Anomalous scattering in supercooled ST2 water

ABSTRACT

Large-scale molecular dynamics (MD) simulations of systems containing up to 256,000 molecules were performed to investigate the scattering behaviour of the ST2 water model at deeply supercooled conditions. The simulations reveal that ST2 exhibits anomalous scattering, reminiscent of that observed in experiment, which is characterised by an increase in the static structure factor at low wavenumbers. This unusual behaviour in ST2 is linked with coupled fluctuations in density and local tetrahedral order in the liquid. The Ornstein–Zernike correlation length estimated from the anomalous scattering component exhibits power-law growth upon cooling, consistent with the existence of a liquid–liquid critical point (LLCP) in the ST2 model at ca. 245 K. Further, spontaneous liquid–liquid phase separation is observed upon thermally quenching a large system with 256,000 water molecules below the predicted critical temperature into the two-phase region. The large-scale MD simulations therefore confirm the existence of a metastable liquid–liquid phase transition in ST2 and support findings from previous computational studies performed using smaller systems containing only a few hundred molecules. We anticipate that our analysis may prove useful in interpreting recent scattering experiments that have been performed to search for an LLCP in deeply supercooled water.     

Dynamics of supercooled confined water measured by deep inelastic neutron scattering

In this paper, we present the results of deep inelastic neutron scattering (DINS) measurements on supercooled water confined within the pores (average pore diameter ~ 20 Å) of a disordered hydrophilic silica matrix obtained through hydrolysis and polycondensation of the alkoxide precursor Tetra-Methyl-Ortho-Silicate via the sol-gel method. Experiments were performed at two temperatures (250 K and 210 K, i.e., before and after the putative liquid–liquid transition of supercooled confined water) on a “wet” sample with hydration h ~ 40% w/w, which is high enough to have water-filled pores but low enough to avoid water crystallization. A virtually “dry” sample at h ~ 7% was also investigated to measure the contribution of the silica matrix to the neutron scattering signal. As is well known, DINS measurements allow the determination of the mean kinetic energy and the momentum distribution of the hydrogen atoms in the system and therefore, allow researchers to probe the local structure of supercooled confined water. The main result obtained is that at 210 K the hydrogen mean kinetic energy is equal or even slightly higher than at 250 K. This is at odds with the predictions of a semiempirical harmonic model recently proposed to describe the temperature dependence of the kinetic energy of hydrogen in water. This is a new and very interesting result, which suggests that at 210 K, the water hydrogens experience a stiffer intermolecular potential than at 250 K. This is in agreement with the liquid–liquid transition hypothesis.     

Fragile to strong crossover and Widom line in supercooled water: A comparative study

The aim of this paper is to discuss the relationship between the dynamics and thermodynamics of water in the supercooled region. Reviewed case studies comprehend bulk water simulated with the SPC/E, TIP4P and TIP4P/2005 potentials, water at protein interfaces, and water in solution with electrolytes. Upon supercooling, the fragile to strong crossover in the α-relaxation of water is found to occur when the Widom line emanating from the liquid-liquid critical point is crossed. This appears to be a general characteristic of supercooled water, not depending on the applied interaction potential and/or different local environments.     

利用扩展系综法计算水的自由能

利用扩展系综法得到了正则系综下水的TIP4P模型的自由能值为-21．485±0．035kJ/mol， 并与其他方法所得的结果作了比较.提出了选择该方法中关键参数(平衡因子)的有效方法， 并讨论了该方法的可移植性. 关键词： 自由能 TIP4P水模型 扩展系综 分子动力学模拟 水分子团     

Comment to “Dynamics of supercooled confined water measured by deep inelastic neutron scattering”

We comment on the findings of “Dynamics of supercooled confined water measured by deep inelastic neutron scattering”, by V. De Michele, G. Romanelli, and A. Cupane [Front. Phys. 13, 138205 (2018)]. We show that the current sensitivity of the deep inelastic neutron scattering (DINS) method, cannot detect with confidence small differences in the proton kinetic energy, Ke(H), involved in a liquid-liquid transition in supercooled water confined in nanoporous silica. We also critisize the calculation of Ke(H) carried out in Front. Phys. 13, 138205 (2018).     

High-temperature dynamic behavior in bulk liquid water: A molecular dynamics simulation study using the OPC and TIP4P-Ew potentials

Classical molecular dynamics simulations were performed to study the high-temperature (above 300 K) dynamic behavior of bulk water, specifically the behavior of the diffusion coefficient, hydrogen bond, and nearest-neighbor lifetimes. Two water potentials were compared: the recently proposed “globally optimal” point charge (OPC) model and the well-known TIP4P-Ew model. By considering the Arrhenius plots of the computed inverse diffusion coefficient and rotational relaxation constants, a crossover from Vogel–Fulcher–Tammann behavior to a linear trend with increasing temperature was detected at T* ≈ 309 and T* ≈ 285 K for the OPC and TIP4P-Ew models, respectively. Experimentally, the crossover point was previously observed at T* ± 315–5 K. We also verified that for the coefficient of thermal expansion α _P (T, P), the isobaric α _P (T) curves cross at about the same T* as in the experiment. The lifetimes of water hydrogen bonds and of the nearest neighbors were evaluated and were found to cross near T*, where the lifetimes are about 1 ps. For T < T*, hydrogen bonds persist longer than nearest neighbors, suggesting that the hydrogen bonding network dominates the water structure at T < T*, whereas for T > T*, water behaves more like a simple liquid. The fact that T* falls within the biologically relevant temperature range is a strong motivation for further analysis of the phenomenon and its possible consequences for biomolecular systems.     

Molecular dynamics simulations on the melting of gold nanoparticles

Molecular dynamics is employed to study the melting of bulk gold and gold nanoparticles. PCFF, Sutton-Chen and COMPASS force fields are adopted to study the melting point of bulk gold and we find out that the Sutton-Chen force field is the most accurate model in predicting the melting point of bulk gold. Consequently, the Sutton-Chen force field is applied to study the melting points of spherical gold nanoparticles with different diameters. Variations of diffusion coefficient, potential energy and translational order parameter with temperature are analyzed. The simulated melting points of gold nanoparticles are between 615～1115 K, which are much lower than that of bulk gold (1336 K). As the diameter of gold nanoparticle drops, the melting point also descends. The melting mechanism is also analyzed for gold nanoparticles.     

Density functional theory of freezing of a system of highly elongated ellipsoidal oligomer solutions

We have used the density functional theory of freezing to study the liquid crystalline phase behavior of a system of highly elongated ellipsoidal conjugated oligomers dispersed in three different solvents namely chloroform, toluene and their equimolar mixture. The molecules are assumed to interact via solvent-implicit coarse-grained Gay–Berne potential. Pair correlation functions needed as input in the density functional theory have been calculated using the Percus–Yevick (PY) integral equation theory. Considering the isotropic and nematic phases, we have calculated the isotropic–nematic phase transition parameters and presented the temperature–density and pressure–temperature phase diagrams. Different solvent conditions are found not only to affect the transition parameters but also determine the capability of oligomers to form nematic phase in various thermodynamic conditions. In principle, our results are verifiable through computer simulations.     

Intermediate range order in supercooled water

ABSTRACT

The temperature dependence of the heights of the first and second x-ray diffraction peaks in supercooled water measured down to 244?K are found to display very different behaviours. While the first peak intensity remains essentially constant, the second peak increases strongly with decreasing temperature. In real space this is concomitant with the reduction of the number of non-bonded interstitial molecules between the first and second shells. It is found that although the first O-O shell in supercooled water is unchanged upon supercooling, the variations in intermediate range order are mainly associated with the growth of a predominantly tetrahedral network that is distinctly different from ice-Ih. Moreover, in this temperature regime we find a direct correlation between the height of the second diffraction peak and the intensity changes in the 2nd, 3rd, 4th and 5th peaks in the oxygen-oxygen pair distribution function.     

Experimental and computer simulation determination of the structural changes occurring through the liquid–glass transition in Cu–Zr alloys

Molecular dynamics (MD) simulations were performed of the structural changes occurring through the liquid–glass transition in Cu–Zr alloys. The total scattering functions (TSF), and their associated primary diffuse scattering peak positions (K _p), heights (K _h) and full-widths at half maximum (K _FWHM) were used as metrics to compare the simulations to high-energy X-ray scattering data. The residuals of difference between the model and experimental TSFs are ～0.03 for the liquids and about 0.07 for the glasses. Over the compositional range studied, Zr_{1? x} Cu _x (0.1 ≤ x ≤ 0.9), K _p, K _h and K _FWHM show a strong dependence on composition and temperature. The simulation and experimental data correlate well between each other. MD simulation revealed that the Cu–Zr bonds undergo the largest changes during cooling of the liquid, whereas the Cu–Cu bonds change the least. Changes in the partial-pair correlations are more readily seen in the second and third shells. The Voronoi polyhedra (VP) in glasses are dominated by only a few select types that are compositionally dependent. The relative concentrations of the dominant VPs rapidly change in their relative proportion in the deeply undercooled liquid. The experimentally determined region of best glass formability, x _Cu ～ 65%, shows the largest temperature dependent changes for the deeply undercooled liquid in the MD simulation. This region also exhibits very strong temperature dependence for the diffusivity and the total energy of the system. These data point to a strong topological change in the best glass-forming alloys and a concurrent change in the VP chemistry in the deeply undercooled liquid.     

Free energy reaction root mapping of alanine tripeptide in water

ABSTRACT

We have investigated the free energy surface of alanine tripeptide in water. To elucidate the secondary structure of the amide chain, information on the free energy surface with explicit water at room temperature, and the multidimensional reaction coordinates are required. We studied the minimum free energy paths (MFEPs) connecting reactants, transition structures (TS) and products. To solve this problem, we used the free energy reaction root mapping (FERRMap) method. This is an automated search method to find MFEPs by using umbrella integration and the scaled hypersphere search method. We calculated the four-dimensional free energy surface for alanine tripeptide in water using FERRMap and found 61 equilibrium structures (EQ) connected by 133 TS points. After elucidating the MFEP network, we analysed the structures of the EQ points and the MFEPs connecting beta-sheet structures and beta-turn structures or left-handed helix structures.     

A molecular level study of liquid water with a flexible water model

The structural properties of water at different temperatures and pressures have been investigated by using a flexible water model and the inherent structure mechanism. The presence of 60$^\circ$ peak in the O--O--O angle distribution function and the behaviours of the hydrogen bonds in the first shell indicate that some water molecules in the second shell move toward the central molecules through the bending (not breaking) of hydrogen bonds and even become first-shell molecules of the central molecule on the basis of the O--O cutoff distance but not first-shell molecules by means of the hydrogen-bond criterion. The inherent-structure analysis of the O--O radial distribution functions at different pressures shows that the first peak is almost independent of the pressure; the position of second peak moves from 0.45 to 0.32nm as the pressure increases from $1\times 10^5$Pa to $1\times 10^9$Pa. This particularly evident pressure effect, i.e. the constant nearest-neighbours and the transformation of outer-neighbours on the basis of O--O distance, together with the results at different temperatures, gives a positive evidence for the two-state outer-neighbour mixture model: liquid water is a mixture of Ice-Ih-type-bonding and Ice-II-type bonding structures.     

过冷液体钾形核初期微观动力学机理的模拟研究

采用分子动力学方法对液态金属钾凝固过程进行了模拟,根据凝固过程体系平均原子能量、原子成键类型和成团类型,以及均方位移和非Gauss参数等动力学参数的演化特征,对过冷熔体形核初期微观动力学机理进行了研究.结果表明:根据过冷液体钾结晶形核过程热力学、动力学和结构特性的演化规律, 其过冷温度区间可以分为两个明显不同的阶段,潜在结晶核心出现在过冷液体较低温区.过冷熔体钾在形核初期,二十面体团簇结构在α-弛豫阶段逐渐解体,同时具有体心立方(bcc)结构的潜在结晶核心逐步形成,其临界晶核包含约300个原子.     

Reply to “Comment to ‘Dynamics of supercooled confined water measured by deep inelastic neutron scattering’ by Y. Finkelstein and R. Moreh”

We reply to the comment [Front. Phys. 14(5), 53605 (2019)] by Y. Finkelstein and R. Moreh on our article Front. Phys. 13(1), 138205 (2018). We agree with some of their criticisms about our calculation of the temperature effect on the kinetic energy of hydrogen atoms of supercooled confined water; we also agree with their statement that, in view of the current sensitivity of the technique, possible effects of the liquid–liquid water transition are hardly detected with deep inelastic neutron scattering (DINS). However, we disagree with their use of the translational mass ratio of a single water molecule and, in general, with their underestimation of collective effects.     

Electrical resistivity of liquid Bi–Sb alloys

The electrical resistivities of liquid Bi–Sb alloys have been measured by DC four-probe method within Bi-rich composition through a wide temperature range. The distinct anomaly of a hump shape was observed on resistivity–temperature (ρndash;T) curves for liquid Bi–Sb alloys on heating at the relatively high temperatures. These anomalies have revealed the temperature-induced liquid–liquid phase transition in Bi–Sb melts. The DSC results for BiSb20wt% alloy further prove the existence of liquid–liquid transition. Measuring the ρ–T curves first on heating and then on cooling we have found that on cooling the ρ–T curve remained linear. It means that the postulated liquid–liquid transition may be irreversible.     

Density and local order dependence of the dynamical features of water pentamers: an instantaneous normal mode investigation

We analyse liquid configurations of simulated water using the instantaneous normal mode approach along three isotherms (T=260, 300, 350 K), with densities ranging from 0.90 to 1.15 g/cm³. We focus our attention to clusters of five hydrogen-bonded molecules and project their instantaneous motion on some a priori modes related to the tetrahedral symmetry of these pentamers. We discuss the dependence of the resulting dynamical features on the density and the structural order set up by the presence of hydrogen bonding.     

Molecular dynamics simulation of ice nanocluster in supercooled water

The properties of the interface between an ice nanocluster and the surrounding water shell were investigated by the molecular simulation method for the SPC/E model in the temperature interval from 200 to 230?K. The melting point of the ice core was determined on the basis of the caloric curve and the behaviour of the diffusion coefficient. The change of the local structure was described by the orientational distribution functions and by the radial profiles of the local density, energy, and normal pressure.     

自由能方法与零压下Al的熔化温度

利用自由能方法的分子动力学模拟,计算了零压下Al的熔化温度.在计算液相自由能的过程中,采用勒纳-琼斯(LJ)液体作为参考系统,同时将计算结果与Mei和Davenport等人的计算结果进行了比较,计算结果表明：1)选用LJ参考系统使液相自由能的计算时间节省一半,并且不影响熔化温度的计算结果;2)采用不同的埋入原子势(EAM)的分子动力学模拟计算得到的熔化温度与实验值都存在偏差,而就金属Al而言,采用Cai等人的EAM势的熔化温度的计算结果比Mei和Davenport及Morris等人采用的势模型的结果略有改 关键词： 熔化温度 自由能方法 分子动力学模拟