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We combine the use of molecular dynamics simulations and the generalized Langevin equation to study the diffusion of a fluid adsorbed within kerogen, the main organic phase of shales. As a class of microporous and amorphous materials that can exhibit significant adsorption-induced swelling, the dynamics of the kerogen's microstructure is expected to play an important role in the confined fluid dynamics. This role is investigated by conducting all-atom simulations with or without solid dynamics. Whenever the dynamics coupling between the fluid and solid is accounted for, we show that the fluid dynamics displays some qualitative differences compared to bulk fluids, which can be modulated by the amount of adsorbed fluid owing to adsorption-induced swelling. We highlight that working with the memory kernel, the central time correlation function of the generalized Langevin equation, allows the fingerprint of the dynamics of the solid to appear on that of the fluid. Interestingly, we observe that the memory kernels of fluid diffusion in kerogen qualitatively behave as those of tagged particles in supercooled liquids. We emphasize the importance of reproducing the velocity–force correlation function to validate the memory kernel numerically obtained as confinement enhances the numerical instabilities. This route is interesting as it opens the way for modeling the impact of fluid concentration on the diffusion coefficient in such ultra-confining cases. [ABSTRACT FROM AUTHOR]

Monte Carlo simulations are widely employed to measure the physical properties of glass-forming liquids in thermal equilibrium. Combined with local Monte Carlo moves, the Metropolis algorithm can also be used to simulate the relaxation dynamics, thus offering an efficient alternative to molecular dynamics. Monte Carlo simulations are, however, more versatile because carefully designed Monte Carlo algorithms can more efficiently sample the rugged free energy landscape characteristic of glassy systems. After a brief overview of Monte Carlo studies of glass-formers, we define and implement a series of Monte Carlo algorithms in a three-dimensional model of polydisperse hard spheres. We show that the standard local Metropolis algorithm is the slowest and that implementing collective moves or breaking detailed balance enhances the efficiency of the Monte Carlo sampling. We use time correlation functions to provide a microscopic interpretation of these observations. Seventy years after its invention, the Monte Carlo method remains the most efficient and versatile tool to compute low-temperature properties in supercooled liquids. [ABSTRACT FROM AUTHOR]

We perform molecular dynamics simulations of a binary mixture of water and trehalose with the TIP4P/Ice water model. We analyze the slow dynamics of trehalose molecules in the mildly supercooled region for concentrations of 3.66 and 18.57 wt. %. We previously studied the dynamics of water in the same mixtures. Supercooled TIP4P/Ice water solvating trehalose molecules was found to follow the Mode Coupling Theory (MCT) and to undergo a transition from a fragile to a strong behavior for both concentrations. Here, we show that also the dynamics of trehalose molecules follows the MCT and displays a fragile to strong crossover (FSC). The results show that trehalose in binary mixtures with water shares with it the dynamical behavior typical of glass forming liquids. Moreover, the FSC for trehalose structural relaxation times is found to occur at temperatures close to those previously obtained for water in the same solutions, showing that the dynamics of the solute is strongly coupled to that of the solvent. We also perform a MCT test showing that the trehalose dynamics obeys the MCT time–temperature superposition principle and that the exponents derived from the theory and the ones obtained from fitting procedure of the relaxation times are comparable, confirming that trehalose molecules in supercooled water solutions follow the MCT of glassy dynamics. Moreover, as predicted by the theory, trehalose particles have MCT parameters comparable to those of water in the same mixtures. This is an important result, given that MCT was originally formulated for monoatomic particles. [ABSTRACT FROM AUTHOR]

A method for characterizing the topological fluctuations in liquids is proposed. This approach exploits the concept of the weighted gyration tensor of a collection of particles and permits the definition of a local configurational unit (LCU). The first principal axis of the gyration tensor serves as the director of the LCU, which can be tracked and analyzed by molecular dynamics simulations. Analysis of moderately supercooled Kob–Andersen mixtures suggests that orientational relaxation of the LCU closely follows viscoelastic relaxation and exhibits a two-stage behavior. The slow relaxing component of the LCU corresponds to the structural, Maxwellian mechanical relaxation. Additionally, it is found that the mean curvature of the LCUs is approximately zero at the Maxwell relaxation time with the Gaussian curvature being negative. This observation implies that structural relaxation occurs when the configurationally stable and destabilized regions interpenetrate each other in a bicontinuous manner. Finally, the mean and Gaussian curvatures of the LCUs can serve as reduced variables for the shear stress correlation, providing a compelling proof of the close connection between viscoelastic relaxation and topological fluctuations in glass-forming liquids. [ABSTRACT FROM AUTHOR]

The distinctive characteristics of water, evident in its thermodynamic anomalies, have implications across disciplines from biology to geophysics. Considered a valid hypothesis to rationalize its unique properties, a liquid--liquid phase transition in water below the freezing point, in the so-called supercooled regime, has nowadays been observed in several molecular dynamics simulations and is being actively researched experimentally. The hypothesis of ferroelectric phase transition in supercooled water can be traced back to 1977, due to Stillinger. In this work, we highlight intriguing and far-reaching implications of water: The ferroelectric and liquid--liquid phase transitions can be designed as two facets of the same underlying phenomenon. Our results are based on the analysis of extensive molecular dynamics simulations and are explained in the context of a classical density functional theory in mean-field approximation valid for a polar liquid, where dipolar interaction is treated perturbatively. The theory underpins the potential role of ferroelectricity in promoting the liquid--liquid phase transition, being the density-polarization coupling inherent in the dipolar interaction potential. The existence of ferroelectric order in supercooled low-density liquid water is confirmed by the observation in molecular dynamics simulations of collective modes in space-time polarization correlation functions, traceable to spontaneous breaking of continuous rotational symmetry. Our work sheds light on water's supercooled behavior and opens the door to experimental investigations of the static and dynamic behavior of water's polarization. [ABSTRACT FROM AUTHOR]

A vast array of phenomena, ranging from chemical reactions to phase transformations, are analyzed in terms of a free energy surface defined with respect to a single or multiple order parameters. Enhanced sampling methods are typically used, especially in the presence of large free energy barriers, to estimate free energies using biasing protocols and sampling of transition paths. Kinetic reconstructions of free energy barriers of intermediate height have been performed, with respect to a single order parameter, employing the steady state properties of unconstrained simulation trajectories when barrier crossing is achievable with reasonable computational effort. Considering such cases, we describe a method to estimate free energy surfaces with respect to multiple order parameters from a steady state ensemble of trajectories. The approach applies to cases where the transition rates between pairs of order parameter values considered is not affected by the presence of an absorbing boundary, whereas the macroscopic fluxes and sampling probabilities are. We demonstrate the applicability of our prescription on different test cases of random walkers executing Brownian motion in order parameter space with an underlying (free) energy landscape and discuss strategies to improve numerical estimates of the fluxes and sampling. We next use this approach to reconstruct the free energy surface for supercooled liquid silicon with respect to the degree of crystallinity and density, from unconstrained molecular dynamics simulations, and obtain results quantitatively consistent with earlier results from umbrella sampling. [ABSTRACT FROM AUTHOR]

The most accepted origin for the water anomalous behavior is the phase transition between two liquids (LLPT) in the supercooled regime connected to the glassy first order phase transition at lower temperatures. Two length scale potentials are an effective approach that has long been employed to understand the properties of fluids with waterlike anomalies and, more recently, the behavior of colloids and nanoparticles. These potentials can be parameterized to have distinct shapes, as a pure repulsive ramp, such as the model proposed by de Oliveira et al. [J. Chem. Phys. 124, 64901 (2006)]. This model has waterlike anomalies despite the absence of LLPT. To unravel how the waterlike anomalies are connected to the solid phases, we employ molecular dynamics simulations. We have analyzed the fluid–solid transition under cooling, with two solid crystalline phases, BCC and HCP, and two amorphous regions being observed. We show how the competition between the scales creates an amorphous cluster in the BCC crystal that leads to amorphization at low temperatures. A similar mechanism is found in the fluid phase, with the system changing from a BCC-like to an amorphous-like structure in the point where a maxima in kT is observed. With this, we can relate the competition between two fluid structures with the amorphous clusterization in the BCC phase. These findings help to understand the origins of waterlike behavior in systems without the liquid–liquid critical point. [ABSTRACT FROM AUTHOR]

The slow relaxation of interface water (IW) across three primary phases of membranes is relevant to understand the influence of IW on membrane functions at supercooled conditions. To this objective, a total of ∼ 16.26 μ s all-atom molecular dynamics simulations of 1,2-dimyristoyl-sn-glycerol-3-phosphocholine lipid membranes are carried out. A supercooling-driven drastic slow-down in heterogeneity time scales of the IW is found at the fluid to the ripple to the gel phase transitions of the membranes. At both fluid-to-ripple-to-gel phase transitions, the IW undergoes two dynamic crossovers in Arrhenius behavior with the highest activation energy at the gel phase due to the highest number of hydrogen bonds. Interestingly, the Stokes–Einstein (SE) relation is conserved for the IW near all three phases of the membranes for the time scales derived from the diffusion exponents and the non-Gaussian parameters. However, the SE relation breaks for the time scale obtained from the self-intermediate scattering functions. The behavioral difference in different time scales is universal and found to be an intrinsic property of glass. The first dynamical transition in the α relaxation time of the IW is associated with an increase in the Gibbs energy of activation of hydrogen bond breaking with locally distorted tetrahedral structures, unlike the bulk water. Thus, our analyses unveil the nature of the relaxation time scales of the IW across membrane phase transitions in comparison with the bulk water. The results will be useful to understand the activities and survival of complex biomembranes under supercooled conditions in the future. [ABSTRACT FROM AUTHOR]

We investigate the properties of water along the liquid/vapor coexistence line in the supercooled regime down to the no-man's land. Extensive molecular dynamics simulations of the TIP4P/2005 liquid/vapor interface in the range 198–348 K allow us to locate the second surface tension inflection point with a high accuracy at 283 ± 5 K, close to the temperature of maximum density. This temperature also coincides with the appearance of a density anomaly at the interface known as the apophysis. We relate the emergence of the apophysis to the observation of high-density liquid (HDL) water adsorption in the proximity of the liquid/vapor interface. [ABSTRACT FROM AUTHOR]

The first paper of this series [J. Chem. Phys. 158, 034103 (2023)] demonstrated that excess entropy scaling holds for both fine-grained and corresponding coarse-grained (CG) systems. Despite its universality, a more exact determination of the scaling relationship was not possible due to the semi-empirical nature. In this second paper, an analytical excess entropy scaling relation is derived for bottom-up CG systems. At the single-site CG resolution, effective hard sphere systems are constructed that yield near-identical dynamical properties as the target CG systems by taking advantage of how hard sphere dynamics and excess entropy can be analytically expressed in terms of the liquid packing fraction. Inspired by classical equilibrium perturbation theories and recent advances in constructing hard sphere models for predicting activated dynamics of supercooled liquids, we propose a new approach for understanding the diffusion of molecular liquids in the normal regime using hard sphere reference fluids. The proposed "fluctuation matching" is designed to have the same amplitude of long wavelength density fluctuations (dimensionless compressibility) as the CG system. Utilizing the Enskog theory to derive an expression for hard sphere diffusion coefficients, a bridge between the CG dynamics and excess entropy is then established. The CG diffusion coefficient can be roughly estimated using various equations of the state, and an accurate prediction of accelerated CG dynamics at different temperatures is also possible in advance of running any CG simulation. By introducing another layer of coarsening, these findings provide a more rigorous method to assess excess entropy scaling and understand the accelerated CG dynamics of molecular fluids. [ABSTRACT FROM AUTHOR]

Understanding the mechanism of metal solidification holds both theoretical significance and practical importance. In this study, we conducted molecular dynamics simulations to investigate the impact of cooling rates on the solidification of a melted ruthenium alloy using the embedded atom method (EAM) potential. The EAM potential is a widely employed interatomic potential for describing the metallic system, which can capture numerous crucial properties, including mechanical properties, the energy of competing crystal structure dynamics, defects, and liquid structures. Our simulations showed that upon quenching with different cooling rates, the system transformed into a supercooled liquid state at 1200 K, and a hexagonal close-packed cluster emerged as a dominant structure that remained stable even in the supercooled state. A critical cooling rate (1011 K/s) marked the transition from crystal to amorphous phase; this transition exhibited an upward trend as the superheating temperature increased until it reached the maximum achievable cooling rate. Our simulations also revealed that the optimal conditions for undercooling and superheating occur at ∼0.4396 and 1.2893 Tm, respectively, where Tm is the melting temperature. Our results provide comprehensive insights into the evolution of melt structures with changing temperatures during deep undercooling, the formation of homogeneous melt-free crystal regions, and the effect of the molten state on solidification phenomena. [ABSTRACT FROM AUTHOR]

Amorphous alloys possess increased mechanical strength. Currently, the processing and shaping of amorphous alloys employ their superplasticity in the supercooled liquid phase, which has pushed research into the evolution behavior of the supercooled liquid area during the processing of amorphous alloys. In this study, a molecular dynamics technique was applied to analyze the development rules of the supercooled liquid area and the "shear transformation zone" during laser-assisted scratching of Cu50Zr50 amorphous alloy. It was discovered through research on temperature fluctuations that laser irradiation raises the temperature of the processing region, and the formation of the supercooled liquid region moves the characteristic point of material removal to an earlier stage. As the local temperature rises, the distribution of the supercooled liquid region changes from dispersed to an angle of about 30° with regard to the scratching direction, and the angle subsequently decreases. The variable laws of the "shear transformation zone" during the scratching process were explored based on the von Mises strain theory. The findings demonstrate that when laser energy rises, the Newtonian layer warms up and the "shear transformation zone" distribution angle gradually decreases. At 20 eV/ps, the angle between the "shear transformation zone" and the scratching direction achieves its smallest value of 33°, resulting in the largest overlap with the supercooled liquid region and the lowest content of the "shear transformation zone". By constructing a mathematical model for material removal efficiency, it was established that energy in the range of 20 eV/ps to 30 eV/ps demonstrates higher removal efficiency and a steady processing process while reducing surface roughness by 6-7%. [ABSTRACT FROM AUTHOR]

Phase‐change materials (PCMs) are an important family of alloys employed in non‐volatile memories and neuromorphic devices. These devices exploit the ability of PCMs to undergo rapid transitions between amorphous and crystalline phases at moderately high temperature. At ambient conditions, both phases are stable. These properties imply a very strong temperature dependence of the crystallization kinetics, which has been attributed to the high fragility of the supercooled liquid phase. In this work, the fragility index of GeTe, an important PCM, is extracted from a computational exploration of the potential energy landscape of the system by molecular dynamics simulations. For this purpose, a neural‐network potential is used to describe the interatomic interactions, which enables the evaluation of the configurational entropy in the deeply supercooled regime. The viscosity and the relaxation times are also calculated in the same temperature range. Finally, the Adam‐Gibbs relation is used to extrapolate the data to low temperatures and estimate the glass transition temperature and the fragility index. The latter quantity turns out to be of the order of 135–140, which shows that liquid GeTe is a highly fragile system. [ABSTRACT FROM AUTHOR]

A supercooled fluid close to the glass transition develops nonlocal shear-stress correlations that anticipate the emergence of elasticity. We performed molecular dynamics simulations of a binary Lennard-Jones mixture at different temperatures and investigated the spatiotemporal autocorrelation function of the shear stress for different wavevectors, q, from a locally measured and Fourier-transformed stress tensor. Anisotropic correlations are observed at non-zero wavevectors, exhibiting strongly damped oscillations with a characteristic frequency ω(q). A comparison with a recently developed hydrodynamic theory [Maier et al., Phys. Rev. Lett. 119, 265701 (2017)] shows a remarkably good quantitative agreement between particle-based simulations and theoretical predictions. [ABSTRACT FROM AUTHOR]

Molecular dynamics computer simulations of a polydisperse soft-sphere model under shear are presented. The starting point for these simulations are deeply supercooled samples far below the critical temperature, Tc, of mode coupling theory. These samples are fully equilibrated with the aid of the swap Monte Carlo technique. For states below Tc, we identify a lifetime τlt that measures the time scale on which the system can be considered as an amorphous solid. The temperature dependence of τlt can be well described by an Arrhenius law. The existence of transient amorphous solid states below Tc is associated with the possibility of brittle yielding, as manifested by a sharp stress drop in the stress–strain relation and shear banding. We show that brittle yielding requires, on the one hand, low shear rates and, on the other hand, the time scale corresponding to the inverse shear rate has to be smaller or of the order of τlt. Both conditions can only be met for a large lifetime τlt, i.e., for states far below Tc. [ABSTRACT FROM AUTHOR]

The spatiotemporal correlations of the local stress tensor in supercooled liquids are studied both theoretically and by molecular dynamics simulations of a two-dimensional (2D) polydisperse Lennard-Jones system. Asymptotically exact theoretical equations defining the dynamical structure factor and all components of the stress correlation tensor for low wave-vector q are presented in terms of the generalized (q-dependent) shear and longitudinal relaxation moduli, G(q, t) and K(q, t). We developed a rigorous approach (valid for low q) to calculate K(q, t) in terms of certain bulk correlation functions (for q = 0), the static structure factor S(q), and thermal conductivity κ. The proposed approach takes into account both the thermostatting effect and the effect of polydispersity. The theoretical results for the (q, t)-dependent stress correlation functions are compared with our simulation data, and an excellent agreement is found for q b ̄ ≲ 0.5 (with b ̄ being the mean particle diameter) both above and below the glass transition without any fitting parameters. Our data are consistent with recently predicted (both theoretically and by simulations) long-range correlations of the shear stress quenched in heterogeneous glassy structures. [ABSTRACT FROM AUTHOR]

Employing dielectric spectroscopy, oscillatory shear rheology, and calorimetry, the present work explores the molecular dynamics of the widely used insecticide imidacloprid above and below its glass transition temperature. In its supercooled liquid regime, the applied techniques yield good agreement regarding the characteristic structural (alpha) relaxation times of this material. In addition, the generalized Gemant–DiMarzio–Bishop model provides a good conversion between the frequency-dependent dielectric and shear mechanical responses in its viscous state, allowing for an assessment of imidacloprid's molecular hydrodynamic radius. In order to characterize the molecular dynamics in its glassy regime, we employ several approaches. These include the application of frequency–temperature superposition (FTS) to its isostructural dielectric and rheological responses as well as use of dielectric and calorimetric physical aging and the Adam–Gibbs–Vogel model. While the latter approach and dielectric FTS provide relaxation times that are close to each other, the other methods predict notably longer times that are closer to those reflecting a complete recovery of ergodicity. This seemingly conflicting dissimilarity demonstrates that the molecular dynamics of glassy imidacloprid strongly depends on its thermal history, with high relevance for the use of this insecticide as an active ingredient in technological applications. [ABSTRACT FROM AUTHOR]

Impulsive stimulated thermal scattering (ISTS) allows one to access the structural relaxation dynamics in supercooled molecular liquids on a time scale ranging from nanoseconds to milliseconds. Till now, a heuristic semi-empirical model has been commonly adopted to account for the ISTS signals. This model implicitly assumes that the relaxation of specific heat, C, and thermal expansion coefficient, γ, occur on the same time scale and accounts for them via a single stretched exponential. This work proposes two models that assume disentangled relaxations, respectively, based on the Debye and Havriliak–Negami assumptions for the relaxation spectrum and explicitly accounting for the relaxation of C and γ separately in the ISTS response. A theoretical analysis was conducted to test and compare the disentangled relaxation models against the stretched exponential. The former models were applied to rationalize the experimental ISTS signals acquired on supercooled glycerol. This allows us to simultaneously retrieve the frequency-dependent specific heat and thermal expansion up to the sub-100 MHz frequency range and further to compare the fragility and time scale probed by thermal, mechanical, and dielectric susceptibilities. [ABSTRACT FROM AUTHOR]

The breakdown of the Stokes–Einstein relation in supercooled liquids, which is the increase in the ratio τ α τ D between the two macroscopic times for structural relaxation and diffusion on decreasing the temperature, is commonly ascribed to dynamic heterogeneities, but a clear-cut microscopic interpretation is still lacking. Here, we tackle this issue exploiting the single-particle cage-jump framework to analyze molecular dynamics simulations of soft disk assemblies and supercooled water. We find that τ α τ D ∝ ⟨ t p ⟩ ⟨ t c ⟩ , where ⟨tp⟩ and ⟨tc⟩ are the cage-jump times characterizing slow and fast particles, respectively. We further clarify that this scaling does not arise from a simple term-by-term proportionality; rather, the relations τ α ∝ ⟨ t p ⟩ ⟨ Δ r J 2 ⟩ and τ D ∝ ⟨ t c ⟩ ⟨ Δ r J 2 ⟩ effectively connect the macroscopic and microscopic timescales, with the mean square jump length ⟨ Δ r J 2 ⟩ shrinking on cooling. Our work provides a microscopic perspective on the Stokes–Einstein breakdown and generalizes previous results on lattice models to the case of more realistic glass-formers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The self-diffusion coefficient of viscous liquids is estimated on the basis of a simple analysis of their rheological shear spectra. To this end, the Almond–West approach, previously employed to access single-particle diffusivities in ionic conductors, is generalized for application to molecular dynamics in supercooled liquids. Rheology based estimates, presented for indomethacin, ortho-terphenyl, and trinaphthylbenzene, reveal relatively small, yet systematic differences when compared with diffusivity data directly measured for these highly viscous liquids. These deviations are discussed in terms of mechanical Haven ratios, introduced to quantify the magnitude of collective translational effects that have an impact on the viscous flow. [ABSTRACT FROM AUTHOR]

The Kauzmann temperature (TK) of a supercooled liquid is defined as the temperature at which the liquid entropy becomes equal to that of the crystal. The excess entropy, the difference between liquid and crystal entropies, is routinely used as a measure of the configurational entropy, whose vanishing signals the thermodynamic glass transition. The existence of the thermodynamic glass transition is a widely studied subject, and of particular recent interest is the role of dimensionality in determining the presence of a glass transition at a finite temperature. The glass transition in water has been investigated intensely and is challenging as the experimental glass transition appears to occur at a temperature where the metastable liquid is strongly prone to crystallization and is not stable. To understand the dimensionality dependence of the Kauzmann temperature in water, we study computationally bulk water (three-dimensions), water confined in the slit pore of the graphene sheet (two-dimensions), and water confined in the pore of the carbon nanotube of chirality (11,11) having a diameter of 14.9 Å (one-dimension), which is the lowest diameter where amorphous water does not always crystallize into nanotube ice in the supercooled region. Using molecular dynamics simulations, we compute the entropy of water in bulk and under reduced dimensional nanoscale confinement to investigate the variation of the Kauzmann temperature with dimension. We obtain a value of TK (133 K) for bulk water in good agreement with experiments [136 K (C. A. Angell, Science 319, 582–587 (2008) and K. Amann-Winkel et al., Proc. Natl. Acad. Sci. U. S. A. 110, 17720–17725 (2013)]. However, for confined water, in two-dimensions and one-dimension, we find that there is no finite temperature Kauzmann point (in other words, the Kauzmann temperature is 0 K). Analysis of the fluidicity factor, a measure of anharmonicity in the oscillation of normal modes, reveals that the Kauzmann temperature can also be computed from the difference in the fluidicity factor between amorphous and ice phases. [ABSTRACT FROM AUTHOR]

It is well-known that time-dependent correlation functions related to temperature and energy can crucially depend on the thermostatting mechanism used in computer simulations of molecular systems. We argue, however, that linear response functions must be considered as universal properties of physical systems. This implies that the classical fluctuation equation for the transient heat capacity, cv(t), is not applicable to the thermostatted molecular dynamics (apart from long enough times). To improve on this point, we derive a number of exact general expressions for the frequency-dependent heat capacity in terms of energy correlation functions, valid for the Nosé–Hoover and some other thermostats. We also establish a general relation between auto- and cross correlation functions of energy and temperature. Recommendations on how to use these relations to maximize the numerical precision are provided. It is demonstrated that our approach allows us to obtain cv(t) for a supercooled liquid system with high precision and over many decades in time reflecting all pertinent relaxation processes. [ABSTRACT FROM AUTHOR]

We report an ab initio molecular dynamics (MD) simulation investigating the effect of a fully hydrated surface of TiO2 on the water dynamics. It is found that the universal relation between the rotational and translational diffusion characteristics of bulk water is broken in the water layers near the surface with the rotational diffusion demonstrating progressive retardation relative to the translational diffusion when approaching the surface. This kind of rotation–translation decoupling has so far only been observed in the supercooled liquids approaching glass transition, and its observation in water at a normal liquid temperature is of conceptual interest. This finding is also of interest for the application-significant studies of the water interaction with fully hydrated nanoparticles. We note that this is the first observation of rotation–translation decoupling in an ab initio MD simulation of water. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Adam–Gibbs (AG) model, linking thermodynamics with molecular dynamics of glass-forming liquids, plays a crucial role in the studies of the glass transition phenomenon. We employ this approach to investigate the relationship between ion dynamics and thermodynamics in three imidazolium-based ionic liquids in the current work. We show that the AG relation, −log10σdc ∝ (TSc)−1 (where σdc, T, and Sc denote the dc-conductivity, absolute temperature, and configurational entropy, respectively), does not work when the whole supercooled liquid state is considered. Meanwhile, a linear relationship between −log10σdc and (TSe)−1 (where Se denotes the excess entropy) was observed in the entire supercooled range. On the other hand, the generalized AG model log10σdc ∝ (TScα)−1 with an additional free parameter α successfully describes the relation between σdc and Sc. The determined α values being less than unity indicate that the configurational entropy is insufficient to govern the ion dynamics. Meanwhile, we found a systematical decrease in α with the elongation of the alkyl chain attached to the imidazolium ring. [ABSTRACT FROM AUTHOR]

Experimental studies of the glassy slowdown in molecular liquids indicate that the high-temperature activation energy E∞ of glass-forming liquids is directly related to their glass transition temperature Tg. To further investigate such a possible relation between high- and low-temperature dynamics in glass-forming liquids, we analyze the glassy dynamics of binary mixtures using molecular dynamics simulations. We consider a binary mixture of charged Lennard-Jones particles and vary the partial charges of the particles and, thus, the high-temperature activation energy and the glass transition temperature of the system. Based on previous results, we introduce a phenomenological model describing relaxation times over the whole temperature regime from high temperatures to temperatures well inside the supercooled regime. By investigating the dynamics of both particle species on molecular and diffusive length scales along isochoric and isobaric pathways, we find a quadratic charge dependence of both E∞ and Tg, resulting in an approximately constant ratio of both quantities independent of the underlying observable, the thermodynamic ensemble, and the particle species, and this result is robust against the actual definition of Tg. This generic relation between the activation energy and the glass transition temperature indicates that high-temperature dynamics and the glassy slowdown are related phenomena, and the knowledge of E∞ may allow us to approximately predict Tg. [ABSTRACT FROM AUTHOR]

We perform classical molecular dynamics simulations of polycrystalline ice at 250 K using the TIP4P/Ice model. The structures of polycrystalline ice are prepared by growing ice particles in supercooled water. An order parameter developed recently is used to characterize local structures in terms of the liquid–liquid phase transition scenario. It is shown that the grain boundaries and triple junctions in ice are structurally similar to low-density liquid water in which most water molecules form four hydrogen bonds and the O–O–O angles deviate from the tetrahedral angle of 109.47°. The thickness of the grain boundaries is ∼1 nm. The diffusion coefficient of water molecules along the grain boundaries calculated in this study, 5.0 × 10−13 m2 s−1, is in good agreement with experimental data. The diffusion along the triple junctions is 3.4 times faster than that along the grain boundaries. We model the grain size dependence of diffusivity of water molecules in polycrystalline ice using the simulation results and find that the impact of the grain boundaries and the triple junctions on the diffusivity is negligible for typical polycrystalline ice samples having grain sizes of the order of millimeters. We also demonstrate that the properties of the grain boundaries are quite different from those of the ice/vapor interface at the same temperature: the quasi-liquid layer at the ice/vapor interface is similar to high-density liquid water and the diffusion coefficient along the ice/vapor interface is two orders of magnitude larger than that along the grain boundaries. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Kinetic rate factors of crystallization have a direct effect on formation and growth of an ordered solid phase in supercooled liquids and glasses. Using the crystallizing Lennard-Jones liquid as an example, in the present work, we perform a direct quantitative estimation of values of the key crystallization kinetic rate factors—the rate g+ of particle attachments to a crystalline nucleus and the rate g− of particle detachments from a nucleus. We propose a numerical approach, according to which a statistical treatment of the results of molecular dynamics simulations was performed without using any model functions and/or fitting parameters. This approach allows one to accurately estimate the critical nucleus size nc. We find that for the growing nuclei, whose sizes are larger than the critical size nc, the dependence of these kinetic rate factors on the nucleus size n follows a power law. In the case of the subnucleation regime, when the nuclei are smaller than nc, the n-dependence of the quantity g+ is strongly determined by the inherent microscopic properties of a system, and this dependence cannot be described in the framework of any universal law (for example, a power law). It has been established that the dependence of the growth rate of a crystalline nucleus on its size goes into the stationary regime at the size n > 3nc particles. [ABSTRACT FROM AUTHOR]

It was found recently that the liquid dynamics of several metals (Li, Zn, Ni, Fe, Tl, Pb) under pressure is characterized by transverse spectral functions containing an additional high-frequency peak. To rationalize the pressure dependence of the contributions from different propagating processes to transverse spectral functions in liquid metals, ab initio molecular dynamics simulations were performed for two typical liquid metals (Na and Al) in a wide range of pressures. The influence of density/pressure is investigated for Na by considering four pressures ranging from 15 to 147 GPa, while the temperature influence is considered for Al between 600 K in the deep supercooled liquid up to 1700 K well above the melting point at ambient pressure. Both temperature and density dependence of the spectra of collective excitations are analyzed with a focus on the appearance of a second high-frequency mode in the transverse spectra. A correspondence between spectra of transverse collective excitations and the peak positions of the Fourier-spectra of velocity autocorrelation functions (vibrational density of states) is found. [ABSTRACT FROM AUTHOR]

As a result of examining the structure of Zr55Cu35Al10 alloy around the glass transition temperature (Tg) using the classical molecular dynamics simulations, it was proven that the atomic bonds in the interconnecting zones (i-zones) became loose with the small amount of energy absorption, and it became free volumes easily when the temperature approached Tg. Instead of i-zones, when clusters were largely separated by free volume networks, the solid amorphous structure was converted into supercooled liquid state, resulting in a sharp strength reduce and the great plasticity change from a limited plastic deformation to superplasticity. [ABSTRACT FROM AUTHOR]

As a result of examining the structure of Zr55Cu35Al10 alloy around the glass transition temperature (Tg) using the classical molecular dynamics simulations, it was proven that the atomic bonds in the interconnecting zones (i-zones) became loose with the small amount of energy absorption, and it became free volumes easily when the temperature approached Tg. Instead of i-zones, when clusters were largely separated by free volume networks, the solid amorphous structure was converted into supercooled liquid state, resulting in a sharp strength reduce and the great plasticity change from a limited plastic deformation to superplasticity. [ABSTRACT FROM AUTHOR]

As a result of examining the structure of Zr55Cu35Al10 alloy around the glass transition temperature (Tg) using the classical molecular dynamics simulations, it was proven that the atomic bonds in the interconnecting zones (i-zones) became loose with the small amount of energy absorption, and it became free volumes easily when the temperature approached Tg. Instead of i-zones, when clusters were largely separated by free volume networks, the solid amorphous structure was converted into supercooled liquid state, resulting in a sharp strength reduce and the great plasticity change from a limited plastic deformation to superplasticity. [ABSTRACT FROM AUTHOR]

Liquid water is the most intriguing liquid in nature, both because of its importance to every known form of life, and its numerous anomalous properties, largely magnified under supercooled conditions. Among the anomalous properties of water is the seeming divergence of the thermodynamic response functions and dynamic properties below the homogenous nucleation temperature (~ 232 K). Furthermore, water exhibits an increasingly decoupling of the viscosity and diffusion, upon cooling, resulting in the breakdown of the Stokes-Einstein relationship (SER). At high temperatures and pressures, however, water behaves more like a "simple" liquid. Nonetheless, experiments at 400 K and GPa pressures (Bove et al. (2011) Phys. Rev. Lett., 111:185,901) showed that although the diffusion decreases monotonically with the pressure, opposite to pressurized supercooled water, a decoupling of the viscosity and diffusion, larger than that found in supercooled water at normal pressure, is observed. Here, we studied the validity of SER and different pressure-dependent thermodynamic response functions, known to exhibit an abnormal behavior upon cooling, including the density, isothermal compressibility, and the thermal expansion coefficient along the 400 K isotherm up to 3 GPa through molecular dynamics simulations. Seven different water models were investigated. A monotonic increase of the density (~ 50%) and decrease of the isothermal compressibility (~ 90%) and thermal expansion (~ 65%) is found. Our results also show that compressed hot water has various resemblances to cool water at normal pressure, with pressure inducing the formation of a new second coordination sphere and a monotonic decrease of the diffusion and viscosity coefficients. Whereas all water models provide a good account of the viscosity, the magnitude of the violation of the SER at high pressures (> ~ 1 GPa) is significantly smaller than that found through experiments. Thus, violation of the SER in simulations is comparable to that observed for liquid supercooled water, indicating possible limitations of the water models to account for the local structure and self-diffusion of superheated water above ~ 1 GPa. [ABSTRACT FROM AUTHOR]

The shadow energy, Es, is the conserved quantity in microcanonical ensemble (NVE) molecular dynamics simulations carried out with the position Verlet central-difference algorithm. A new methodology for calculating precise and accurate values of Es is presented. It is shown for the first time that Es rather than E is constant during structural changes occurring within a supercooled liquid. It is also explained how to prepare and conduct microsecond range bulk-phase NVE simulations with essentially zero energy drift without the need for thermostating. The drift is analyzed with block averaging and new drift functions of the shadow energy. With such minimal drift, extremely small and accurate standard errors in the mean for quantities like Es, E, and temperature, T, can be obtained. Values of the standard error for Es of ≈10−10 in molecule-based reduced units can be routinely achieved for simulations of 108 time steps. This corresponds to a simulation temperature drift of ≈10−6 K/μs, six orders of magnitude smaller than generally considered to be acceptable for protein simulations. We also show for the first time how these treatments can be extended with no loss of accuracy to polyatomic systems with both flexible degrees of freedom and arbitrary geometric constraints imposed via the SHAKE algorithm. As a bonus, estimates of simulation-average kinetic and total energies from high order velocity expressions can be obtained to a good approximation from 2nd order velocities and the average mean square force (for polyatomics, this refers to per site, including any constraint forces). [ABSTRACT FROM AUTHOR]

The transport of small penetrants through disordered materials with glassy dynamics is encountered in applications ranging from drug delivery to chemical separations. Nonetheless, understanding the influence of the matrix structure and fluctuations on penetrant motions remains a persistent challenge. Here, we use event-driven molecular dynamics to investigate the transport of small, hard-sphere tracers embedded in matrices of square-well particles. Short-range attractions between matrix particles give rise to reentrant dynamics in the supercooled regime, in which the liquid's relaxation time increases dramatically upon heating or cooling. Heating results in a "repulsive" supercooled liquid where relaxations are frustrated by steric interactions between particles, whereas cooling produces an "attractive" liquid in which relaxations are hindered by long-lived interparticle bonds. Further cooling or heating, or compression, of the supercooled liquids results in the formation of distinct glasses. Our study reveals that tracer transport in these supercooled liquids and glasses is influenced by the matrix structure and dynamics. The relative importance of each factor varies between matrices and is examined in detail by analyzing particle mean-square displacements, caging behavior, and trajectories sampled from the isoconfigurational ensemble. We identify features of tracer dynamics that reveal the spatial and temporal heterogeneity of the matrices and show that matrix arrest is insufficient to localize tracers. [ABSTRACT FROM AUTHOR]

In2Te5 is a stoichiometric compound in the In–Te system of interest for applications in phase change electronic memories and thermoelectrics. Here, we perform a computational study of the structural, dynamical, and electronic properties of the liquid, supercooled liquid, and amorphous phases of this compound by means of density functional molecular dynamics simulations. Models of the supercooled liquid and amorphous phases have been generated by quenching from the melt. The structure of the liquid phase is characterized by a mixture of defective octahedral and tetrahedral local environments of In atoms, while the amorphous phase displays a mostly tetrahedral local geometry for In atoms with corner and edge sharing tetrahedra similar to those found in the crystalline phases of the In2Te5, InTe, and In2Te3 compounds. Comparison with our previous results on liquid and amorphous In2Te3 and further data on the structural properties of liquid In2Te3 are also discussed. The analysis of the electronic properties highlights the opening of a mobility gap in In2Te5 at about 150 K below the liquidus temperature. [ABSTRACT FROM AUTHOR]

We investigate a LiCl:6D2O water solution in the deep undercooled regime as a function of pressure by neutron diffraction, small angle neutron scattering, and molecular dynamics simulations. We probe the structure of the undercooled liquid and the existence of density fluctuations in the system along isotherms just above the observed first-order-like polymorphic transition in the solid between a high density amorphous (s-HDA) and a very high density amorphous (s-VHDA) form [L. E. Bove et al., Phys. Rev. Lett. 106, 125701 (2011)]. We observe a continuous pressure evolution of the structure factor of the liquid and the absence of density fluctuations when crossing the continuation line of the s-HDA and s-VHDA boundary. These results indicate that no liquid-liquid transition is observed in the system in correspondence with the solid polyamorphism. [ABSTRACT FROM AUTHOR]

We propose a method to detect alternating diffusive states undergoing a free diffusive state and a trapped state described by the Ornstein-Uhlenbeck process. Using a stochastic model with alternating diffusive states, a phenomenological model of glassy dynamics, we show that control parameters in the method may be determined by the mean square displacement and the non-Gaussianity parameter. Our method works when diffusivities for the two states are clearly distinct and all the states last longer than a specified relaxation time. Applying our method to molecular dynamics simulation data of supercooled liquids, we show that trapped states last for a long time and the sojourn-time distribution for trapped states becomes a power-law form as the temperature approaches the glass temperature. [ABSTRACT FROM AUTHOR]

We use molecular dynamics simulations to ascertain the effects of geometrical restriction on glass-forming tetrahedral liquids. Striving for a broad approach, we study families of waterlike and silicalike liquids, for which we systematically scale the partial charges and, hence, the relevance of the tetrahedral networks. The confined liquids and the confining matrices consist of the same type of particles to avoid disruptive interactions and distorted structures at the interfaces. Spatially resolved analyses show that these neutral confinements still impose static mobility gradients and density correlations on the liquids. We quantify the increasing degree and range of the altered properties upon cooling. For both families of models, common relations describe the confinement effects of all systems with tetrahedral order, while deviations occur for systems with lower polarities and different structures. The observations are rationalized by considering the fact that a pinned wall imprints a static energy landscape to a neighboring liquid. We explore the properties of this landscape based on changes in vibrational motion and structural relaxation and find that typical barrier heights amount to two to three times the activation energy of bulk dynamics. Combining the present and previous results, we predict the evolution of confinement effects down to the glass transition temperature for liquids without fragile-to-strong crossover. In addition, it is found for silicalike liquids that the temperature dependence of dynamic and static correlation lengths from confinement studies is not affected when cooling through fragile-to-strong transitions of the bulk materials, casting doubt on the relevance of these length scales for the glassy slowdown. [ABSTRACT FROM AUTHOR]

In the liquid-liquid phase transition scenario, supercooled water separates into the high density liquid (HDL) and low density liquid (LDL) phases at temperatures lower than the second critical point. We investigate the effects of hydrophilic and hydrophobic solutes on the liquid-liquid phase transition using molecular dynamics simulations. It is found that a supercooled aqueous NaCl solution separates into solute-rich HDL and solute-poor LDL parts at low pressures. By contrast, a supercooled aqueous Ne solution separates into solute-rich LDL and solute-poor HDL parts at high pressures. Both the solutes increase the high temperature limit of the liquid-liquid separation. The degree of separation is quantified using the local density of solute particles to determine the liquid-liquid coexistence region in the pressure-temperature phase diagram. The effects of NaCl and Ne on the phase diagram of supercooled water are explained in terms of preferential solvation of ions in HDL and that of small hydrophobic particles in LDL, respectively. [ABSTRACT FROM AUTHOR]

We use molecular dynamics simulations to study the behavior of supercooled water subject to different stimuli from a diluted azobenzene hydrophobic probe. When the molecular motor does not fold, it acts as a passive probe, modifying the structure of water around it, while when the motor is active, it induces elementary diffusion processes inside the medium acting mainly on the dynamics. We study two particular densities, the density of ambient water and a lower density around the ambient pressure ice density, chosen to favor high density liquid and low density liquid (LDL) water, respectively. We find that the passive probe induces ever an acceleration or a slowing down of the diffusion process around it depending on the density of water, while the active probe induces acceleration only. We find a crossover between the diffusion coefficients for the two densities near the passive probe, around T = 215 K. This dynamical crossover is associated with a modification of the structure of water near the probe. Structure calculations show a crossover of the proportion of LDL water around the same temperature, suggesting that it induces the observed dynamical crossover. In opposition with these results, the active stimuli increase diffusion for both densities and decrease the proportion of LDL water at low temperatures. However, we also find for the active stimuli a crossover of the LDL proportion between the two densities of study, showing remarkable similarities between active and passive stimulus results. [ABSTRACT FROM AUTHOR]

A broad variety of liquids conform to density scaling: relaxation times can be expressed as a function of the ratio of temperature to density, the latter raised to a material constant γ. For atomic liquids interacting only through simple pair potentials, the exponent γ is very nearly equal to n/3, where n is the steepness of the intermolecular potential, while for molecular liquids having rigid bonds and built using the same interatomic potential, γ > n/3. We find that for this class of molecular liquids, γ = n/δ, where the parameter δ relates the intermolecular distance to the density along an isomorph (the line of approximately constant dynamics and structure). δ depends only on the molecular structure and not the interatomic potential. [ABSTRACT FROM AUTHOR]

We apply a recently developed optimization scheme to obtain effective potentials for alkali and alkaline-earth aluminosilicate glasses that contain lithium, sodium, potassium, or calcium as modifiers. As input data for the optimization, we used the radial distribution functions of the liquid at high temperature generated by means of ab initio molecular dynamics simulations and density and elastic modulus of glass at room temperature from experiments. The new interaction potentials are able to reproduce reliably the structure and various mechanical and vibrational properties over a wide range of compositions for binary silicates. We have tested these potentials for various ternary systems and find that they are transferable and can be mixed, thus allowing us to reproduce and predict the structure and properties of multicomponent glasses. [ABSTRACT FROM AUTHOR]

We perform molecular dynamics simulations of binary glass-forming liquids with high dynamical contrast. In detail, we mix water-like molecules with various polarities, which exhibit strongly different dynamics but very similar structures as neat liquids. It is found that these mixtures show concentration fluctuations, which strongly grow upon cooling towards an expected mixing-demixing phase transition. Because of the developing microphase segregation, the composition and temperature ranges accessible to our approach are limited, but the studied mixtures are ideal models to ascertain the relation between concentration fluctuations and molecular dynamics, in particular, the glassy slowdown of binary liquids, including aqueous solutions. We find that the dynamics of the components decouple upon cooling. While the dynamics of the slow component resembles that of neat glass formers, that of the fast component shows different features, including sub-linear diffusion on length and time scales beyond that of local particle cages and quasi-logarithmic decays of correlation functions. We show that this "anomalous dynamics" of the fast component receives a simple explanation based on a strong dependence of the particle mobility on the local concentration in combination with microphase segregation and dynamical contrast. Explicitly, spatially heterogeneous dynamics, which are more extended and tenacious than in neat glass formers, result from growing concentration fluctuations upon cooling. Due to a high dynamical asymmetry of the components at low temperatures, the slow species sustains the microphase segregation in the mixture, while the fast component moves along the concentration and mobility gradients, leading to remarkable dynamical features. [ABSTRACT FROM AUTHOR]

How glasses relax at room temperature is still a great challenge for both experimental and simulation studies due to the extremely long relaxation time-scale. Here, by employing a modified molecular dynamics simulation technique, we extend the quantitative measurement of relaxation process of metallic glasses to room temperature. Both energy relaxation and dynamics, at low temperatures, follow a stretched exponential decay with a characteristic stretching exponent β = 3/7, which is distinct from that of supercooled liquid. Such aging dynamics originates from the release of energy, an intrinsic nature of out-of-equilibrium system, and manifests itself as the elimination of defects through localized atomic strains. This finding is also supported by long-time stress-relaxation experiments of various metallic glasses, confirming its validity and universality. Here, we show that the distinct relaxation mechanism can be regarded as a direct indicator of glass transition from a dynamic perspective. The mechanism governing structural relaxation in metallic glasses remains elusive, hampering their stability and engineering applications. Here, the authors reveal a distinct relaxation mechanism with a stretching exponent of 3/7, providing new insight for understanding the nature of glass. [ABSTRACT FROM AUTHOR]

• Simulation deviations of melting/solidification temperatures are within 3.24 ℃. • Large electrostatic energy, low van der Waals and binding energies cause supercooling. • Large thermal conductivity of SiO 2 and micro-convection reduce melting temperatures. • Distributions of molecules and ions around SiO 2 affect solidification temperatures. Hydrated salts possess great competitiveness in terms of enhancing utilization efficiency of heat due to large densities of heat storage and low costs. However, the existence of supercooling degree is the common phenomenon for hydrated salts, which might restrict the application prospects in partial fields. This work focuses on hydrated salts with relatively larger latent heats, namely Na 2 CO 3 ·10H 2 O and Na 2 HPO 4 ·14H 2 O in the absence and presence of SiO 2 as well as eutectic hydrated salt prepared with 40 wt% Na 2 CO 3 ·10H 2 O and 60 wt% Na 2 HPO 4 ·12H 2 O. Their melting/solidification temperatures were determined and the supercooling behavior was analyzed based on molecular dynamics simulations. The simulation results of melting/solidification temperatures present small deviations of 0.17–3.24 ℃ in comparison with the experimental data. Additionally, the aggregation phenomenon among salt ions and dispersion phenomena among water molecules and between water molecules and salt ions for supercooled liquid hydrated salts are revealed, which are attributed to stronger electrostatic interactions and weaker van der Waals interactions as well as lower binding energies. Moreover, SiO 2 not only lowers the melting temperatures due to its larger thermal conductivity than hydrated salts and the existence of micro-convection, but affects solidification behavior of hydrated salts. The different influences of SiO 2 on the solidification temperatures of Na 2 HPO 4 ·14H 2 O and Na 2 CO 3 ·10H 2 O were associated with dispersion degree between water molecules and salt ions around SiO 2 , which would be attributed to different strengths of interaction forces between SiO 2 and salt ions. On these bases, this work provides new perspectives for clarifying the phase change characteristics of nano-hydrated salts. [ABSTRACT FROM AUTHOR]

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.   • Supercooled aqueous solutions of water and trehalose are useful for cryoprotection. • The phase diagram in the liquid and supercooled region still shows anomalies. • The liquid-liquid critical point is still present for the lowest concentration. • Trehalose favors the high density liquid phase of water. [ABSTRACT FROM AUTHOR]