High-density Liquid Research Articles

Hydration level dependent dynamics of supercooled water confined in water harvester metal organic framework-303

Crystallization of bulk water cannot be avoided in the temperature range 150–235 K, which is known as no man’s land in the literature focused on the dynamics of the supercooled water. Dynamics of the supercooled water in this temperature range can be accessed by confining water in the nanopores. In the present study, we have investigated dynamics of the supercooled water confined in the pore network of a water harvester metal organic framework (MOF-303) at two different hydration levels (63.4 and 37.0 %). The framework was chosen as a confining host, because the hydrogen bonded–structure of the confined water molecules during adsorption–desorption (i.e. at different hydration levels) is explicitly known. All the relaxation processes observed in the supercooled region are comparatively slower than the bulk–like water dynamics due to the modified hydrogen-bonded structure and the interactions with the pore walls. The confined water, not directly interacting with the pore walls, undergoes a liquid-to-liquid transition (Ts) at ∼186 K followed by its glassy freezing with glass transition temperature Tg ∼ 108 K which is consistent with high density liquid phase of amorphous ice. Both Ts as well as Tg are observed to increase with the decrease in hydration level. It confirmed that the evolved hydrogen-bonded network and the interactions with the pore walls play deterministic roles in the glassy freezing of the confined water. No crystallization as well as the glassy freezing of the water molecules directly interacting with the pore walls through the hydrogen bonding is observed at both the hydration level. The observed relaxation processes corresponding to these molecules in the supercooled region can be attributed to rearrangement of their hydrogen-bonded structure.

Two-state free-volume model for anomalous dynamics in supercooled water macromolecules undergoing liquid–liquid phase transition

Modelling anomalous dynamics of complex liquid water is a huge challenge due to its various condensed molecular structures and the associated liquid–liquid phase transitions (LLPTs). In this study, we considered, for the first time, the influences of free volume and entropy on LLPTs for both low-density liquid (LDL) and high-density liquid (HDL) waters. Firstly, we proposed a two-state free-volume model and combined with the Adam-Gibbs models to investigate the dynamic equilibria with free diffusion coefficient, viscosity, glass transition temperature and the anomalous dynamics in the two-state water. Free-energy equation was then developed to explore the constitutive relationships of free volume, entropy and anomalously dynamic behaviors. Finally, analytical results of the proposed two-state free-volume model were compared with the experimental data of two-state water reported in literature, and good agreements between them were demonstrated. This study offers a new physical insight into free volume for the anomalous dynamics and LLPTs in the two-state water.

Thermodynamics, phase diagram, structure and anomalies of supercooled aqueous solutions of trehalose: A molecular dynamics study

We perform molecular dynamics simulations of supercooled solutions of trehalose in TIP4P/2005 water, with concentrations of 20 and 40 wt% (weight percentage) in trehalose, a disaccharide often used in cryopreserving solutions. We analyze the thermodynamics and the structure of these solutions. From the isochores we find that in the 20 wt% solution the temperature of maximum density (TMD) line is still present and it is shifted to lower temperatures with respect to that of pure water. We also find that the Widom line and the liquid-liquid critical point (LLCP) are still present for this concentration and we estimate the position of the LLCP to be shifted at lower temperatures and slightly lower pressures with respect to that of pure water. In the 40 wt% solution we find that the TMD line still persists, but the region of density anomaly shrinks. No LLCP is observed down to the lowest temperature investigated, and maxima of the isothermal compressibility, used as proxy for the Widom line, become weaker. The water oxygen - water oxygen radial distribution functions keep on showing the typical distinction between high density and low density liquid found in pure water, but at low temperatures the presence of trehalose appears to favor the high density phase of water more than in pure water. The number of water molecules of the solvation shell confirms this picture.

Two state model for the ML-BOP potential

The coarse-grained machine-learning derived ML-BOP model [Chan et al., Nature Commun. 10, 379 (2019)] provides a monoatomic representation of the water-water interaction potential in which orientational interactions are included as three-body contributions. Despite its simplicity, the model reproduces the phase diagram of water and its anomalies. Here, we show that a two-state Gibbs free energy expression – fitted simultaneously on the temperature and pressure dependence of the density and internal energy – predicts the existence of a liquid–liquid critical point, with critical parameters consistent with previous estimates. We also show that in this model: (i) while the low density liquid is pre-empted by crystal nucleation, the high-density liquid and its spinodal are accessible in numerical studies down to 100 K; (ii) crystallisation requires the presence of a local low density region. Thus, for densities larger than the critical density, spinodal decomposition (or nucleation of the low-density liquid) is a pre-requisite for ice nucleation.

Techno-economic, energy, and environmental impact assessment of hydrogen supply chain: A comparative study of large-scale production and long-distance transportation

Hydrogen energy has made significant progress as one of the technological pathways that can facilitate the green transformation of various sectors, including the chemical industry, steel production, transportation, and power generation. However, areas with high demand for hydrogen are typically located thousands of kilometers away from large-scale production facilities. Hydrogen transported from the most cost-competitive large production sites to areas that lack hydrogen resources requires converting gaseous hydrogen into a high-density liquid. Thus, global market trade is important for hydrogen carriers in long-distance and large-scale transportation. In this study, liquefied hydrogen (LH2) and ammonia (NH3), which are hydrogen-based energy carriers, are analyzed and compared in terms of economic costs, energy efficiency, and carbon dioxide (CO2) emissions. It has been demonstrated that the LH2 supply chain is more energy-efficient and has higher CO2 emissions compared to the NH3 supply chain. Furthermore, this study shows that the levelized cost of hydrogen transportation (LCoHT) delivered from Australia to Ningbo, China, is lower for NH3 (19.95 yuan/kg-H2) compared to LH2 (22.83 yuan/kg-H2). Meanwhile, the LCoHT for the two supply chains is in a similar range (27.82 yuan/kg-H2 and 21.53 yuan/kg-H2 for LH2 and NH3, respectively) from Norway to Ningbo, China. The impacts of important parameters on the LCoHT, energy efficiency, and CO2 emissions of the LH2/NH3 supply chain are also considered through a sensitivity analysis.

Models to predict configurational adiabats of Lennard-Jones fluids and their transport coefficients.

A comparison is made between three simple approximate formulas for the configurational adiabat (i.e., constant excess entropy, sex) lines in a Lennard-Jones (LJ) fluid, one of which is an analytic formula based on a harmonic approximation, which was derived by Heyes et al. [J. Chem. Phys. 159, 224504 (2023)] (analytic isomorph line, AIL). Another is where the density is normalized by the freezing density at that temperature (freezing isomorph line, FIL). It is found that the AIL formula and the average of the freezing density and the melting density ("FMIL") are configurational adiabats at all densities essentially down to the liquid-vapor binodal. The FIL approximation departs from a configurational adiabat in the vicinity of the liquid-vapor binodal close to the freezing line. The self-diffusion coefficient, D, shear viscosity, ηs, and thermal conductivity, λ, in macroscopic reduced units are essentially constant along the AIL and FMIL at all fluid densities and temperatures, but departures from this trend are found along the FIL at high liquid state densities near the liquid-vapor binodal. This supports growing evidence that for simple model systems with no or few internal degrees of freedom, isodynes are lines of constant excess entropy. It is shown that for the LJ fluid, ηs and D can be predicted accurately by an essentially analytic procedure from the high temperature limiting inverse power fluid values (apart from at very low densities), and this is demonstrated quite well also for the experimental argon viscosity.

Liquid-Liquid Transition in a Bose Fluid near Collapse.

Discovering novel emergent behavior in quantum many-body systems is a main objective of contemporary research. In this Letter, we explore the effects on phases and phase transitions of the proximity to a Ruelle-Fisher instability, marking the transition to a collapsed state. To accomplish this, we study by quantum MonteCarlo simulations a two-dimensional system of soft-core bosons interacting through an isotropic finite-ranged attraction, with a parameter η describing its strength. If η exceeds a characteristic value η_{c}, the thermodynamic limit is lost, as the system becomes unstable against collapse. We investigate the phase diagram of the model for η≲η_{c}, finding-in addition to a liquid-vapor transition-a first-order transition between two liquid phases. Upon cooling, the high-density liquid turns superfluid, possibly above the vapor-liquid-liquid triple temperature. As η approaches η_{c}, the stability region of the high-density liquid is shifted to increasingly higher densities, a behavior at variance with distinguishable quantum or classical particles. Finally, for η larger than η_{c} our simulations yield evidence of collapse of the low-temperature fluid for any density; the collapsed system forms a circular cluster whose radius is insensitive to the number of particles.

Melting Curve of Black Phosphorus: Evidence for a Solid-Liquid-Liquid Triple Point.

Black phosphorus (bP) is a crystalline material that can be seen as an ordered stacking of two-dimensional layers, which results in outstanding anisotropic physical properties. The knowledge of its pressure (P)-temperature (T) phase diagram, and in particular, of its melting curve is fundamental for a better understanding of the synthesis and stability conditions of this element. Despite the numerous studies devoted to this subject, significant uncertainties remain regarding the determination of the position and slope of its melting curve. Here we measured the melting curve of bP in an extended P, T region from 0.10(3) to 5.05(40) GPa and from 914(25) to 1788(70) K, using in situ high-pressure and high-temperature synchrotron X-ray diffraction. We employed an original metrology based on the anisotropic thermoelastic properties of bP to accurately determine P and T. We observed a monotonic increase of the melting temperature with pressure and the existence of two distinct linear regimes below and above 1.35(15) GPa, with respective slopes of 348 ± 21 and of 105 ± 12 K·GPa-1. These correspond to the melting of bP toward the low-density liquid and the high-density liquid, respectively. The triple point at which solid bP and the two liquids meet is located at 1.35(15) GPa and 1350(25) K. In addition, we have characterized the solid phases after crystallization of the two liquids and found that, while the high-density liquid transforms back to solid bP, the low-density liquid crystallizes into a more complex, partly crystalline and partly amorphous solid. The X-ray diffraction pattern of the crystalline component could be indexed as a mixture of red and violet P.

Advancing the process understanding and localization in jet electrochemical machining with electrolyte confinement through fluid dynamic simulations and experiments

Functionality-oriented micro-structures, such as micro-dimples and grooves, are widely used in tribology and heat transfer. Jet electrochemical machining (JEM) is effective to fabricate micro-structures on metallic surfaces. However, in traditional JEM, the unrestricted electrolyte flowing can induce stray corrosion on workpiece, and thus, both surface quality and machining localization are reduced. In this paper, a novel electrolyte-confinement technique is proposed for JEM, a high-density liquid (perfluorotripropylamine, FTPA) is used to confine the electrolyte flowing region on workpiece when electrolyte exits nozzle, facilitating reduction in stray corrosion on workpiece and overcut of micro-structures. A multi-physics model including two-phase flow field and electric field is developed to analyze the electrolyte confined by FTPA, and both simulation and observation results show that the area of electrolyte flowing on the workpiece is confined well by FTPA, and the current density distribution becomes concentrated, which enhances the machining localization. Compared to traditional JEM, the etch factor of micro-dimple is improved by 2.5 times and there is no stray corrosion. The material removal rate is increased due to the concentration of current distribution on the workpiece surface. Furthermore, profile evolution of micro-dimples revealed that with feed depth increased, FTPA could flow into the micro-dimple to protect the sidewall from continuous dissolution, thus forming vertical sidewall. Additionally, electrolyte flowing region is still confined during the scanning motion of nozzle, and the etch factor increases from 0.41 to 8.8 compared to traditional JEM. Moreover, increasing inter-electrode gap could reduce electrolyte flowing region on workpiece, further enhancing machining localization.

The structure of water from hot to supercooled temperatures based on analysis of experimental X-ray scattering data

An increasing number of studies support the existence of a liquid–liquid critical point (LLCP) in water, implying that under ambient conditions, water is a supercritical mixture of low-density (LDL) and high-density liquid (HDL) structures. Here, we analyse X-ray scattering data in terms of this model. Small-angle X-ray scattering data shows that anomalies are seen at temperatures as high as 360 K. By assuming the local structure in LDL-domains resembles that of the corresponding amorphous solid, we estimate the population of LDL-domains at different temperatures from pair-correlation functions (PCFs). By subtracting the LDL contribution, we isolate the PCF of the HDL component. The temperature-dependent HDL-PCFs show a feature around 4 Å in the supercooled regime, interpreted as interstitials resulting from collapse of the second coordination shell. We define the first coordination shell as neighbours up to 3.3 Å, based on an isosbestic point in the running coordination number. Our analysis shows that the HDL-domains have on average two strongly and two weakly H-bonded neighbours. The LDL population in TIP4P/2005-water is less temperature dependent than in real water, and the HDL component contains more structures with three strongly H-bonded neighbours, leading to a slight overestimation of the first peak of the PCF.

Early prediction of spinodal-like relaxation events in supercooled liquid water.

Several computational studies on different water models reported evidence of a phase transition in supercooled conditions between two liquid states of water differing in density: the high-density liquid (HDL) and the low-density liquid (LDL). Yet, conclusive experimental evidence of the existence of a phase transition between the two liquid water phases could not be obtained due to fast crystallization in the region where the phase transition should occur. For the same reason, the investigation of possible transition mechanisms between the two phases is committed to computational investigations. In this work, we simulate an out-of-equilibrium temperature-induced transition from the LDL to the HDL-like state in the TIP4P/2005 water model. To structurally characterize the system relaxation, we use the node total communicability (NTC) we recently proposed as an effective order parameter to discriminate the two liquid phases differing in density. We find that the relaxation process is compatible with a spinodal-like scenario. We observe the formation of HDL-like domains in the LDL phase and we characterize their fluctuating behavior and subsequent coarsening and stabilization. Furthermore, we find that the formation of stable HDL-like domains is favored in the regions where the early formation of small patches of highly connected HDL-like molecules (i.e., with very high NTC values) is observed. Besides characterizing the LDL- to HDL-like relaxation from a structural point of view, these results also show that the NTC order parameter can serve as an early-time predictor of the regions from which the transition process initiates.

Unveiling a common phase transition pathway of high-density amorphous ices through time-resolved x-ray scattering.

Here, we investigate the hypothesis that despite the existence of at least two high-density amorphous ices, only one high-density liquid state exists in water. We prepared a very-high-density amorphous ice (VHDA) sample and rapidly increased its temperature to around 205 ± 10K using laser-induced isochoric heating. This temperature falls within the so-called "no-man's land" well above the glass-liquid transition, wherein the IR laser pulse creates a metastable liquid state. Subsequently, this high-density liquid (HDL) state of water decompresses over time, and we examined the time-dependent structural changes using short x-ray pulses from a free electron laser. We observed a liquid-liquid transition to low-density liquid water (LDL) over time scales ranging from 20ns to 3 μs, consistent with previous experimental results using expanded high-density amorphous ice (eHDA) as the initial state. In addition, the resulting LDL derived both from VHDA and eHDA displays similar density and degree of inhomogeneity. Our observation supports the idea that regardless of the initial annealing states of the high-density amorphous ices, the same HDL and final LDL states are reached at temperatures around 205K.

Simulating the Lyotropic Phase Behavior of a Partially Self-Complementary DNA Tetramer.

DNA oligomers in solution have been found to develop liquid crystal phases via a hierarchical process that involves Watson-Crick base pairing, supramolecular assembly into columns of duplexes, and long-range ordering. The multiscale nature of this phenomenon makes it difficult to quantitatively describe and assess the importance of the various contributions, particularly for very short strands. We performed molecular dynamics simulations based on the coarse-grained oxDNA model, aiming to depict all of the assembly processes involved and the phase behavior of solutions of the DNA GCCG tetramers. We find good quantitative matching to experimental data at both levels of molecular association (thermal melting) and collective ordering (phase diagram). We characterize the isotropic state and the low-density nematic and high-density columnar liquid crystal phases in terms of molecular order, size of aggregates, and structure, together with their effects on diffusivity processes. We observe a cooperative aggregation mechanism in which the formation of dimers is less thermodynamically favored than the formation of longer aggregates.

Continuous casting process based on an optimized submerged entry nozzle

Continuous casting process is an important part in the steel manufacturing process. The slab production quality is commonly influenced by mould and flow pattern of submerged entry nozzles (SEN). Hence, the present work aimed to attain a more liquid flow rate to increase the slab production quantity within a certain period. Moreover, a novel African Buffalo Algorithm (ABA) is presented in this work to optimise the SEN parameters and the SEN was designed in the MATLAB environment. The developed ABA model optimises the design parameters of SEN such as the size of the nozzle, the shape of the port, length, submergence depth, etc. The fitness of buffalo effectively optimises the SEN parameters and by utilising the optimised design parameters, the SEN was resigned in the MATLAB to identify the proposed framework's effectiveness. The results demonstrated that the liquid flow rate is increased and surface tension gets minimised. When the flow rate of liquid grows, the wave amplitude also upturns. The effect of liquid and air flow rate, port shape and size, maximum wave amplitude, surface tension, etc., were calculated with 0° parallel port. In addition, the presented SEN's results were compared with other existing SEN to determine the optimised SEN's effectiveness. The comparison result demonstrated that the designed optimised SEN has a high liquid flow rate, casting speed, density and viscosity.

Liquid–liquid transition and ice crystallization in a machine-learned coarse-grained water model

Mounting experimental evidence supports the existence of a liquid-liquid transition (LLT) in high-pressure supercooled water. However, fast crystallization of supercooled water has impeded identification of the LLT line TLL(p) in experiments. While the most accurate all-atom (AA) water models display a LLT, their computational cost limits investigations of its interplay with ice formation. Coarse-grained (CG) models provide over 100-fold computational efficiency gain over AA models, enabling the study of water crystallization, but have not yet shown to have a LLT. Here, we demonstrate that the CG machine-learned water model Machine-Learned Bond-Order Potential (ML-BOP) has a LLT that ends in a critical point at pc = 170 ± 10 MPa and Tc = 181 ± 3 K. The TLL(p) of ML-BOP is almost identical to the one of TIP4P/2005, adding to the similarity in the equation of state of liquid water in both models. Cooling simulations reveal that ice crystallization is fastest at the LLT and its supercritical continuation of maximum heat capacity, supporting a mechanistic relationship between the structural transformation of water to a low-density liquid (LDL) and ice formation. We find no signature of liquid-liquid criticality in the ice crystallization temperatures. ML-BOP replicates the competition between formation of LDL and ice observed in ultrafast experiments of decompression of the high-density liquid (HDL) into the region of stability of LDL. The simulations reveal that crystallization occurs prior to the coarsening of the HDL and LDL domains, obscuring the distinction between the highly metastable first-order LLT and pronounced structural fluctuations along its supercritical continuation.

Isocompositional liquid-liquid transition in dilute aqueous LiCl solutions

We here demonstrate that small amounts of LiCl dissolved in water extend the existence window of deeply supercooled liquid water. This shift allows us to observe the isocompositional, sharp liquid-liquid transition (LLT), while this is not possible in pure water. Upon heating at ambient pressure, the hyperquenched and densified glass first turns into the high-density liquid (HDL) and then experiences the LLT to the low-density liquid (LDL) at 137–141 K and ambient pressure. At the LLT the viscosity suddenly jumps up by an order of magnitude, from ∼4.3×109Pas in HDL to ∼3.7×1010Pas in LDL for the 3.1 mol % solution based on our calorimetric analysis. That is, the LLT takes place clearly in the ultraviscous liquid domain at ambient pressure. By contrast, the viscosity of the emerging LDL at the LLT is still in the domain of the soft glass for pure water and for solutions exceeding 5 mol % LiCl that were studied in the past. This is owing to our observation that, first, the LDL's glass transition is lowered by about 8 K to 126 K in the presence of small amounts LiCl, whereas the HDL's glass transition remains at 118 K. Second, the stability of HDL at ambient pressure is increased by high-pressure annealing, shifting the LLT to higher temperatures. Published by the American Physical Society 2024

Ultrafine Pt particles on hollow hierarchical porous β zeolites for selective hydrogenation of naphthalene

It is crucial to develop effective heterogeneous catalysts for the production of decalin as a high-density liquid hydrogen storage material from naphthalene hydrogenation. In this work, hollow hierarchical Pt/Hβ catalysts with ultrafine Pt particles were successfully synthesized successfully. The construction of hollow cavity optimized the metal-support interaction and reduced the resistance of mass diffusion. The ultrafine Pt species could expose more active sites for effective hydrogenation of naphthalene. The optimized Pt/Hβ-2 with hollow structure catalyst displayed the remarkable catalytic performance with the naphthalene conversion of 94.2 % and the decalin yield of 81.4 % at 220 °C, which was superior to the reaction performance of the catalysts reported in the literature under similar reaction condition. Besides, Pt/Hβ-2 presents higher turnover frequency (25.4 h−1) and reaction rate constant (9.9 × 10-3 min−1), which is because of the relatively low activation energy (30.86 kJ mol−1) for the naphthalene hydrogenation.

Molecular dynamics study on relaxation of supercooled liquid water at different cooling rates

The relaxation in supercooled liquids is a phenomena by which the thermodynamic system starts transforming from one state to other after losing its high end thermodynamic properties (i.e. density, potential energy, enthalpy, etc.) with a significant change. Since the relaxation phenomena is quite challenging to observe, specifically near the transition region of liquid water. Hence, the thermodynamic analysis for understanding the relaxation phenomena of liquid water in supercooled region from the high density liquid (HDL) state to low density amorphous (LDA) state are important from both fundamental and as well as applied perspectives. Therefore, we focused our study on performing thermodynamic analysis on the relaxation of super-cooled liquid water by using a monatomic water (mW) potential model with an isothermal-isobaric (NPT) molecular dynamics (MD) simulation technique at zero pressure under different cooling rates. In this work, we examine the relaxation in terms of root mean square (RMS) fluctuation of two and three-body potential energy of the maximum size of four coordinated cluster under different cooling rates. The predicted data over this study shows the independency of the cooling rate, where the relaxation of the liquid state of mW water results in a sharp irreversible drop in potential energy and large fluctuation in two and three body potential energy of the maximum size of four coordinated cluster. Further, we found the relaxation of liquid state of mW water associated with the sharp and continuous increase in the fraction of four coordinated particles as a function of temperature for different cooling rates. The predicted results of this study are consistent with the literature, which helps to understand the relaxation phenomena of mW water and other such thermodynamic systems more significantly.

Deciphering Density Fluctuations in the Hydration Water of Brownian Nanoparticles via Upconversion Thermometry.

We investigate the intricate relationship among temperature, pH, and Brownian velocity in a range of differently sized upconversion nanoparticles (UCNPs) dispersed in water. These UCNPs, acting as nanorulers, offer insights into assessing the relative proportion of high-density and low-density liquid in the surrounding hydration water. The study reveals a size-dependent reduction in the onset temperature of liquid-water fluctuations, indicating an augmented presence of high-density liquid domains at the nanoparticle surfaces. The observed upper-temperature threshold is consistent with a hypothetical phase diagram of water, validating the two-state model. Moreover, an increase in pH disrupts the organization of water molecules, similar to external pressure effects, allowing simulation of the effects of temperature and pressure on hydrogen bonding networks. The findings underscore the significance of the surface of suspended nanoparticles for understanding high- to low-density liquid fluctuations and water behavior at charged interfaces.

A close look at the hydration layer and at the premelting layer of K-feldspar

In this work, we perform molecular dynamics simulations to investigate the structural properties of water at the interface with K-feldspar. We aim at understanding the modifications induced on water structure by the presence of this mineral. We analysed both the hydration layer of K-feldspar in supercooled water and the premelting layer at the interface between K-feldspar and hexagonal ice. We studied the density profile, the orientational tetrahedral order parameter, the distribution of the cos ⁡ ( γ ) function, the hydrogen bond distribution, the number of water molecules belonging to different phases close to the surface of the feldspar. We focused on the interplay between high-density water and low-density water to inquire on local modifications of water structure with respect to the bulk phase. We investigated four different pressures where the structure of bulk water spans from high-density liquid (locally more disordered) to low-density liquid (locally more ordered). The investigation showed that both the hydration and the premelting layers present a low-density liquid water component and similarities as a function of pressure with the corresponding bulk. We also find that the premelting layer shows structural features intermediate between the liquid and the crystalline state acting as a matching layer.