Parametric crossover model and physical limit of stability in supercooled water

Minimal Microscopic Model for Liquid Polyamorphism and Waterlike Anomalies

Liquid polyamorphism is the intriguing possibility for a single component substance to exist in multiple liquid phases. We propose a minimal model for this phenomenon. Starting with a binary lattice model with critical azeotropy and liquid-liquid demixing, we allow interconversion of the two species, turning the system into a single-component fluid with two states differing in energy and entropy. Unveiling the phase diagram of the noninterconverting binary mixture gives unprecedented insight on the phase behaviors accessible to the interconverting fluid, such as a liquid-liquid transition with a critical point, or a singularity-free scenario, exhibiting thermodynamic anomalies without polyamorphism. The model provides a unified theoretical framework to describe supercooled water and a variety of polyamorphic liquids with waterlike anomalies.

Breakdown of the Stokes-Einstein relation in supercooled water: the jump-diffusion perspective

Although water is the most ubiquitous liquid it shows many thermodynamic and dynamic anomalies. Some of the anomalies further intensify in the supercooled regime. While many experimental and theoretical studies have focused on the thermodynamic anomalies of supercooled water, fewer studies explored the dynamical anomalies very extensively. This is due to the intricacy of the experimental measurement of the dynamical properties of supercooled water. Violation of the Stokes-Einstein relation (SER), an important relation connecting the diffusion of particles with the viscosity of the medium, is one of the major dynamical anomalies. In absence of experimentally measured viscosity, researchers used to check the validity of SER indirectly using average translational relaxation time or α-relaxation time. Very recently, the viscosity of supercooled water was accurately measured at a wide range of temperatures and pressures. This allowed direct verification of the SER at different temperature-pressure thermodynamic state points. An increasing breakdown of the SER was observed with decreasing temperature. Increasing pressure reduces the extent of breakdown. Although some well-known theories explained the above breakdown, a detailed molecular mechanism was still elusive. Recently, a translational jump-diffusion (TJD) approach has been able to quantitatively explain the breakdown of the SER in pure supercooled water and an aqueous solution of methanol. The objective of this article is to present a detailed and state-of-the-art analysis of the past and present works on the breakdown of SER in supercooled water with a specific focus on the new TJD approach for explaining the breakdown of the SER.

Water: A Tale of Two Liquids

Water is the most abundant liquid on earth and also the substance with the largest number of anomalies in its properties. It is a prerequisite for life and as such a most important subject of current research in chemical physics and physical chemistry. In spite of its simplicity as a liquid, it has an enormously rich phase diagram where different types of ices, amorphous phases, and anomalies disclose a path that points to unique thermodynamics of its supercooled liquid state that still hides many unraveled secrets. In this review we describe the behavior of water in the regime from ambient conditions to the deeply supercooled region. The review describes simulations and experiments on this anomalous liquid. Several scenarios have been proposed to explain the anomalous properties that become strongly enhanced in the supercooled region. Among those, the second critical-point scenario has been investigated extensively, and at present most experimental evidence point to this scenario. Starting from very low temperatures, a coexistence line between a high-density amorphous phase and a low-density amorphous phase would continue in a coexistence line between a high-density and a low-density liquid phase terminating in a liquid–liquid critical point, LLCP. On approaching this LLCP from the one-phase region, a crossover in thermodynamics and dynamics can be found. This is discussed based on a picture of a temperature-dependent balance between a high-density liquid and a low-density liquid favored by, respectively, entropy and enthalpy, leading to a consistent picture of the thermodynamics of bulk water. Ice nucleation is also discussed, since this is what severely impedes experimental investigation of the vicinity of the proposed LLCP. Experimental investigation of stretched water, i.e., water at negative pressure, gives access to a different regime of the complex water diagram. Different ways to inhibit crystallization through confinement and aqueous solutions are discussed through results from experiments and simulations using the most sophisticated and advanced techniques. These findings represent tiles of a global picture that still needs to be completed. Some of the possible experimental lines of research that are essential to complete this picture are explored.

Connecting the Water Phase Diagram to the Metastable Domain: High-Pressure Studies in the Supercooled Regime

Pressure is extremely efficient to tune intermolecular interactions, allowing the study of the mechanisms regulating, at the molecular level, the structure and dynamics of condensed phases. Among the simplest molecules, water represents in many respects a mystery despite its primary role in ruling most of the biological, physical, and chemical processes occurring in nature. Here we report a careful characterization of the dynamic regime change associated with low-density and high-density forms of liquid water by measuring the line shape of the OD stretching mode of HOD in liquid water along different isotherms as a function of pressure. Remarkably, the high-pressure studies have been here extended down to 240 K, well inside the supercooled regime. Supported by molecular dynamics simulations, a correlation among amorphous and crystalline solids and the two different liquid water forms is attempted to provide a unified picture of the metastable and thermodynamic regimes of water.

Thermodynamics of supercooled water

We review the available experimental information on the thermodynamic properties of supercooled water and demonstrate the possibility of modeling these thermodynamic properties on a theoretical basis. We show that by assuming the existence of a liquid-liquid critical point in supercooled water, the theory of critical phenomena can give an accurate account of the experimental thermodynamic-property data up to a pressure of 150 MPa. In addition, we show that a phenomenological extension of the theoretical model can account for all currently available experimental data in the supercooled region, up to 400 MPa. The stability limit of the liquid state and possible coupling between crystallization and liquid-liquid separation are also discussed. It is concluded that critical-point thermodynamics describes the available thermodynamic data for supercooled water within experimental accuracy, thus establishing a benchmark for further developments in this area.

Correspondence between phase diagrams of the TIP5P water model and a spherically symmetric repulsive ramp potential with two characteristic length scales

We perform molecular dynamics simulations of the TIP5P water model and derive the physical parameters for a simple two-scale repulsive ramp potential model. We find that the regions of anomalous behavior in the phase diagrams of both systems can be mapped onto each other if (i) pressure P and temperature T are replaced by T-T(C) and P-P(C), respectively, where (T(C),P(C)) are the coordinates of the liquid-liquid critical point of the corresponding system; and (ii) a single ramp particle corresponds effectively to two TIP5P molecules. We present heuristic arguments supporting point (ii). We also argue that the waterlike anomalies in the ramp potential are due to the ability of the particles to reproduce, upon compression or heating, the migration of water molecules from the second shell to its first shell.

Four phases of amorphous water: Simulations versus experiment

Multiplicity of the liquid-liquid phase transitions in supercooled water, first obtained in computer simulations [Brovchenko et al., J. Chem. Phys. 118, 9473 (2003)], has got strong support from the recent experimental observation of the two phase transitions between amorphous ices [Loerting et al., Phys. Rev. Lett. 96, 025702 (2006)]. These experimental results allow assignment of the four amorphous water phases (I-IV) obtained in simulations to the three kinds of amorphous ices. Water phase I (rho approximately 0.90 gcm(3)) corresponds to the low-density amorphous ice, phase III (rho approximately 1.10 gcm(3)) to the high-density amorphous ice, and phase IV (rho approximately 1.20 gcm(3)) to the very-high-density amorphous ice. Phase II of model water with density rho approximately 1.00 gcm(3) corresponds to the normal-density water. Such assignment is confirmed by the comparison of the structural functions of the amorphous phases of model water and real water. In phases I and II the first and second coordination shells are clearly divided. Phase I consists mainly of the four coordinated tetrahedrally ordered water molecules. Phase II is enriched with molecules, which have tetrahedrally ordered four nearest neighbors and up six molecules in the first coordination shell. Majority of the molecules in phase III still have tetrahedrally ordered four nearest neighbors. Transition from phase III to phase IV is characterized by a noticeable drop of tetrahedral order, and phase IV consists mainly of molecules with highly isotropic angular distribution of the nearest neighbors. Relation between the structures of amorphous water phases, crystalline ices, and liquid water is discussed.

Studying thermodynamics of metastable states

Simple classical thermodynamic approach to the general description of metastable states is presented. It makes it possible to calculate the explicit dependence of the Gibbs free energy on temperature, to calculate the heat capacity, the thermodynamic barrier, dividing metastable and more stable states, and the thermal expansion coefficient. Thermodynamic stability under mechanical loading is considered. The influence of the heating (cooling) rate on the measured dynamic heat capacity is investigated. A phase shift of the temperature oscillations of an ac heated sample is shown to be determined by the relaxation time of the relaxation of the metastable nonequilibrium state back to the metastable equilibrium one. This dependence allows one to calculate the relaxation time. A general description of the metastable phase equilibrium is proposed. Metastable states in AB3 alloys are considered. Reasons for the change from the diffusional mechanism of the supercritical nucleus growth to the martensitic one as the heating rate increases are discussed. The Ostwald stage rule is derived.

Nucleation rates of water and heavy water using equations of state

The original formula of Gibbs for the reversible work of critical nucleus formation is evaluated in three approximate ways for ordinary and heavy water. The least approximate way employs an equation of state to evaluate the pressure difference between the new and old phases. This form of the theory yields a temperature dependence for the nucleation rate close to that observed experimentally. This is a substantial improvement over the most commonly used (and most approximate) form of classical theory.