two-structure thermodynamics to the description of recently published 17,42,43 and new, previously unpublished extensive simulation data from the Princeton group on the thermodynamic properties of the TIP4P/2005 classical water model over a wide range of temperatures and pressures. TIP4P/2005 is one of the best available models of water, and, in particular, it reproduces well the thermodynamic anomalies of real water at low temperatures, 44 and the sound velocity in stretched water.…”

Compressibility Anomalies in Stretched Water and Their Interplay with Density Anomalies

“…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 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.…”

Communication: Re-entrant limits of stability of the liquid phase and the Speedy scenario in colloidal model systems

“…In addition, simulations based on classical models of water do not show any retracing. 30,32,33 Thus, with the exception of lattice models of water-like fluids, [34][35][36] where the spinodal was found to retrace but only at negative pressures, Speedy's hypothesis remains a fascinating scenario that has never been realized, not even in silico. In this communication we fill this gap, reporting two examples of a one-component system exhibiting re-entrant limit of stability of the liquid phase, covering both the cases presented in Fig.…”