Volume of supercooled water under pressure and the liquid-liquid critical point

The volume of water (H(2)O) was obtained at about 200-275 K and 40-400 MPa by using emulsified water. The plot of volume against temperature showed slightly concave-downward curvature at pressures higher than ≈200 MPa. This is compatible with the liquid-liquid critical-point hypothesis, but hardly with the singularity-free scenario. When the critical point is assumed to exist at ≈50 MPa and ≈223 K, the experimental volume and the derived compressibility are qualitatively described by the modified Fuentevilla-Anisimov scaling equation.     

Collective dynamics of supercooled water close to the liquid-liquid coexistence lines

We report molecular dynamics simulation results for the collective dynamical properties of supercooled bulk water at 180 K at three different densities, corresponding to different phases whose coexistence has recently been discovered in the supercooled regime. In this study, we focus on the behaviour of the longitudinal and transverse current correlation functions and their relative spectra, which we analyze in detail to understand the dynamical processes responsible for the main features observed. Despite the considerable differences in the structure and densities of the three thermodynamic states considered, the obtained current correlation functions show rather similar behaviour in every case. We show that the longitudinal spectra can only be described in terms of three Lorentzian lines, while the accurate reproduction of the transverse spectra requires at least four separate spectral lines. In fact, the behaviour of the peak frequency of the modes necessary to reproduce the spectra as a function of the wavevector indicates in a clear way the nature of the dynamical process. We demonstrate the presence of collective modes associated with the propagation of both longitudinal and transverse sound along with the important contribution of "optical-like" modes, which point out the relevance of localized motions for a right interpretation of the spectral line shape. The wavevector dependence of the relative contributions of the various modes to the total spectral area is also discussed.     

Effect of confinement on coil-globule transition

The equilibrium thermodynamic properties of a linear polymer chain confined to a space between two impenetrable walls (lines) at a distance D under various solvent conditions have been studied using series analysis and exact enumeration technique. We have calculated the end-to-end distance of polymer chain, which shows a nonmonotonic behavior with inter wall separation D. The density distribution profile shows a maxima at a particular value of (D=)D*. Around this D*, our results show that the collapse transition occurs at higher temperature as compared to its bulk value of 2d and 3d. The variation of theta-temperature with D shows a re-entrance behavior. We also calculate the force of compression exerted by the walls (lines) on the polymer.     

Fragile-to-strong liquid transition in deeply supercooled confined water

Confining water in lab synthesized nanoporous silica matrices MCM-41-S with pore diameters of 18 and 14 A, we have been able to study the molecular dynamics of water in deeply supercooled states, down to 200 K. Using quasielastic neutron scattering and analyzing the data with the relaxing cage model, we determined the temperature variation of the average translational relaxation time and its Q-dependence. We find a clear evidence of an abrupt change of the relaxation time behavior at T approximately equal to 225 K, which we interpreted as the predicted fragile-to-strong liquid-liquid transition.     

Phosphorus: first principle simulation of a liquid-liquid phase transition

We report a Car-Parrinello molecular dynamics study of the liquid-liquid phase transition in phosphorus. We employed a gradient corrected density functional (B-LYP) to describe the electronic structure and performed simulations at constant pressure. Upon increasing pressure we observed, along the 1500 K isotherm, a structural transition converting the molecular P4 liquid into an atomic liquid with a network structure. Our calculations suggest this transition to be first order with a discontinuous density increase accompanied by an insulator into metal transition. The transition pressure is significantly higher than obtained by employing the less accurate local density functional (LDA) [Morishita, Phys. Rev. Lett. 87, 105701 (2001)], which matches the experimental value for the pressure. We argue why the LDA result should be considered fortuitous. The change of the calculated structure factor upon the transition shows the same trend as experimentally observed. Analysis of the structural changes during the phase transition revealed that a chain of linked and opened up ("butterfly") P4 molecules may serve as a seed triggering the transition from the molecular to the network phase.     

Communication: probable scenario of the liquid-liquid phase transition of SnI4

We have shown from in situ synchrotron x-ray diffraction measurements that there are two thermodynamically stable liquid forms of SnI(4), depending on the pressure. Based on the liquid-liquid critical point scenario, our recent measurements suggest that the second critical point, if it exists, may be located in a region close to the point at which the melting curve of the crystalline phase abruptly breaks. This region is, unlike that of water, experimentally accessible with relative ease.     

The putative liquid-liquid transition is a liquid-solid transition in atomistic models of water

We use numerical simulation to examine the possibility of a reversible liquid-liquid transition in supercooled water and related systems. In particular, for two atomistic models of water, we have computed free energies as functions of multiple order parameters, where one is density and another distinguishes crystal from liquid. For a range of temperatures and pressures, separate free energy basins for liquid and crystal are found, conditions of phase coexistence between these phases are demonstrated, and time scales for equilibration are determined. We find that at no range of temperatures and pressures is there more than a single liquid basin, even at conditions where amorphous behavior is unstable with respect to the crystal. We find a similar result for a related model of silicon. This result excludes the possibility of the proposed liquid-liquid critical point for the models we have studied. Further, we argue that behaviors others have attributed to a liquid-liquid transition in water and related systems are in fact reflections of transitions between liquid and crystal.     

Liquid-vapor and liquid-liquid phase equilibria of the Brodholt-Sampoli-Vallauri polarizable water model

Liquid-vapor and liquid-liquid phase equilibria of the polarizable Brodholt-Sampoli-Vallauri water model have been investigated by Gibbs ensemble Monte Carlo computer simulations. The coexisting liquid and vapor densities and energy of vaporization of the model is found to be in a reasonable agreement with experimental data in the entire temperature range of liquid-vapor coexistence. The critical temperature and density of the model are found to be 615 K and 0.278 gcm(3), respectively, close to the experimental values of 647.1 K and 0.322 gcm(3). In the supercooled state two distinct liquid-liquid coexistence regions are observed. The existence of liquid-liquid phase separation of a polarizable water model is demonstrated for the first time.     

Effect of change of water structure on the phase transition of liposomes of dipalmitoyl phosphatidylcholine

The role of water structure around model membrane systems (e.g., liposomes) on phase transition of the lipid dipalmitoyl phosphatidylcholine was investigated. Water structure was altered by changing pH and by addition of solutes which are known breakers and makers of water structure. The structure makers broadened the zone of transition and changed the overall phase transition profile, while the main effect of structure breakers was to cause a shift in the transition temperature. The observed variation of Chapman transition temperature and broadening of zone of transition with varying pH is discussed in terms of altered water structure around the membrane–aqueous interface. Binding studies with the dye 1-anilino-8-napthalene sulfonate showed that structure makers or breakers did not bind to the liposome surface directly. Thus the structure makers and breakers act on the membranes perhaps by altering the water structure differentially. Possible associated mechanisms of action are discussed.     

Liquid-liquid phase transitions in supercooled water studied by computer simulations of various water models

Liquid-liquid and liquid-vapor coexistence regions of various water models were determined by Monte Carlo (MC) simulations of isotherms of density fluctuation-restricted systems and by Gibbs ensemble MC simulations. All studied water models show multiple liquid-liquid phase transitions in the supercooled region: we observe two transitions of the TIP4P, TIP5P, and SPCE models and three transitions of the ST2 model. The location of these phase transitions with respect to the liquid-vapor coexistence curve and the glass temperature is highly sensitive to the water model and its implementation. We suggest that the apparent thermodynamic singularity of real liquid water in the supercooled region at about 228 K is caused by an approach to the spinodal of the first (lowest density) liquid-liquid phase transition. The well-known density maximum of liquid water at 277 K is related to the second liquid-liquid phase transition, which is located at positive pressures with a critical point close to the maximum. A possible order parameter and the universality class of liquid-liquid phase transitions in one-component fluids are discussed.     

Liquid-liquid transition in supercooled water investigated by interaction with LiCl and Xe

The hypothesis that supercooled water consists of two distinct liquid phases has been explored on the basis of their ability to hydrate nonpolar (Xe) and electrolytic (LiCl) species. Xe incorporated in the bulk of amorphous solid water survives in the deeply supercooled regime above the glass-transition temperature of 136 K and is finally dehydrated at 165 K, whereas LiCl dissolves only in the liquid phase appearing above 165 K. The second liquid phase connects with normal water as inferred from high (poor) solubility of LiCl (Xe). This result also suggests that decoupling of translational diffusion and viscosity in the deeply supercooled regime is caused by domain structures of the two liquid phases formed during a possible liquid-liquid transition.     

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.     

Molecular dynamics study of phase transition and nucleation in supercooled clusters of potassium iodide

In a series of molecular dynamics (MD) runs on (KI)₁₀₈ clusters, the Born–Mayer–Huggins potential function is employed to study structural, energetic, and kinetic aspects of phase change and the homogeneous nucleation of KI clusters. Melting and freezing are reproducible when clusters are heated and cooled. The melted clusters are not spherical in shape no matter the starting cluster is cubic or spherical. Quenching a melted (KI)₁₀₈ cluster from 960 K in a bath with temperature range 200–400 K for a time period of 80 ps both nucleation and crystallization are observed. Nucleation rates exceeding 10³⁶ critical nuclei m⁻³ s⁻¹ are determined at 200, 250, 300, 350, and 400 K. Results are interpreted in terms of the classical theory of nucleation of Turnbull and Fisher and of Buckle. Interfacial free energies of the liquid–solid phase derived from the nucleation rates are 7–10 mJ m⁻². This quantity is 0.19 of the heat of transition per unit area from solid to liquid, or about two-thirds of the corresponding ratio which Turnbull proposed for freezing transition. The temperature dependence of σ_sl(T) of (KI)₁₀₈ clusters can be expressed as σ_sl(T)∝T^0.34.     

Dynamics near a liquid-liquid phase transition in a non-tetrahedral liquid: the case of gallium

We use molecular simulation to analyze liquid dynamics in the vicinity of the liquid-liquid phase transition (LLPT) recently discovered in the modified embedded-atom model for elemental gallium. For this purpose we analyze the diffusive behavior in terms of the mean-squared displacement and self-intermediate scattering functions for two systems obtained by cooling the stable liquid through the LLPT at different cooling rates. The results show a pronounced heterogeneity of the dynamics upon the onset of the LLPT. Furthermore, it is found that this heterogeneity is closely correlated to the structural properties of the 9-fold coordinated high-density and 8-fold coordinated low-density liquid forms involved in the transition, showing a mixture of domains with very different diffusion time scales. The dynamics of the low-density liquid is found to be much more sluggish than that of the high-density form. Analysis of the energetics suggests that the origin of this difference is rooted in the fact that the cohesion in the former is significantly stronger than that in the latter.     

Effect of molecular structure on the crystal-nematic phase transition

A systematic study of the crystal-nematic phase transition for a series of unsymmetricalp-phenylene bis (p-substituted benzoates) is reported. Differences in molecular structure along the series are in the nature of one small, compact terminal substituent. Enthalpy and entropy measurements are reported for the crystal-nematic transition as obtained by differential thermal analysis. A correlation between the thermodynamic data and molecular structure is presented.

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Zusammenfassung Eine systematische Untersuchung über den kristallin-nematischen Phasenübergang einer Reihe asymmetrischerp-Phenylen-bis-(p-substituierter Benzoate) wird bekanntgegeben. Die Unterschiede der Molekularstruktur der Serienglieder bestehen in der Beschaffenheit eines kleinen, kompakten, terminalen Substituenten. Die mittels Differentialthermoanalyse erhaltenen Enthalpie- und Entropiemessungen des kristallin-nematischen Überganges werden beschrieben. Eine Korrelation zwischen den thermodynamischen Daten und der Molekularstruktur wird gegeben.

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Résumé On communique les résultats d'une étude systématique de la transition de phase entre l'état cristallin et l'état nématique d'une série de benzoatesp-phénylène bisp-substitués asymétriques. Les différences de structure moléculaire des membres de la série sont dues à la nature d'un substituant terminal compact de faible dimension. On décrit les mesures d'enthalpie et d'entropie de la transition entre l'état cristallin et l'état nématique, effectuées par ATD. On présente une corrélation entre les données thermodynamiques et la structure moléculaire.

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The freezing of supercooled water

The onset of crystallization of supercooled water upon lowering the temperature is highly unpredictable, depending strongly on the specific sample and its treatment. The mechanism causing this is to be investigated and may be found in terms of the dependence of the transition temperature on the shear acting in the convecting liquid. This effect of shear on the fluctuation spectrum is considered qualitatively.     

Influence of alcohols on the crystallization of supercooled water

T. G. Shevchenko Kiev State University. Translated from Zhurnal Strukturnoi Khimii, Vol. 31, No. 3, pp. 141–143, May–June, 1990.     

Structural study of supercooled liquid transition metals

Local structural models for supercooled liquid transition metals (Ti, Ni, and Zr) are proposed based on a reverse Monte Carlo analysis of high-energy x-ray diffraction data taken from the electrostatically levitated liquids. The resulting structures are characterized by their bond angle distributions, Honeycutt and Andersen indices [J. Phys. Chem. 91, 4950 (1987)], and bond orientational order parameters. All analyses suggest that an icosahedral short-range order is present in these supercooled liquids, but it is distorted in liquid Ti. These results are in agreement with the observed evolution of the high-q shoulder on the second peak in the structure factor S(q) and with an earlier analysis based on a local cluster model.     

Self-diffusion of supercooled o-terphenyl near the glass transition temperature

Self-diffusion coefficients for the low molecular weight glass former o-terphenyl have been measured near Tg by isothermally desorbing thin film bilayers of deuterio and protio o-terphenyl in a vacuum chamber. We observe translational diffusion that is about 100 times faster at Tg + 3 K than the Stokes-Einstein prediction. Predictions from random first order transition theory and a dynamic facilitation approach are in reasonable agreement with our results; in these approaches, enhanced translational diffusion is associated with spatially heterogeneous dynamics. Self-diffusion controls crystallization in o-terphenyl for most of the supercooled liquid regime, but at temperatures below Tg + 10 K, the reported crystallization rate increases suddenly while the self-diffusion coefficient does not. This work and previous work on trisnaphthylbenzene both find a self-diffusion-controlled crystal growth regime and an enhancement in self-diffusion near Tg, suggesting that these phenomena are general characteristics of fragile low molecular weight glass formers. We discuss the width of the relaxation time distributions of o-terphenyl and trisnaphthylbenzene as they relate to the observation of enhanced translational diffusion.