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

Abstract

One of the most promising frameworks for understanding the anomalies of cold and supercooled water postulates the existence of two competing, interconvertible local structures. If the non-ideality in the Gibbs energy of mixing overcomes the ideal entropy of mixing of these two structures, a liquid-liquid phase transition, terminated at a liquid-liquid critical point, is predicted. Various versions of the "twostructure equation of state" (TSEOS) based on this concept have shown remarkable agreement with both experimental data for metastable, deeply supercooled water and simulations of molecular water models. However, existing TSEOSs were not designed to describe the negative pressure region and do not account for the stability limit of the liquid state with respect to the vapor. While experimental data on supercooled water at negative pressures may shed additional light on the source of the anomalies of water, such data are very limited. To fill this gap, we have analyzed simulation results for TIP4P/2005, one of the most accurate classical water models available. We have used recently published simulation data, and performed additional simulations, over a broad range of positive and negative pressures, from ambient temperature to deeply supercooled conditions. We show that, by explicitly incorporating the liquid-vapor spinodal into a TSEOS, we are able to match the simulation data for TIP4P/2005 with remarkable accuracy. In particular, this equation of state quantitatively reproduces the lines of extrema in density, isothermal compressibility, and isobaric heat capacity. Contrary to an explanation of the thermodynamic anomalies of water based on a "retracing spinodal," the liquid-vapor spinodal in the present TSEOS continues monotonically to lower pressures upon cooling, influencing but not giving rise to density extrema and other thermodynamic anomalies. Published by AIP Publishing.

Supercooled and glassy water: Metastable liquid(s), amorphous solid(s), and a no-man’s land

“…Their extended model estimate for the LLCP locus is 0.057 GPa and 214 K (56). Mean-field free energies based on the idea of a chemical equilibrium between two main water classes, supported by the previously mentioned order parameter analysis of computer simulations data, have also been shown to be compatible with the LLCP hypothesis (29,30,54).…”

“…Support for the LLCP scenario in water, in addition to the numerical studies of several water (30,(49)(50)(51)(52)(53)(54) and water-like (55) potentials, comes from the work of Holten et al (56). They presented an equation of state, built under the assumption of the presence of a critical component in the free energy, compatible with all available experimental thermodynamic data.…”

A Two-State Picture of Water and the Funnel of Life

“…An alternative is to remove thermal energy by simulating deeply supercooled and pressurized water. In this case, many different water models have indicated the existence of an LLCP associated with a liquid-liquid transition [12][13][15][16]24,29,39,42,[145][146][147][148][149][150][151][152][153][154][155][156] . However, since the LLCP in the models is located at very low temperature, these simulations are challenging and require very long equilibration times.…”

“…35 . In the following, we will present and discuss evidence from different experimental techniques and theoretical simulations, which point to fluctuations between two well-defined local environments. Before embarking on this endeavor, however, we first point out that twostate behavior has been shown to be fully consistent with thermodynamics and a prerequisite for a theory that describes the anomalies of water [17][18][19][39][40][41][42][43] . Secondly, we note that the two-state picture of water has a long history [44][45][46][47][48][49] , and may be considered required to explain the properties of supercooled water 13 .…”

Thermodynamic metric geometry of the two-state ST2 model for supercooled water

“…of the Holten-Anisimov-Sengers type [31], is necessary. Based on the two-structure concept, TSEOS's were developed so far only for the mW [53], ST2 [54] and the TIP4P/2005 [44] applied for exploring the doubly metastable region, where liquid water is both supercooled and under tension. With our focus on systems with a clear evidence for an LLCP we chose the ST2-TSEOS [54] as a starting point for investigations, accepting that the potential exhibits a number of quantitative deviations from the behavior of real water [55].…”