Relation between structural and dynamical anomalies in supercooled water

Aqueous D2O solutions of lithium (0.5, 1, and 3 M) and sodium (1 and 3 M) perchlorate have been studied by FT-IR spectroscopy in their glassy states at 80 K, and their IR spectra compared with those of 1.0 M D2O solutions recorded from 300 to 235 K. Evidence for strongly increasing contact-ion pairing in going from ambient temperature to the glassy state is based on the development of a second band in the ν1 band region centered at high frequency of the band from "free" perchlorate and complete loss of degeneracy in the ν3 and ν4 band region. On the basis of curve resolution of the ν1 band region, increasing contact-ion pairing is much more pronounced for sodium than for lithium perchlorate. The spectral features of contact-ion paired perchlorate are consistent with at most C2v symmetry with bidentate binding. Anomalous increase in contact-ion pairing on hyperquenching into the glassy state is attributed to water's density maximum and the anomalies of supercooled water and "dilute" aqueous solutions. Spectral features of ion-pairing are weakly observable already at 300 K in the ν1 and ν3 band region and become more pronounced on cooling to 235 K. Significance of the results is discussed with respect to (i) differences in local geometry of contact-ion pairs in aprotic donor solvents, (ii) vapor-deposited alkali metal perchlorates isolated in a water matrix, (iii) thermodynamic continuity of states between vapor-deposited amorphous water, hyperquenched glassy water and normal supercooled water, and (iv) cryofixation of polyelectrolyte-type biomolecules such as nucleic acid sodium salts.

Water displays a vast array of unique properties, known as water's anomalies, whose origin remains subject to hot debate. Our aim in this article is to provide a unified microscopic physical picture of water's anomalies in terms of locally favored structures, encompassing both thermodynamic and dynamic anomalies, which are often attributed to different origins. We first identify locally favored structures via a microscopic structural descriptor that measures local translational order and provide direct evidence that they have a hierarchical impact on the anomalies. At each state point, the strength of thermodynamic anomalies is directly proportional to the amount of locally favored structures, while the dynamic properties of each molecule depend on the local structure surrounding both itself and its nearest neighbors. To incorporate this, we develop a novel hierarchical two-state model. We show by extensive simulations of two popular water models that both thermodynamic and kinetic anomalies can be almost perfectly explained by the temperature and pressure dependence of these local and non-local versions of the same structural descriptor, respectively. Moreover, our scenario makes three unique predictions in supercooled water, setting it apart from other scenarios: (1) Presence of an "Arrhenius-to-Arrhenius" crossover upon cooling, as the origin of the apparent "fragile-to-strong" transition; (2) maximum of dynamic heterogeneity around 20 K below the Widom line and far above the glass transition; (3) violation of the Stokes-Einstein-Debye relation at ∼2T g, rather than 1.2T g typical of normal glass-formers. These predictions are verified by recent measurement of water's diffusion at very low temperatures (point 1) and discoveries from our extensive simulations (points 2-3). We suggest that the same scenario may generally apply to water-like anomalies in liquids tending to form locally favored structures, including not only other important tetrahedral liquids such as silicon, germanium, and silica, but also metallic and chalcogenide liquids.

Constant temperature and constant pressure molecular dynamics calculations have been performed on the extended simple point charge (SPC/E) model water to understand the effects of pressure on its structural and dynamical properties. The effect of pressure is to increase the presence of interstitial water molecules, which is shown to be responsible for the experimentally observed increase in the first x-ray diffraction peak and a broadening of the OOO∧ bond angle distribution. These structural changes lead to weaker hydrogen bonding and the observed minimum in the rotational correlation time as a function of pressure. The weak maximum in the measured translational diffusion coefficient at around 1.5 kbar was not reproduced. The temperature of the density maximum was estimated to occur at 245±4 K, which suggests that structural changes with increasing pressure and decreasing temperature are not quantitatively reproduced by the SPC/E model.

Rotational dynamics in a simulated supercooled network-forming liquid

In our previous study [Plankova et al., EPJWeb. Conf. 92 , 02071 (2015)], several molecular simulations of vapor-liquid phase interfaces for pure water were performed using the DL_POLY Classic software. The TIP4P/2005 molecular model was successfully used for the modeling of the density profile and the thickness of phase interfaces together with the temperature dependence of the surface tension. In the current study, the extended simple point charge (SPC/E) model for water was employed for the investigation of vapor-liquid phase interfaces over a wide temperature range from 250 K to 600 K. The TIP4P/2005 model was also used with the temperature step of 25 K to obtain more consistent data compared to our previous study. Results of the new simulations are in a good agreement with most of the literature data. TIP4P/2005 provides better results for the saturated liquid density with its maximum close to 275 K, while SPC/E predicts slightly better saturated vapor density. Both models give qualitatively correct representation for the surface tension of water. The maximum absolute deviation from the IAPWS standard for the surface tension of ordinary water is 10.4 mN · m−1 and 4.1 mN · m−1 over the temperature range from 275 K to 600 K in case of SPC/E and TIP4P/2005, respectively.

The anomalies of supercooled water may be explained by an underlying liquid-liquid phase transition (LLPT) between high- and low-density states. Recently, its observation at 185 K was inferred using solutions containing aqueous ionic liquids at a solute mole fraction of x=0.156 [Woutersen et al., Science 359, 1127 (2018)10.1126/science.aao7049]. We employ x-ray diffraction, calorimetry, and dilatometry on these hydrazinium trifluoroacetate solutions at x=0.00-0.40 to show that the transition at 185 K is not related to a genuine LLPT of water. Continuous densification upon compression, continuous changes of halo position, and absence of thermal signatures for a high- to low-density transition rule out the possibility of an LLPT for x≥0.13. The data show that employing sophisticated solutions adds a layer of complexity that hampers extrapolation of the LLPT concept from one- to two-component systems. The possibility of an LLPT can only be probed for pure water or sufficiently dilute aqueous solutions.

We investigate the equation of state, diffusion coefficient, and structural order of a family of spherically symmetric potentials consisting of a hard core and a linear repulsive ramp. This generic potential has two characteristic length scales: the hard and soft core diameters. The family of potentials is generated by varying their ratio, lambda. We find negative thermal expansion (thermodynamic anomaly) and an increase of the diffusion coefficient upon isothermal compression (dynamic anomaly) for 0< or =lambda<6/7. As in water, the regions where these anomalies occur are nested domes in the (T, p) or (T, P) planes , with the thermodynamic anomaly dome contained entirely within the dynamic anomaly dome. We calculate translational and orientational order parameters (t and Q6), and project equilibrium state points onto the (t, Q6) plane, or order map. The order map evolves from waterlike behavior to hard-sphere-like behavior upon varying lambda between 4/7 and 6/7. Thus, we traverse the range of liquid behavior encompassed by hard spheres (lanbda=1) and waterlike (lambda approximately 4/7) with a family of tunable spherically symmetric potentials by simply varying the ratio of hard to soft-core diameters. Although dynamic and thermodynamic anomalies occur almost across the entire range 0< or=lambda< or=1, waterlike structural anomalies (i.e., decrease in both t and Q6 upon compression and strictly correlated T and Q6 in the anomalous region) occur only around lambda=4/7. Waterlike anomalies in structure, dynamics and thermodynamics arise solely due to the existence of two length scales, with their ratio lambda being the single control parameter, orientation-dependent interactions being absent by design.

We study the potential energy surface (PES) sampled by a liquid modeled via the widely studied extended simple point charge (SPC/E) model for water. We characterize the curvature of the PES by calculating the instantaneous normal mode (INM) spectrum for a wide range of densities and temperatures. We discuss the information contained in the INM density of states, which requires additional processing to be unambiguously associated with the long-time dynamics. For the SPC/E model, we find that the slowing down of the dynamics in the supercooled region-where the ideal mode coupling theory has been used to describe the dynamics-is controlled by the reduction in the number of directions in configuration space that allow a structural change. We find that the fraction f(dw) of the double-well directions in configuration space determines the value of the diffusion constant D, thereby relating a property of the PES to a macroscopic dynamic quantity; specifically, it appears that square root D is approximately linear in f(dw). Our findings are consistent with the hypothesis that, at the mode coupling crossover temperature, dynamical processes based on the free exploration of configuration space vanish, and processes requiring activation dominate. Hence, the reduction of the number of directions allowing free exploration of configuration space is the mechanism of diffusion implicitly implemented in the ideal mode coupling theory. Additionally, we find a direct relationship between the number of basins sampled by the system and the number of free directions. In this picture, diffusion appears to be related to geometrical properties of the PES, and to be entropic in origin.

We investigate the properties of geometrically modified water models by performing molecular dynamics simulations of perturbations of the extended simple point charge (SPC/E) model of water over a wide range of temperatures at 1 bar. The geometric modification consists of altering the H-O-H angle in SPC/E. The dipole moment is held constant by altering the O-H bond length, while the electrostatic charges are left unchanged. We find that a H-O-H angle of at least 100 degrees is necessary for the appearance of density anomalies and of solubility extrema with respect to temperature for small apolar solutes. We observe the occurrence of two incompatible types of structural order in these models: Tetrahedral, with waterlike translational order for bent models with H-O-H angles in excess of 100 degrees ; and linear, with Lennard-Jones-like orientationally averaged translational order for smaller H-O-H angles. Increasing the H-O-H angle causes the density to increase, while at the same time shifting waterlike anomalies to progressively higher temperatures. For bent models with H-O-H angle greater than SPC/E's, we observe arrest of translational motion at 300 K (115 degrees) and 330 K (120 degrees).

Path integral molecular dynamics (PIMD) simulations of the extended simple point charge (SPC/E) model of liquid water (both H2O and D2O) have been performed in order to find the temperature at which the maximum liquid density is obtained. For comparison, purely classical SPC and SPC/E water have also been simulated over long periods (more than 200 ps). Structural properties and various temperature dependent quantities are reported. Special attention has been given to the fluctuations of pressure and volume in simulations run in the canonical (constant NVT) and isothermal–isobaric (constant NPT) ensembles, respectively. Although a density maximum is detected by monitoring energy–volume correlations as a function of temperature, the large statistical uncertainty in the correlations reduces the significance of this finding. The main conclusions of this work are threefold. First, the use of the energy–volume correlation moment to determine the density maximum appears to be a useful method, although a number of very long simulations of between 0.5–1.0 ns over a fairly wide temperature range are necessary to obtain accurate results. Second, neither the SPC nor the SPC/E water models are able to reproduce the density of cold water accurately. Finally, a quantum mechanical treatment of the water model is necessary to correctly represent the structure, energy, and fluctuations of the condensed phase.

Improved SPC force field of water based on the dielectric constant: SPC/[formula omitted

Water is a notoriously difficult substance to model both accurately and efficiently. Here, we focus on descriptions with a single coarse-grained particle per molecule using the so-called approximate non-conformal and generalized Stockmayer potentials as the starting points. They are fitted using the radial distribution function and the liquid-gas density profile of the atomistic extended simple point charge (SPC/E) model by downhill simplex optimization. We compare the results with monatomic water (mW), ELBA, and direct iterative Boltzmann inversion of SPC/E. The results show that symmetrical potentials result in non-transferable models, that is, they need to be reparametrized for new state points. This indicates that transferability may require more complex models. Furthermore, the results also show that the addition of a point dipole is not sufficient to make the potentials accurate and transferable to different temperatures (300 K-500 K) and pressures without an appropriate choice of properties as targets during model optimization.

Anomalous behaviour in density, diffusivity, and structural order is investigated for silicon modeled by the Stillinger-Weber potential by performing molecular dynamics simulations. As previously reported in the case of water [J. R. Errington and P. G. Debenedetti, Nature (London) 409, 318 (2001)] and silica [M. S. Shell, P. G. Debenedetti, and A. Z. Panagiotopoulos, Phys. Rev. E 66, 011202 (2002)], a cascading of thermodynamic, dynamic, and structural anomalous regions is also observed in liquid silicon. The region of structural anomaly includes the region of diffusivity anomaly, which in turn encompasses the region of density anomaly (which is unlike water but similar to silica). In the region of structural anomaly, a tight correlation between the translational and tetrahedrality order parameter is found, but the correlation is weaker when a local orientational order parameter (q3) is used as a measure of tetrahedrality. The total excess entropy and the pair correlation entropy are computed across the phase diagram and the correlation between the excess entropy and the regions of anomalies in the phase diagram of liquid silicon is examined. Scaling relations associating the excess entropy with the diffusion coefficient show considerable deviation from the quasi-universal behaviour observed in hard-sphere and Lennard-Jones liquids and some liquid metals. Excess entropy based criteria for diffusivity and structural anomalies fail to capture the observed regions of anomaly.

Numerical modeling of the effects hydration and number of hydrogen bonds on the mechanical properties of the tropocollagen molecule.

Unveiling relaxation and crystal nucleation interplay in supercooled germanium liquid