Low-density Liquid Research Articles

Field-mediated interactions of passive and conformation-active particles: multibody and retardation effects.

Particles in soft matter interact through the deformation field they create, as in the "cheerios" effect or the curvature-mediated interactions of membrane proteins. Using a simple model for field-mediated interactions between passive particles, or active particles that switch conformation randomly or synchronously, we derive generic results concerning multibody interactions, activity driven patterns, and retardation effects.

Phase diagram and critical properties of a two-dimensional associating lattice gas.

We revisit the associating lattice gas (ALG) introduced by Henriques etal. [Phys. Rev. E 71, 031504 (2005)PLEEE81539-375510.1103/PhysRevE.71.031504] in its symmetric version. In this model, defined on the triangular lattice, interaction between molecules occupying nearest-neighbor sites depends on their relative orientation, mimicking the formation of hydrogen bonds in network-forming fluids. Although all previous studies of this model agree that it has a disordered fluid (DF), a low-density liquid (LDL), and a high-density liquid (HDL) phase, quite different forms have been reported for its phase diagram. Here, we present a thorough investigation of its phase behavior using both transfer matrix calculations and Monte Carlo (MC) simulations, along with finite-size scaling extrapolations. Results in striking agreement are found using these methods. The critical point associated with the DF-HDL transition at full occupancy, identified by Furlan et al. [Phys. Rev. E 100, 022109 (2019)2470-004510.1103/PhysRevE.100.022109] is shown to be one terminus of a critical line separating these phases. In opposition to previous simulation studies, we find that the transition between the DF and LDL phases is always discontinuous, similar to the LDL-HDL transition. The associated coexistence lines meet at the point where the DF-HDL critical line ends, making it a critical-end-point. Overall, the form of the phase diagram observed in our simulations is very similar to that found in the exact solution of the model on a Husimi lattice. Our results confirm that, despite the existence of some waterlike anomalies in this model, it is unable to reproduce key features of the phase behavior of liquid water.

Segregation on the nanoscale coupled to liquid water polyamorphism in supercooled aqueous ionic-liquid solution.

The most intriguing hypothesis explaining many water anomalies is a metastable liquid-liquid phase transition (LLPT) at high pressure and low temperatures, experimentally hidden by homogeneous nucleation. Recent infrared spectroscopic experiments showed that upon addition of hydrazinium trifluoroacetate to water, the supercooled ionic solution undergoes a sharp, reversible LLPT at ambient pressure, possible offspring of that in pure water. Here, we calculate the temperature-dependent signature of the OH-stretching band, reporting on the low/high density phase of water, in neat water and in the same experimentally investigated ionic solution. The comparison between the infrared signature of the pure liquid and that of the ionic solution can be achieved only computationally, providing insight into the nature of the experimentally observed phase transition and allowing us to investigate the effects of ionic compounds on the high to low density supercooled liquid water transition. We show that the experimentally observed crossover behavior in the ionic solution can be reproduced only if the phase transition between the low- and high-density liquid states of water is coupled to a mixing-unmixing transition between the water component and the ions: at low temperatures, water and ions are separated and the water component is a low density liquid. At high temperatures, water and ions get mixed and the water component is a high-density liquid. The separation at low temperatures into ion-rich and ion-poor regions allows unveiling the polyamorphic nature of liquid water, leading to a crossover behavior resembling that observed in supercooled neat water under high pressure.

Liquid–liquid phase transition in simulations of ultrafast heating and decompression of amorphous ice

• Ultrafast heating experiments on amorphous ice clarified by simulations. • High density amorphous ice heated to liquid state by ultrafast laser pulse. • Decompression of high density liquid reveals liquid–liquid transition. • Simulations detect structural transition along out-of-equilibrium paths. A recent experiment [K. H. Kim, et al. , Science 370 , 978 (2020)] showed that it may be possible to detect a liquid–liquid phase transition (LLPT) in supercooled water by subjecting high density amorphous ice (HDA) to ultrafast heating, after which the sample reportedly undergoes spontaneous decompression from a high density liquid (HDL) to a low density liquid (LDL) via a first-order phase transition. Here we conduct computer simulations of the ST2 water model, in which a LLPT is known to occur. We subject various HDA samples of this model to a heating and decompression protocol that follows a thermodynamic pathway similar to that of the recent experiments. Our results show that a signature of the underlying equilibrium LLPT can be observed in a strongly out-of-equilibrium process that follows this pathway despite the very high heating and decompression rates employed here. Our results are also consistent with the phase diagram of glassy ST2 water reported in previous studies.

Tetrahedral structure of supercooled water at ambient pressure and its influence on dynamic relaxation: Comparative study of water models

In this paper, we investigated the tetrahedral structure of supercooled water at ambient pressure and its influence on dynamic relaxation by comparing simulation results of TIP4P/2005 and SPC/E water models. The globally tetrahedral structure of supercooled water was characterized with the second-peak maximum and a deep first minimum in the radial distribution function g(r) of O-atoms and the reverse order in magnitude between the first two peaks of structure factor. The locally tetrahedral structure was specified by molecules, which and their neighbors up to the second hydration shell all have four H-bond coordinators. These molecules are referred as low-density liquid (LDL) and the others as high-density liquid (HDL). The water dynamics relaxation was studied through the self-intermediate scattering function, the non-Gaussian parameter, and the polarizability anisotropy time correlation function. Indicated by our simulations, the temperature dependence of the stretched exponent describing the α-relaxation displayed a small peak in the supercooled regime above the Widom line (WL), where LDL at the peak temperature was roughly one fourth of the total. The stretched exponent depicting the polarizability anisotropy relaxation was found to be insensitive to temperature, consistent with the experimental results. Above the WL, all relaxation times studied displayed a power-law temperature dependence with a consistent singular temperature for each model. The inverse relaxation times showed exponential functions of two-body excess entropy due to translational motions, where the entropy exhibited a logarithmic temperature behavior with a singular temperature close to that of relaxation time. This result leads to a conclusion that excess entropy is a quantity to describe dynamic relaxation of supercooled water in the thermodynamic region where the mode-coupling theory works. The water structure that causes the two-body excess entropy is illustrated and the contributions of HDL, LDL, and their mixing are also shown, where the mixing contributes significantly as near the WL. Below the WL, the formula based on the two-body excess entropy may no longer be valid.

Dynamical crossover and its connection to the Widom line in supercooled TIP4P/Ice water.

We perform molecular dynamics simulations with the TIP4P/Ice water model to characterize the relationship between dynamics and thermodynamics of liquid water in the supercooled region. We calculate the relevant properties of the phase diagram, and we find that TIP4P/Ice presents a retracing line of density maxima, similar to what was previously found for atomistic water models and models of other tetrahedral liquids. For this model, a liquid-liquid critical point between a high-density liquid and a low-density liquid was recently found. We compute the lines of the maxima of isothermal compressibility and the minima of the coefficient of thermal expansion in the one phase region, and we show that these lines point to the liquid-liquid critical point while collapsing on the Widom line. This line is the line of the maxima of correlation length that emanates from a second order critical point in the one phase region. Supercooled water was found to follow mode coupling theory and to undergo a transition from a fragile to a strong behavior right at the crossing of the Widom line. We find here that this phenomenology also happens for TIP4P/Ice. Our results appear, therefore, to be a general characteristic of supercooled water, which does not depend on the interaction potential used, and they reinforce the idea that the dynamical crossover from a region where the relaxation mechanism is dominated by cage relaxation to a region where cages are frozen and hopping dominates is correlated in water to a phase transition between a high-density liquid and a low-density liquid.

Free energy convergence in short- and long-length hydrophobic hydration

We show, through molecular dynamics, that the hydration free energy of model hydrocarbons, exhibits a convergence temperature in supercooled water, TG∗ ~ 255 K ± 10 K, around which, hydration entropy and enthalpy minima are observed. Further, although entropy convergence at TS∗ ~ 400 K ± 10 K is found to be violated by large non-spherical hydrocarbons, due to “folding”, a free energy maximum is still observed, opposite to hard spheres. Evidence is provided that the entropy and enthalpy minima are associated with shell-bulk water structural differences, also observed between low-density liquid and high-density liquid water, around TG∗, specifically, the occurrence of an orientational order maximum.

Cold denaturation induced helix-to-helix transition and its implication to activity of helical antifreeze protein

Study of cold denaturation is relevant for understanding the freeze tolerant activity of organism at sub-zero environment. The proteins responsible for the survival of these organisms contain abundance of alpha-helix. Here we study the effect of cold temperature on the alpha-helical region of a widely used model system, Trp-cage miniprotein by means of extensive atomistic molecular dynamics simulations in conjunction with replica exchange scheme. We find a helix-to-helix transition when the usual ~3.59 alpha-helix transforms to ~3.7 helix upon cooling below ~230 K. This transition is primarily driven by enthalpy gain from water-water interactions with the significant enhancement of low-density liquid (LDL)-like water at cold temperature. Because the transition from ~3.59-helix to ~3.7-helix occurs at cold temperature, it may be accompanied by cold denaturation of protein. Like the cold denatured state of Trp-cage, type-1 antifreeze proteins (AFP) also have ~3.7 or more residues per helical turn. However, unlike Trp-cage, AFP has the 3.7-helix in its biological active form. Such helical arrangement in AFP facilitates the hydrogen bonding with the surrounding clathrate or ice-like water molecules, which is crucial for the antifreeze activity of AFP.

Two Liquid-Liquid Phase Transitions in Confined Water Nanofilms.

The stories behind supercooled bulk and confined water can be different. Bulk water has a metastable liquid-liquid phase transition at deeply supercooled conditions, but the existence of such a phenomenon in confined water is in question. Herein we show simulation results of first-order phase transitions between high- and low-density liquid (HDL and LDL) in confined water in both positive and negative pressures. A mid-density state between these two local states appears, which lets the transition show the hysteresis loop with transiently stable intermediate states. On the basis of Landau theory that we have adapted for mixing of moieties with high- and low-density states, we explain the phase transitions with the order parameter-dependent free energy change which is governed by second- to higher-order interactions among those moieties.

Structure of levitated Si–Ge melts studied by high-energy x-ray diffraction in combination with reverse Monte Carlo simulations

The short-range order in liquid Si, Ge and binary Si x –Ge1−x alloys (x = 0.25, 0.50, 0.75) was studied by x-ray diffraction and reverse Monte Carlo simulations. Experiments were performed in the normal and supercooled liquid states by using the containerless technique of aerodynamic levitation with CO2 laser heating, enabling deeper supercooling of liquid Si and Si–Ge alloys than previously reported. The local atomic structure of liquid Si and Ge resembles the β-tin structure. The first coordination numbers of about 6 for all compositions are found to be independent of temperature indicating the supercooled liquids studied retain this high-density liquid (HDL) structure. However, there is evidence of developing local tetrahedral ordering, as manifested by a shoulder on the right side of the first peak in S(Q) which becomes more prominent with increasing supercooling. This result is potentially indicative of a continuous transition from the stable HDL β-tin (high pressure) phase, towards a metastable low-density liquid phase, reminiscent of the diamond (ambient pressure) structure.

Thorough Analysis of the Phase Diagram for the Bell–Lavis Model: An Entropic Simulational Study

In this work, we investigate the Bell-Lavis model using entropic simulations for several values of the energy parameters. The $T\times\mu$ phase diagram and the ground state configurations are analyzed thoroughly. Besides, we examine the particle density and specific heat behavior for different values of the chemical potential $\mu$ as functions of temperature. We also obtain configurations that maximize the canonical probability for several values of chemical potential and temperature, enabling the identification of the low density ($LDL$) and high-density liquid ($HDL$) phases, among others, in the critical regions. We found a second-order phase transition from the $LDL-HDL_0$ to $LDL-HDL$ coexistence in the range of $0<\mu<1.05503$. In the $1.05503<\mu<1.48024$ range, the transition between the $LDL-HDL_0$ and $LDL-HDL_0-empty$ coexistence presents discontinuous and continuous transitions characteristics. Finally, for $1.48024<\mu<1.5$, the phase transition between $LDL$ and $empty$ phases is of first-order.

Formation of a Low-Density Liquid Phase during the Dissociation of Gas Hydrates in Confined Environments.

The large amounts of natural gas in a dense solid phase stored in the confined environment of porous materials have become a new, potential method for storing and transporting natural gas. However, there is no experimental evidence to accurately determine the phase state of water during nanoscale gas hydrate dissociation. The results on the dissociation behavior of methane hydrates confined in a nanosilica gel and the contained water phase state during hydrate dissociation at temperatures below the ice point and under atmospheric pressure are presented. Fourier transform infrared spectroscopy (FTIR) and powder X-ray diffraction (PXRD) were used to trace the dissociation of confined methane hydrate synthesized from pore water confined inside the nanosilica gel. The characterization of the confined methane hydrate was also analyzed by PXRD. It was found that the confined methane hydrates dissociated into ultra viscous low-density liquid water (LDL) and methane gas. The results showed that the mechanism of confined methane hydrate dissociation at temperatures below the ice point depended on the phase state of water during hydrate dissociation.

Relations between thermodynamics, structures, and dynamics for modified water models in their supercooled regimes.

We use molecular dynamics simulations to study relations between thermodymamic, structural, and dynamical properties of TIP4P/2005 water models with systematically reduced partial charges and, thus, weaker hydrogen bonds. Observing a crossing of isochores in the P-T diagram, we show that these water-like models have a readily accessible liquid-liquid critical point (LLCP) associated with a transition between high-density liquid (HDL) and low-density liquid (LDL) forms and determine the dependence of the critical temperature Tc, pressure Pc, and density ρc on the charge-scaling factor from fits to a two-structure equation of states. The results indicate that the water-like models exhibit liquid polyamorphism in a wide range of interaction parameters. Considering elongated systems, we observe a decomposition into extended and stable HDL-like and LDL-like regions at appropriate pressures and low temperatures and analyze the respective structural and dynamical properties. We show that the diverse local order results in very different correlation times of local dynamics, while the fragility is hardly changed. The results yield insights into the origin of a dynamical crossover, which is observed when lowering the temperature along isobars and was previously interpreted in terms of a fragile-to-strong transition. Our findings imply that the effect does not involve two liquid phases with an exceptionally large difference of the fragility but rather a high temperature dependence near the LLCP results from a rapid conversion from HDL-like environments with faster dynamics to LDL-like ones with slower dynamics.

Experimental observation of the liquid-liquid transition in bulk supercooled water under pressure.

We prepared bulk samples of supercooled liquid water under pressure by isochoric heating of high-density amorphous ice to temperatures of 205 ± 10 kelvin, using an infrared femtosecond laser. Because the sample density is preserved during the ultrafast heating, we could estimate an initial internal pressure of 2.5 to 3.5 kilobar in the high-density liquid phase. After heating, the sample expanded rapidly, and we captured the resulting decompression process with femtosecond x-ray laser pulses at different pump-probe delay times. A discontinuous structural change occurred in which low-density liquid domains appeared and grew on time scales between 20 nanoseconds to 3 microseconds, whereas crystallization occurs on time scales of 3 to 50 microseconds. The dynamics of the two processes being separated by more than one order of magnitude provides support for a liquid-liquid transition in bulk supercooled water.

Liquid-liquid transitions under pressure

Water Phases Theoretical simulations suggest that deeply supercooled water undergoes a transition between high- and low-density forms, but this transition is difficult to study experimentally because it occurs under conditions in which ice crystallization is extremely rapid. Kim et al. combined x-ray lasers for rapid structure determination with infrared femtosecond pulses for rapid heating of amorphous ice layers formed at about 200 kelvin. The heating process created high-density liquid water at increased pressures. As the layer expanded and decompressed, low-density liquid domains appeared and grew on time scales between 20 nanoseconds and 3 microseconds, which was much faster than competing ice crystallization. Science , this issue p. [978][1] [1]: /lookup/doi/10.1126/science.abb9385

Some Aspects of the Liquid Water Thermodynamic Behavior: From The Stable to the Deep Supercooled Regime.

Liquid water is considered to be a peculiar example of glass forming materials because of the possibility of giving rise to amorphous phases with different densities and of the thermodynamic anomalies that characterize its supercooled liquid phase. In the present work, literature data on the density of bulk liquid water are analyzed in a wide temperature-pressure range, also including the glass phases. A careful data analysis, which was performed on different density isobars, made in terms of thermodynamic response functions, like the thermal expansion and the specific heat differences , proves, exclusively from the experimental data, the thermodynamic consistence of the liquid-liquid transition hypothesis. The study confirms that supercooled bulk water is a mixture of two liquid “phases”, namely the high density (HDL) and the low density (LDL) liquids that characterize different regions of the water phase diagram. Furthermore, the isobars behaviors clearly support the existence of both a liquid–liquid transition and of a liquid–liquid critical point.

Supercooled water structures

Water Structure Water displays a number of anomalous properties that are further enhanced in its supercooled state, but experimental studies at ambient pressure must obtain data before the onset of rapid crystallization at temperatures below ∼240 kelvin. Kringle et al. obtained infrared spectra of supercooled water films at temperatures between 135 and 235 kelvin that formed for a few nanoseconds by ultrafast heating and cooling. Supercooled water thermally equilibrates before crystallization above 170 kelvin, and over the range of temperatures studied, the structure of water was shown to be a linear combination of a high-density and a low-density liquid. Science , this issue p. [1490][1] [1]: /lookup/doi/10.1126/science.abb7542

Reversible structural transformations in supercooled liquid water from 135 to 245 K.

A fundamental understanding of the unusual properties of water remains elusive because of the limited data at the temperatures and pressures needed to decide among competing theories. We investigated the structural transformations of transiently heated supercooled water films, which evolved for several nanoseconds per pulse during fast laser heating before quenching to 70 kelvin (K). Water's structure relaxed from its initial configuration to a steady-state configuration before appreciable crystallization. Over the full temperature range investigated, all structural changes were reversible and reproducible by a linear combination of high- and low-temperature structural motifs. The fraction of the liquid with the high-temperature motif decreased rapidly as the temperature decreased from 245 to 190 K, consistent with the predictions of two-state "mixture" models for supercooled water in the supercritical regime.

Unraveling liquid polymorphism in silicon driven out-of-equilibrium.

Using nonequilibrium molecular dynamics simulations, we study the properties of supercooled liquids of Si under shear at T = 1060 K over a range of densities encompassing the low-density liquid (LDL) and high-density liquid (HDL) forms. This enables us to generate nonequilibrium steady-states of the LDL and HDL polymorphs that remain stabilized in their liquid forms for as long as the shear is applied. This is unlike the LDL and HDL forms at rest, which are metastable under those conditions and, when at rest, rapidly undergo a transition toward the crystal, i.e., the thermodynamically stable equilibrium phase. In particular, through a detailed analysis of the structural and energetic features of the liquids under shear, we identify the range of densities, as well as the range of shear rates, which give rise to the two forms. We also show how the competition between shear and tetrahedral order impacts the two-body entropy in steady-states of Si under shear. These results open the door to new ways of utilizing shear to stabilize forms that are metastable at rest and can exhibit unique properties, since, for instance, experiments on Si have shown that HDL is metallic with no bandgap, while LDL is semimetallic with a pseudogap.

Decoding a Percolation Phase Transition of Water at ∼330 K with a Nanoparticle Ruler.

Liquid water, despite its simple molecular structure, remains one of the most fascinating and complex substances. Most notably, many questions continue to exist regarding the phase transitions and anomalous properties of water, which are subtle to observe experimentally. Here, we report a sharp transition in water at 330 K unveiled through experimental measurements of the instantaneous Brownian velocity of NaYF4:Yb/Er upconversion nanoparticles in water. Our experimental investigations, corroborated by molecular dynamics simulations, elucidate a geometrical phase transition where a low-density liquid (LDL) clusters become percolated below 330 K. Around this critical temperature, we find the sizes of the LDL clusters to be similar to those of the nanoparticles, confirming the role of the upconversion nanoparticle as a powerful ruler for measuring the extensiveness of the LDL hydrogen-bond network and nanometer-scale spatial changes (20-100 nm) in liquids. Additionally, a new order parameter that unequivocally classifies water molecules into two local geometric states is introduced, providing a new tool for understanding and modeling water's many anomalous properties and phase transitions.