High-density Liquid Phase Research Articles

Molecular dynamics simulations for optical Kerr effect of TIP4P/2005 water in liquid and supercooled states

The optical Kerr effect (OKE) spectroscopy measured with heterodyne detection (HD) is a useful tool to provide information regarding intermolecular vibrations and structural relaxations in liquid water. Recently, the measurements of the OKE spectroscopy have been extended to the supercooled regime of water. Though the measured results can be well described by using a phenomenological model, the time-resolved OKE spectroscopy of liquid and supercooled water still need a comprehensive understanding. In this paper, we investigated the OKE nuclear response functions of this peculiar liquid and their reduced spectral densities by performing molecular dynamics simulations with the TIP4P/2005 water model. The collective polarizability of water was computed via a dipolar induction scheme, which involves the intrinsic polarizability and the first-order hyperpolarizability tensor of water molecule. Our simulation results were qualitatively consistent with the HD-OKE experimental observations for displaying that the polarizability anisotropy relaxation of supercooled water in the high-density liquid phase was fragile-like by following a stretching exponential decay with an exponent βs insensitive to temperature and the temperature dependence of the relaxation time exhibited a power-law divergence at a singular temperature Ts with a critical exponent γs. Indicated by our quantitative results, Ts was predominately determined by the structural arrest, but βs and γs were not only related to the structural relaxation but also influenced by the collective polarizability of the liquid. For all investigations, the effects due to the first-order hyperpolarizability tensor were examined.

Phase behaviors of deeply supercooled bilayer water unseen in bulk water

Akin to bulk water, water confined to an isolated nanoslit can show a wealth of new 2D phases of ice and amorphous ice, as well as unusual phase behavior. Indeed, 2D water phases, such as bilayer hexagonal ice and monolayer square ice, have been detected in the laboratory, confirming earlier computational predictions. Herein, we report theoretical evidence of a hitherto unreported state, namely, bilayer very low density amorphous ice (BL-VLDA), as well as evidence of a strong first-order transition between BL-VLDA and the BL amorphous ice (BL-A), and a weak first-order transition between BL-VLDA and the BL very low density liquid (BL-VLDL) water. The diffusivity of BL-VLDA is typically in the range of 10-9 cm2/s to 10-10 cm2/s. Similar to bulk (3D) water, 2D water can exhibit two forms of liquid in the deeply supercooled state. However, unlike supercooled bulk water, for which the two forms of liquid can coexist and merge into one at a critical point, the 2D BL-VLDL and BL high-density liquid (BL-HDL) phases are separated by the highly stable solid phase of BL-A whose melting line exhibits the isochore end point (IEP) near 220 K in the temperature-pressure diagram. Above the IEP temperature, BL-VLDL and BL-HDL are indistinguishable. At negative pressures, the metastable BL-VLDL exhibits a spatially and temporally heterogeneous structure induced by dynamic changes in the nanodomains, a feature much less pronounced in the BL-HDL.

Evidence of low-density and high-density liquid phases and isochore end point for water confined to carbon nanotube

Possible transition between two phases of supercooled liquid water, namely the low- and high-density liquid water, has been only predicted to occur below 230 K from molecular dynamics (MD) simulation. However, such a phase transition cannot be detected in the laboratory because of the so-called "no-man's land" under deeply supercooled condition, where only crystalline ices have been observed. Here, we show MD simulation evidence that, inside an isolated carbon nanotube (CNT) with a diameter of 1.25 nm, both low- and high-density liquid water states can be detected near ambient temperature and above ambient pressure. In the temperature-pressure phase diagram, the low- and high-density liquid water phases are separated by the hexagonal ice nanotube (hINT) phase, and the melting line terminates at the isochore end point near 292 K because of the retracting melting line from 292 to 278 K. Beyond the isochore end point (292 K), low- and high-density liquid becomes indistinguishable. When the pressure is increased from 10 to 600 MPa along the 280-K isotherm, we observe that water inside the 1.25-nm-diameter CNT can undergo low-density liquid to hINT to high-density liquid reentrant first-order transitions.

Lattice model for water-solute mixtures.

A lattice model for the study of mixtures of associating liquids is proposed. Solvent and solute are modeled by adapting the associating lattice gas (ALG) model. The nature of interaction of solute/solvent is controlled by tuning the energy interactions between the patches of ALG model. We have studied three set of parameters, resulting in, hydrophilic, inert, and hydrophobic interactions. Extensive Monte Carlo simulations were carried out, and the behavior of pure components and the excess properties of the mixtures have been studied. The pure components, water (solvent) and solute, have quite similar phase diagrams, presenting gas, low density liquid, and high density liquid phases. In the case of solute, the regions of coexistence are substantially reduced when compared with both the water and the standard ALG models. A numerical procedure has been developed in order to attain series of results at constant pressure from simulations of the lattice gas model in the grand canonical ensemble. The excess properties of the mixtures, volume and enthalpy as the function of the solute fraction, have been studied for different interaction parameters of the model. Our model is able to reproduce qualitatively well the excess volume and enthalpy for different aqueous solutions. For the hydrophilic case, we show that the model is able to reproduce the excess volume and enthalpy of mixtures of small alcohols and amines. The inert case reproduces the behavior of large alcohols such as propanol, butanol, and pentanol. For the last case (hydrophobic), the excess properties reproduce the behavior of ionic liquids in aqueous solution.

Evidence for a liquid-liquid critical point in supercooled water within the E3B3 model and a possible interpretation of the kink in the homogeneous nucleation line.

Supercooled water exhibits many thermodynamic anomalies, and several scenarios have been proposed to interpret them, among which the liquid-liquid critical point (LLCP) hypothesis is the most commonly discussed. We investigated Widom lines and the LLCP of deeply supercooled water, by using molecular dynamics simulation with a newly reparameterized water model that explicitly includes three-body interactions. Seven isobars are studied from ambient pressure to 2.5 kbar, and Widom lines are identified by calculating maxima in the coefficient of thermal expansion and the isothermal compressibility (both with respect to temperature). From these data we estimate that the LLCP of the new water model is at 180 K and 2.1 kbar. The oxygen radial distribution function is calculated along the 2 kbar isobar. It shows a steep change in the height of its second peak between 180 and 185 K, which indicates a transition between the high-density liquid and low-density liquid phases and which is consistent with the ascribed location of the critical point. The good agreement of the height of the second peak of the radial distribution function between simulation and experiment at 1 bar, as a function of temperature, supports the validity of the model. The location of the LLCP within the model is close to the kink in the experimental homogeneous nucleation line. We use existing experimental data to argue that the experimental LLCP is at 168 K and 1.95 kbar and speculate how this LLCP and its Widom line might be responsible for the kink in the homogeneous nucleation line.

Dynamic crossover in deeply cooled water confined in MCM-41 at 4 kbar and its relation to the liquid-liquid transition hypothesis.

With quasi-elastic neutron scattering, we study the single-particle dynamics of the water confined in a hydrophilic silica material, MCM-41, at 4 kbar. A dynamic crossover phenomenon is observed at 219 K. We compare this dynamic crossover with the one observed at ambient pressure and find that (a) above the crossover temperature, the temperature dependence of the characteristic relaxation time at ambient pressure exhibits a more evident super-Arrhenius behavior than that at 4 kbar. Especially, at temperatures below about 230 K, the relaxation time at 4 kbar is even smaller than that at ambient pressure. This feature is different from many other liquids. (b) Below the crossover temperature, the Arrhenius behavior found at ambient pressure has a larger activation energy compared to the one found at 4 kbar. We ascribe the former to the difference between the local structure of the low-density liquid (LDL) phase and that of the high-density liquid (HDL) phase, and the latter to the difference between the strength of the hydrogen bond of the LDL and that of the HDL. Therefore, we conclude that the phenomena observed in this paper are consistent with the LDL-to-HDL liquid-liquid transition hypothesis.

Thermal and nonthermal melting of silicon under femtosecond x-ray irradiation

As it is known from visible light experiments, silicon under femtosecond pulse irradiation can undergo the so-called 'nonthermal melting' if the density of electrons excited from the valence to the conduction band overcomes a certain critical value. Such ultrafast transition is induced by strong changes in the atomic potential energy surface, which trigger atomic relocation. However, heating of a material due to the electron-phonon coupling can also lead to a phase transition, called 'thermal melting'. This thermal melting can occur even if the excited-electron density is much too low to induce non-thermal effects. To study phase transitions, and in particular, the interplay of the thermal and nonthermal effects in silicon under a femtosecond x-ray irradiation, we propose their unified treatment by going beyond the Born-Oppenheimer approximation within our hybrid model based on tight binding molecular dynamics. With our extended model we identify damage thresholds for various phase transitions in irradiated silicon. We show that electron-phonon coupling triggers the phase transition of solid silicon into a low-density liquid phase if the energy deposited into the sample is above $\sim0.65$ eV per atom. For the deposited doses of over $\sim0.9$ eV per atom, solid silicon undergoes a phase transition into high-density liquid phase triggered by an interplay between electron-phonon heating and nonthermal effects. These thresholds are much lower than those predicted with the Born-Oppenheimer approximation ($\sim2.1$ eV/atom), and indicate a significant contribution of electron-phonon coupling to the relaxation of the laser-excited silicon. We expect that these results will stimulate dedicated experimental studies, unveiling in detail various paths of structural relaxation within laser-irradiated silicon.

Pattern formation in Langmuir–Blodgett films of tricycloquinazoline based discotic liquid crystal molecules

The assembly of molecules on the surface has drawn considerable attention because of its potential to tailor the physicochemical, electronic, and magnetic properties of materials. The structure of aggregates of molecules can be influenced by the molecular interactions and the external parameters like temperature, pressure and ion contents. We have found a stable Langmuir monolayer of amphiphilic tricycloquinazoline based discotic liquid crystal (AmTCQ) molecules at air–water (A–W) interface. The monolayer exhibits gas phase, low density liquid phase and high density liquid phase. The structural evolution in Langmuir–Blodgett (LB) films of the AmTCQ molecules as a function of target surface pressure is studied using an atomic force microscope. We found aggregation of the discotic molecules leading to nucleation sites at a lower surface pressure. With the increasing surface pressure, each nucleation site grows to form a stable triangular structure. The shape and size of the triangular structure remain stable till the target surface pressure for LB films deposition exceeds the equilibrium surface pressure of the AmTCQ molecules. The number of triangles in the film increases with increasing target surface pressure of LB deposition. The target surface pressure of LB deposition acts as a controlling parameter for obtaining desired number of triangular domains in the film.

From parallel to single crystallization kinetics in high-density amorphous ice

The isobaric transformation behavior of unannealed (uHDA) and expanded (eHDA) high-density amorphous ice at pressures up to 0.20 GPa is compared using powder x-ray diffraction and dilatometry. eHDA shows high thermal stability and crystallizes to a single ice phase only, whereas uHDA shows much lower thermal stability and always crystallizes to a mixture of ice phases. Unexpectedly, at low temperatures hexagonal ice grows first from uHDA, whereas this phase never crystallizes from eHDA. This leads us to conclude that hidden structural order in the form of nanocrystalline domains is present in uHDA, which triggers growth of hexagonal ice. By contrast, these ordered domains are absent in eHDA, which appears to be a homogeneous material and, thus, could be considered as a candidate for the low-temperature proxy of the proposed high-density liquid phase of water. The present work provides the basis for further experimental studies aiming at investigating this possibility since it establishes that the well-studied uHDA is not the right material to be studied in this context, whereas the more recently discovered eHDA is.

Free energy surface of ST2 water near the liquid-liquid phase transition

We carry out umbrella sampling Monte Carlo simulations to evaluate the free energy surface of the ST2 model of water as a function of two order parameters, the density and a bond-orientational order parameter. We approximate the long-range electrostatic interactions of the ST2 model using the reaction-field method. We focus on state points in the vicinity of the liquid-liquid critical point proposed for this model in earlier work. At temperatures below the predicted critical temperature we find two basins in the free energy surface, both of which have liquid-like bond orientational order, but differing in density. The pressure and temperature dependence of the shape of the free energy surface is consistent with the assignment of these two basins to the distinct low density and high density liquid phases previously predicted to occur in ST2 water.

Thermodynamic, dynamic, structural, and excess entropy anomalies for core-softened potentials

Using molecular dynamic simulations, we study three families of continuous core-softened potentials consisting of two length scales: a shoulder scale and an attractive scale. All the families have the same slope between the two length scales but exhibit different potential energy gap between them. For each family three shoulder depths are analyzed. We show that all these systems exhibit a liquid-liquid phase transition between a high density liquid phase and a low density liquid phase ending at a critical point. The critical temperature is the same for all cases suggesting that the critical temperature is only dependent on the slope between the two scales. The critical pressure decreases with the decrease of the potential energy gap between the two scales suggesting that the pressure is responsible for forming the high density liquid. We also show, using the radial distribution function and the excess entropy analysis, that the density, the diffusion, and the structural anomalies are present if particles move from the attractive scale to the shoulder scale with the increase of the temperature indicating that the anomalous behavior depends only in what happens up to the second coordination shell.

Ultrasoft primitive model of polyionic solutions: Structure, aggregation, and dynamics

We introduce an ultrasoft core model of interpenetrating polycations and polyanions, with continuous Gaussian charge distributions, to investigate polyelectrolyte aggregation in dilute and semi-dilute salt-free solutions. The model is studied by a combination of approximate theories (random phase approximation and hypernetted chain theory) and numerical simulations. The calculated pair structure, thermodynamics, phase diagram, and polyion dynamics of the symmetric version of the model (the "ultrasoft restricted primitive model" or UPRM) differ from the corresponding properties of the widely studied "restricted primitive model" (RPM) where ions have hard cores. At sufficiently low temperatures and densities, oppositely charged polyions form weakly interacting, polarizable neutral pairs. The clustering probabilities, dielectric behavior, and electrical conductivity point to a line of sharp conductor-insulator transitions in the density-temperature plane. At very low temperatures, the conductor-insulator transition line terminates near the top of a first order coexistence curve separating a high-density liquid phase from a low-density vapor phase. The simulation data hint at a tricritical behavior, reminiscent of that observed for the two-dimensional Coulomb gas, which contrasts with the Ising criticality of its three-dimensional counterpart, the RPM.

Effect of hydrophobic solutes on the liquid-liquid critical point

Jagla ramp particles, interacting through a ramp potential with two characteristic length scales, are known to show in their bulk phase thermodynamic and dynamic anomalies, similar to what is found in water. Jagla particles also exhibit a line of phase transitions separating a low density liquid phase and a high density liquid phase, terminating in a liquid-liquid critical point in a region of the phase diagram that can be studied by simulations. Employing molecular dynamics computer simulations, we study the thermodynamics and the dynamics of solutions of hard spheres (HS) in a solvent formed by Jagla ramp particles. We consider the cases of HS mole fraction xHS=0.10, 0.15, and 0.20, and also the case xHS=0.50 (a 1:1 mixture of HS and Jagla particles). We find a liquid-liquid critical point, up to the highest HS mole fraction; its position shifts to higher pressures and lower temperatures upon increasing xHS. We also find that the diffusion coefficient anomalies appear to be preserved for all the mole fractions studied.

Dynamic transitions in a two dimensional associating lattice gas model

Using Monte Carlo simulations we investigate some new aspects of the phase diagram and the behavior of the diffusion coefficient in an associating lattice gas (ALG) model on different regions of the phase diagram. The ALG model combines a two dimensional lattice gas where particles interact through a soft core potential and orientational degrees of freedom. The competition between soft core potential and directional attractive forces results in a high density liquid phase, a low density liquid phase, and a gas phase. Besides anomalies in the behavior of the density with the temperature at constant pressure and of the diffusion coefficient with density at constant temperature are also found. The two liquid phases are separated by a coexistence line that ends in a bicritical point. The low density liquid phase is separated from the gas phase by a coexistence line that ends in tricritical point. The bicritical and tricritical points are linked by a critical lambda-line. The high density liquid phase and the fluid phases are separated by a second critical tau-line. We then investigate how the diffusion coefficient behaves on different regions of the chemical potential-temperature phase diagram. We find that diffusivity undergoes two types of dynamic transitions: a fragile-to-strong transition when the critical lambda-line is crossed by decreasing the temperature at a constant chemical potential; and a strong-to-strong transition when the critical tau-line is crossed by decreasing the temperature at a constant chemical potential.

The anomalous behavior of the density of water in the range 30 K < T < 373 K

The temperature dependence of the density of water, rho(T), is obtained by means of optical scattering data, Raman and Fourier transform infrared, in a very wide temperature range, 30 < T < 373 K. This interval covers three regions: the thermodynamically stable liquid phase, the metastable supercooled phase, and the low-density amorphous solid phase, at very low T. From analyses of the profile of the OH stretching spectra, we determine the fractional weight of the two main spectral components characterized by two different local hydrogen bond structures. They are, as predicted by the liquid-liquid phase transition hypothesis of liquid water, the low- and the high-density liquid phases. We evaluate contributions to the density of these two phases and thus are able to calculate the absolute density of water as a function of T. We observe in rho(T) a complex thermal behavior characterized not only by the well known maximum in the stable liquid phase at T = 277 K, but also by a well defined minimum in the deeply supercooled region at 203 +/- 5 K, in agreement with suggestions from molecular dynamics simulations.

Liquid-Liquid Phase Transformation in Silicon: Evidence from First-Principles Molecular Dynamics Simulations

We report results of first principles molecular dynamics simulations that confirm early speculations on the presence of liquid-liquid phase transition in undercooled silicon. However, we find that structural and electronic properties of both low-density liquid (LDL) and high-density liquid (HDL) phases are quite different from those obtained by empirical calculations, the difference being more pronounced for the HDL phase. The discrepancy between quantum and classical simulations is attributed to the inability of empirical potentials to describe changes in chemical bonds induced by density and temperature variations.

Contrasting Cu-complexing behaviour in vapour and liquid fluid inclusions from the Yankee Lode tin deposit, Mole Granite, Australia

Synchrotron Radiation-induced X-Ray Fluorescence (SR-XRF) and X-ray Absorption Near Edge Structure (XANES) experiments were performed on coexisting low density vapour and high density liquid inclusions from the Yankee Lode, Mole Granite, Australia. The chosen sample is a quartz crystal from a cassiterite–quartz-vein, showing a complex hydrothermal growth history characterized by successive pulses of fluid trapping from 600 to 200 °C. The liquid and vapour inclusions analysed predated cassiterite precipitation and show a homogenisation temperature of about 450 °C. In vapour inclusions, SR-XRF mapping showed that Cu is associated with S together with K, Mn, Fe, Zn and As, likely as a sulphide mineral. Elemental quantification deduced from XRF maps performed at room temperature on heterogeneous and on inclusions homogenised by heating indicate that Cu was concentrated in the low density vapour inclusions whereas Cl, K, Mn, Fe, Zn, Br and Rb were partitioned in the high density liquid phase. S and As show partition coefficients close to 1:1, with As displaying a stronger affinity for the liquid phase than S. Micro-XANES experiments at the Cu Kα edge show that Cl-ligand complexes dominate upon heating in both inclusion types up to 450 °C. SR-XRF mapping during heating revealed that the vapour inclusion was not completely homogenised at 450 °C. XANES analysis of the vapour inclusion showed that a new Cu-bearing phase formed above 450 °C, which is not detectable in the liquid inclusion. To our knowledge, species detected in the XANES spectra acquired at 506 °C do not correspond to any Cu–Cl complex published so far in the literature. We propose that a Cu 1+ complex of still unknown stoichimetry, including sulfur as a ligand and possibly Cl as well, could account for Cu complexation at high temperature. This observation supports the hypothesis that sulfur could play a central role for Cu-complexing in high temperature porphyry-type environments. Experimental data in the system Cu–S–Cl–CO 2–H 2O are needed to document fully this newly discovered Cu complex.

Equilibrium Limit of Diffusion Equation for Interacting Particles: Are Bifurcations and Multiplicity of Correlation Functions an Artifact or Real?

The structure of a fluid is analyzed by taking the equilibrium limit of a diffusion equation including the Giacomin-Lebowitz term for intermolecular interactions. This equation represents the differential mass balance in fluids with the Metropolis algorithm for fluxes; it allows a new qualitative yet analytical approximation for the direct correlation function over the entire range of fluid densities and temperatures. This approximation is analogous to a classical Ono-Kondo model for adsorption if the distribution of molecules around a central molecule is viewed as the adsorption of molecules on a central molecule. While this model qualitatively predicts known behavior for both gas and liquid phases, approaching a phase transition (e.g., condensation of gas into liquid) results in a bifurcation and multiplicity of the direct correlation function. The model predicts a sequence in the transformation of correlation functions from that of a gas to that of a liquid. This sequence starts with the appearance of an isolated loop in the direct correlation function, indicating states that are stable but cannot be achieved without perturbation of the system. However, the system seems to sense its proximity to a phase transition and reflects the distance to the phase boundary by the size and shape of this isolated loop in the direct correlation function. At lower temperatures, this loop merges with the gas-phase peak, indicating that clusters can form spontaneously. Then, these clusters grow into a high-density (liquid) phase. The notion of bifurcations and multiplicity in correlation functions is an unusual and controversial concept. Certainly, it is unexpected and raises important questions: (a) if such a behavior is not real, why does the diffusion equation predict such behavior, i.e., is it a mathematical artifact or is it due to conflicting physical assumptions? (b) if this behavior is real, how does one interpret it at a molecular level? Here, we present some interpretations, but they are open for discussion.

The Widom line of supercooled water

Water can be supercooled to temperatures as low as−92 °C, the experimental crystal homogeneous nucleation temperatureTH at 2 kbar. Within the supercooled liquid phase its response functions show an anomalousincrease consistent with the presence of a liquid–liquid critical point located in a regioninaccessible to experiments on bulk water. Recent experiments on the dynamics ofconfined water show that a possible way to understand the properties of wateris to investigate the supercooled phase diagram in the vicinity of the Widomline (locus of maximum correlation length) that emanates from the hypothesizedliquid–liquid critical point. Here we explore the Widom line for a Hamiltonianmodel of water using an analytic approach, and discuss the plausibility of thehypothesized liquid–liquid critical point, as well as its possible consequences, on the basisof the assumptions of the model. The present analysis allows us (i) to find ananalytic expression for the spinodal line of the high-density liquid phase, withrespect to the low-density liquid phase, showing that this line becomes flat in theP–T phase diagram in the physical limit of a large number of available orientationsfor the hydrogen bonds, as recently seen in simulations and experiments (Xu et al 2005 Proc. Natl Acad. Sci. 102 16558); (ii) to find an estimate of the values forthe hypothesized liquid–liquid critical point coordinates that compare very wellwith Monte Carlo results; and (iii) to show how the Widom line can be locatedby studying the derivative of the probability of forming hydrogen bonds withlocal tetrahedral orientation which can be calculated analytically within thisapproach.

Effect of split injection on stratified charge formation of direct injection spark ignition engines

The effects of split injection with various dwells and mass ratios on the spray and mixture characteristics in an ambient environment similar to the late stage of compression stroke in direct injection spark ignition (DISI) engines were investigated by using the laser absorption scattering (LAS) technique. Through splitting the fuel injection process with appropriate dwells and mass ratios, some benefits for the stratified charge formation of DISI engines can be achieved. First, the phenomenon of high-density liquid phase fuel piling up at the leading edge of the spray can be circumvented and the subsequent reduction in the spray penetration length for both liquid and vapour phases is seen. Second, the radial width of the ‘combustible mixture’ (equivalence ratio of vapour φv in a range of 0.7 ≤ φv ≤ 1.3) is significantly extended. Finally, the quantity of ‘over lean’ (φv &lt; 0.7) mixture in the spray is significantly reduced. These results are believed to contribute to the stratified lean operation and the reduction in smoke and unburned hydrocarbons (UBHC) emissions of DISI engines. Further, the mechanism behind these effects of the split injection was clarified by analysing the interactions between the two pulsed sprays and the spray-induced ambient air motion using the LIF-PIV technique.