The increase of fossil fuels use is responsible for the increased greenhouse gas emissions which have a great impact on the environment leading to global warming. Consequently, substantial efforts are devoted to decrease our dependence on fossil fuels and there is a growing interest in harnessing the chemical energy from other carbon neutral sources like biomass. Supercritical water gasification (SCWG), which is a catalyst-mediated process, provides a promising method to transform organic streams and wastes into fuels due to the utilization of water as a non-toxic green solvent. However, the process is limited by sulfur impurities that deactivate the catalysts required for efficient conversion. Τhe optimization of SCWG remains challenging because the solvent properties of aqueous mixtures in the supercritical regime are unknown, and they have significant differences while crossing the Widom line, which separates liquid- and gas-like states. In this study, we propose a framework to determine accurate thermodynamic properties for complex systems under supercritical conditions, where experimental data are scarce. We demonstrate that a methodology combining structural characterization experiments, atomistic simulations, and machine learning enables the development of reliable equations of state for systems with previously unknown properties. This approach reveals how the Widom line governs the dissolving power of water and, in turn, controls sulfur speciation in SCWG. By enabling selective sulfur removal and preventing catalyst poisoning, our framework advances the SCWG method as a clean energy technology. More broadly, the methodology is generalizable and can be applied to a wide range of complex fluid systems with poorly understood thermodynamic behavior.

As a part of a systematic study on the structural changes of Gd-transition metal (TM) alloy glasses induced by cryogenic rejuvenation, high-energy x-ray diffraction (HEXRD) and anomalous x-ray scattering (AXS) measurements were performed on a GdCumetallic glass (MG) possessing minimal elastic heterogeneity, as indicated by dynamical mechanical analysis. Significant structural changes were observed in this MG as a result of temperature cyclings between liquid Nand room temperatures 40 times. (1) HEXRD revealed minor changes in the first nearest neighbour range of the pair distribution function,, consistent with observations in other GdTMglasses. (2) The partial structural information obtained from HEXRD and AXS showed that Gd-Cu and Cu-Cu partial correlations dominate the first peak region of the total, whereas Gd-Gd correlations contribute mainly to the second peak. (3) Additionally, some of the Gd-Cu and Cu-Cu correlations extend to the second peak of the total, unlike in GdCoand GdNiMGs. (4) Futhermore, Cu atoms shift farther from each other by cryogenic rejuvenation. (5) Heterogeneities in the Cu elemental composition and the total number density at nm length scale increase by cryogenic rejuvenation, similar to the GdCoMG but opposite to the trend observed in the GdNiMG. Accordingly, these findings suggest that structural heterogeneity in MGs is not systematically related to the extent of elastic heterogeneity.

Subsequent to our previous work on structural changes of a Gd65Co35 metallic glass (MG) by rejuvenation with temperature cycling (cryogenic rejuvenation) (Hosokawa et al., Acta Mater. 284 (2025) 120616), we carried out high energy X-ray diffraction (HEXRD) and anomalous X-ray scattering (AXS) experiments on a Gd65Ni35 MG having a larger elastic heterogeneity suggested by dynamical mechanical analysis to examine the relationship between the extent of the cryogenic rejuvenation effect in atomic structures and the elastic heterogeneities in MGs. We found rather small changes in the structures of this MG, i.e., by repeated temperature changes between liquid N2 and room temperatures 40 times, (1) the HEXRD experiment revealed that tiny structural changes occur in the first nearest-neighbor range. (2) The partial structural information obtained by HEXRD and AXS showed that small movements of both the Gd and Ni atoms are induced in the first- and second-nearest neighbor shells around the Gd atoms. (3) The composition heterogeneity in nanometer length scale slightly decreases for all of the constituent elements, which can be explained that the glass approaches liquid-like features by rejuvenation. Accordingly, it can be concluded that in glasses, the structural heterogeneity is not directly related to the elastic heterogeneity.

Molecular dynamics computer simulations have been conducted on neat liquid methanol, using three different “united atom” (three site) interatomic potentials: TraPPE [Chen et al., J. Phys. Chem. B 105, 3093 (2001)], UAM-I [García-Melgarejo et al., J. Mol. Liq. 323, 114576 (2021)], and OPLS/2016 [D. Gonzalez-Salgado and C. Vega, J. Chem. Phys. 145, 034508 (2016)]. The effects of pressure, between 1 bar and 6 kbar, have been evaluated on total scattering structure factors, partial radial distribution functions, and on collective characteristics such as ring-size distributions and cluster-size distributions. Agreement with experimental density is nearly quantitative for all three force fields, and major trends observed for recent pressure-dependent neutron diffraction data are reproduced qualitatively. In general, the OPLS/2016 force field generates properties that are markedly different from results originating from the other potentials. Pressure effects are hardly noticeable on most partial radial distribution functions and on the distribution of the number of hydrogen-bonded neighbors. On the other hand, collective structural properties, such as cluster- and ring-size distributions, exhibit significant changes with increasing pressure: larger clusters become more numerous, whereas the number of cyclic clusters, i.e., rings, decreases. The self-diffusion coefficient decreases with increasing pressure, and the same is valid for the average lifetime of hydrogen bonds.

To experimentally clarify the changes in structural and dynamic heterogeneities in a metallic glass (MG), Gd65Co35, by rejuvenation with temperature cycling (cryogenic rejuvenation), high-energy X-ray diffraction (HEXRD), anomalous x-ray scattering (AXS), and inelastic x-ray scattering (IXS) experiments were carried out. By repeated temperature changes between liquid N2 and room temperatures 40 times, we observed tiny but clear structural changes by HEXRD even in the first-nearest neighbor range. The partial structural information obtained by AXS revealed that slight movements of Gd and Co atoms occur in the first- and second-nearest neighbor shells around the central Gd atom. The composition heterogeneity in nanometer size is markedly increased for the Gd atoms by temperature cycling, whereas the other heterogeneities are negligible. A distinct change was detected in microscopic elastic property by IXS: The width of longitudinal acoustic excitation broadens by about 20%, indicating an increase in the elastic heterogeneity of this MG induced by thermal treatments. These static and dynamic results explicitly clarify the features of the cryogenic rejuvenation effect experimentally.

To investigate the relationship between the partial structures and the stiffness transition in AsxSe1-x glasses, anomalous X-ray scattering (AXS) and X-ray and neutron diffraction (XRD and ND) experiments were carried out. For the AXS experiments, anomalous terms near the absorption edges were experimentally obtained instead of the theoretical values with large ambiguities. The results were analyzed by reverse Monte Carlo (RMC) modeling to obtain partial structure factors, Sie(Q), partial pair distribution functions, gie(r), and three-dimensional atomic configurations. The Sie(Q) and gie(r) functions gradually vary with x; however, an important change was observed in the intermediate-range element-selective atomic structures (the so-called hyper-ordered structures) near the stiffness transition composition. With decreasing x across the so-called intermediate phase compositions, a rapid decrease of the As-As wrong bonds is visualized. However, the other anomalies found in Ge-Se glasses are not clearly observed, such as a rapid decrease in pre-shoulder position in SSeSe(Q), a rapid decrease in the number of edge-sharing connections, and an exclusion tendency of the connections between the As(Ge) atoms sharing two Se atoms, which may be related to the anisotropic pyramidal atomic configurations around the As atoms in the As-Se glasses in contrast to the isotropic tetrahedral ones around the Ge atoms in the Ge-Se glasses.

Controlling Li ion transport in glasses at atomic and molecular levels is key to realizing all-solid-state batteries, a promising technology for electric vehicles. In this context, Li3PS4 glass, a promising solid electrolyte candidate, exhibits dynamic coupling between the Li+ cation mobility and the PS43− anion libration, which is commonly referred to as the paddlewheel effect. In addition, it exhibits a concerted cation diffusion effect (i.e., a cation–cation interaction), which is regarded as the essence of high Li ion transport. However, the correlation between the Li+ ions within the glass structure can only be vaguely determined, due to the limited experimental information that can be obtained. Here, this study reports that the Li ions present in glasses can be classified by evaluating their valence oscillations via Bader analysis to topologically analyze the chemical bonds. It is found that three types of Li ions are present in Li3PS4 glass, and that the more mobile Li ions (i.e., the Li3-type ions) exhibit a characteristic correlation at relatively long distances of 4.0–5.0 Å. Furthermore, reverse Monte Carlo simulations combined with deep learning potentials that reproduce X-ray, neutron, and electron diffraction pair distribution functions showed an increase in the number of Li3-type ions for partially crystallized glass structures with improved Li ion transport properties. Our results show order within the disorder of the Li ion distribution in the glass by a topological analysis of their valences. Thus, considering the molecular vibrations in the glass during the evaluation of the Li ion valences is expected to lead to the development of new solid electrolytes.

Ab initio molecular dynamics (AIMD) simulations have been performed on aqueous solutions of four simple sugars, alpha-D-glucose, beta-D-glucose, alpha-D-mannose, and alpha-D-galactose. Hydrogen-bonding (HB) properties, such as the number of donor- and acceptor-type HB-s, and the lengths and strengths of hydrogen bonds between sugar and water molecules, have been determined. Related electronic properties, such as the dipole moments of water molecules and partial charges of the sugar O atoms, have also been calculated. The hydrophilic and hydrophobic shells were characterized by means of spatial distribution functions. beta-D-Glucose was found to form the highest number of hydrophilic and the smallest number of hydrophobic connections to neighboring water molecules. The average sugar-water H-bond length was the shortest for beta-D-glucose, which suggests that these are the strongest such H-bonds. Furthermore, beta-D-glucose appears to stand out in terms of the symmetry properties of both its hydrophilic and hydrophobic hydration shells. In summary, in all aspects considered here, there seems to be a correlation between the distinct characteristics of beta-D-glucose reported here and its outstanding solubility in water. Admittedly, our findings represent only some of the important factors that influence the solubility.

Ab initio molecular dynamics (AIMD) simulations have been performed on aqueous solutions of four simple sugars, α-D-glucose, e̱ṯa̱-D-glucose, α-D-mannose and α-D-galactose. Hydrogen bonding (HB) properties, such as the number of donor and acceptor type HB-s, and the lengths and strengths of hydrogen bonds between sugar and water molecules, have been determined. Related electronic properties, such as the dipole moments of water molecules and partial charges of the sugar O-atoms, have also been calculated. The hydrophilic and hydrophobic shells were characterized by means of spatial distribution functions. e̱ṯa̱-D-glucose has been found to form the highest number of hydrophilic and the smallest number of hydrophobic connections to neighboring water molecules. The average sugar-water H-bond length was the shortest for e̱ṯa̱-D-glucose, which suggests that these are the strongest such H-bonds. Furthermore, e̱ṯa̱-D-glucose appears to stand out in terms of symmetry properties of both its hydrophilic and hydrophobic hydration shells. In summary, in all aspects considered here, there seems to be a correlation between the distinct characteristics of e̱ṯa̱-D-glucose and its outstanding solubility in water.

A self-consistent scheme is presented that is applicable for revealing details of the microscopic structure of hydrogen-bonded liquids, including the description of the hydrogen-bonded network. The scheme starts with diffraction measurements, followed by molecular dynamics simulations. Computational results are compared with the experimentally accessible information on the structure, which is most frequently the total scattering structure factor. In the case of an at least semiquantitative agreement between experiment and simulation, sets of particle coordinates from the latter may be exploited for revealing nonmeasurable structural details. Calculations of some properties concerning the hydrogen-bonded network are also described, in the order of increasing complexity: starting with the definition of a hydrogen bond, first and second neighborhoods are described via spatial correlations functions. Attention is then turned to cyclic and noncyclic hydrogen-bonded clusters, before cluster size distributions and percolation are discussed. We would like to point out that, as a result of applying the novel protocol, these latter, rather abstract, quantities become consistent with diffraction data: it may thus be argued that the approach reviewed here is the first one that establishes a direct link between measurements and elements of network theories. Applications for liquid water, simple alcohols, and alcohol-water liquid mixtures demonstrate the usefulness of the aforementioned characteristics. The procedure can readily be applied to more complicated hydrogen-bonded networks, like mixtures of polyols (diols, triols, sugars, etc.) and water, and complex aqueous solutions of even larger molecules (even of proteins).

The hydrogen-bonded structure of methanol - water mixtures is investigated over the entire alcohol concentration range (from xMethanol = 0.1 to 1.0) at several temperatures, from 300 K down to the freezing point of the given mixture. Classical molecular dynamics simulations have been carried out, using the all-atom OPLS-AA force field for methanol and the TIP4P/2005 model for water molecules. Simulation trajectories ('particle configurations') obtained have been analyzed, in order to characterize the hydrogen-bonded network in the mixtures. The temperature and concentration dependence of the average hydrogen bond (H-bond) numbers between different types of molecules, the donor/acceptor roles of water and methanol molecules, and hydrogen bond number distributions have been revealed. The topology of the total system, as well as that of the water and methanol subsystems, has been investigated by calculating the cluster size distributions, the number of primitive rings, and ring size and ring type distributions. It has been found that upon cooling, the average number of Hbonded water molecules increases at every concentration and temperature investigated. As far as the connectivity of the hydrogen-bonded network is concerned, the percolation threshold has been shown to be above xM = 0.9 already at room temperature.

To understand the relation of the glass-forming ability (GFA) to the local atomic configurations of a Pd42.5Ni7.5Cu30P20 (PNCP) metallic glass having the best GFA at present, the local structures were investigated by combining data obtained from anomalous X-ray scattering, X-ray and neutron diffraction, and applying reverse Monte Carlo modeling. By comparing the results of PNCP with Pd40Ni40P20 (PNP) and Pd40Cu40P20 (PCP) having a slightly and much worse GFAs, respectively, characteristic features were observed in the hyper-ordered atomic structures. Firstly, the concentration inhomogeneity of Ni/Cu in PNCP is larger than that of Ni in PNP and Cu in PCP. Secondly, a Voronoi tessellation showed that the fraction of pure icosahedral arrangements around the Cu atoms increases significantly in PNCP by adding icosahedral-preferred Ni atoms in PCP. Finally, a persistent homology (PH) analysis reveals the largest intermediate-size Cu PH rings in PNCP among the PH rings in these Pd-based BMGs. The structural heterogeneity for the excellent GFA of PNCP would be considered by an incompatible mixture of specific Pd-P configurations and icosahedral clusters around the secondary Ni and Cu metals.

In this chapter, Reverse Monte Carlo (RMC) modeling is introduced. Following a brief description of the algorithm, a rather extensive range of applications of the method is shown. We also introduce recent topological analysis tools that help to understand the "order within disorder" in noncrystalline solids.

The structural properties of two Ge-As-Se glass compositions (Ge10As10Se80 and Ge21As21Se58) are investigated from a combination of density-functional-based molecular dynamics simulations and neutron/x-ray scattering experiments. We first focus on structural characteristics, including structure factors, pair distribution functions, angular distributions, coordination numbers, and neighbor distributions, and compare our results with the experimental data. Results leave anticipated coordinations from the octet rule (Se-II, As-III, and Ge-IV) unchanged, and these are contrasted with respect to glasses having the same average coordination number (r) over bar such as binary As30Se70 and Ge33Se67. The increase of (As,Ge) content induces a growth of ring structures that are dominated by edge-sharing motifs (four-membered rings) having mostly heteropolar bonds, while As-As and As-Ge homopolar bonds are clearly more favored than Ge-Ge. These features signal that both topological (rings) and chemical (bonds) features are different with respect to related binaries. The validity of the so-called vibrational isocoordination rule stating that properties of multicomponent chalcogenides depend solely on (r) over bar is checked, and results from a vibrational analysis indicates that this rule is merely satisfied for the Se-rich composition. An inspection of correlations via the Bhatia-Thornton formalism shows that topological ordering is not only different between Ge10As10Se80 and Ge21As21Se58 but also radically contrasts with respect to the isocoordinated binary glasses and displays an obvious reduced directional bonding.

Apart from the well‐known molten white phosphorus, existing at temperatures around 50 °C under atmospheric pressure, early in this millennium, new high‐pressure, high‐temperature phases have been discovered. One group of the newly found liquids can be identified as being formed by P 4 molecules, just like common molten white phosphorus. The structures of these (“old” and “new”) forms have not yet been compared in detail: this comparison is in the focus of the present work. Orientational correlations between P 4 tetrahedra, as a function of the distance between centers of tetrahedra, have been revealed. It is found that face‐to‐face type contacts occur at much lower center–center distances in the newly discovered liquids. As an addition, new estimates, based on series of Reverse Monte Carlo calculations, for the densities of the high‐temperature phases are provided; this step is necessary because in this respect, sizeable uncertainties have been reported previously.

It is shown that the dipole moment of polar (water, methanol, formamide, acetone and acetonitrile) molecules in the neighborhood of a cation is increased primarily by polarization from the bare electrostatic charge of the cation, although the effective value of the latter is somewhat reduced by "back donation" of electrons from neighbouring polar molecules. In other words, the classical picture may be viewed as if a point charge slightly smaller than the nominal charge of the cation would be placed at the cation site. It was found that the geometrical arrangement of the polar molecules in the first solvation shell is such that their mutual polarization reduces the dipole moments of individual molecules, so that in some cases they become smaller than the dipole moment of the free protic or aprotic molecule. We conjecture that this behavior is essentially a manifestation of the Le Chatellier-Braun principle.

Following a demonstration of how neutron diffraction with polarization analysis may be applied for the accurate determination of the coherent static structure factor of disordered materials containing substantial amounts of proton nuclei (Temleitner et al., Phys. Rev. B 92, 014201, 2015), we now focus on the incoherent scattering. Incoherent contributions are responsible for the great difficulties while processing standard (non-polarized) neutron diffraction data from hydrogenous materials, hence the importance of the issue. Here we report incoherent scattering intensities for liquid acetone, cyclohexane, methanol and water, as function of the 1H/H ratio. The incoherent intensities are determined directly by polarized neutron diffraction. This way, possible variations of the incoherent background due to the changing chemical environment may be monitored. In addition, for some of the water samples, incoherent intensities as a function of the wavelength of the incident neutron beam (at 0.4, 0.5 and 0.8 Å) have also been measured. It is found that in each case, the incoherent intensity can be described by a single Gaussian function, within statistical errors. The (full) width (at half maximum) of the Gaussians clearly depends on the applied wavelength. On the other hand, the different bonding environments of hydrogen atoms do not seem to affect the width of the Gaussian.

Topological descriptors related to the size distribution of hydrogen bonded clusters are scrutinized systematically, primarily from the point of view of their applicability for locating the percolation transition point. As a first step, we focus on regular, relatively simple systems like ice polymorphs (cubic, hexagonal, ice VI and ice VII), in order to monitor the evolution of these descriptors with varying the hydrogen bonding probability. Further quantities are also introduced, like the ratio of the number of molecules that do not belong to any ring structure, by which it is possible to determine a critical bonding probability below that the system shows dominantly chain-like behaviour. By means of calculating the smallest eigenvalues of the Laplace spectra of hydrogen bonded networks, yet another way has been developed for determining the percolation transition point. Results concerning the percolation transition agree well with those arising from more traditional 'toolbox' calculations. Finally, the aforementioned descriptors have been calculated for more realistic systems, with the aim of understanding better the properties of hydrogen bonded networks in important alcohol-water solutions. (C) 2022 Published by Elsevier B.V.