Diffusion dynamics of supercooled water modeled with the cage-jump motion and hydrogen-bond rearrangement

Unraveling the dynamic slowdown in supercooled water: The role of dynamic disorder in jump motions

When a liquid is rapidly cooled below its melting point without inducing crystallization, its dynamics slow down significantly without noticeable structural changes. Elucidating the origin of this slowdown has been a long-standing challenge. Here, we report a theoretical investigation into the mechanism of the dynamic slowdown in supercooled water, a ubiquitous yet extraordinary substance characterized by various anomalous properties arising from local density fluctuations. Using molecular dynamics simulations, we found that the jump dynamics, which are elementary structural change processes, deviate from Poisson statistics with decreasing temperature. This deviation is attributed to slow variables competing with the jump motions, i.e., dynamic disorder. The present analysis of the dynamic disorder showed that the primary slow variable is the displacement of the fourth nearest oxygen atom of a jumping molecule, which occurs in an environment created by the fluctuations of molecules outside the first hydration shell. As the temperature decreases, the jump dynamics become slow and intermittent. These intermittent dynamics are attributed to the prolonged trapping of jumping molecules within extended and stable low-density domains. As the temperature continues to decrease, the number of slow variables increases due to the increased cooperative motions. Consequently, the jump dynamics proceed in a higher-dimensional space consisting of multiple slow variables, becoming slower and more intermittent. It is then conceivable that with further decreasing temperature, the slowing and intermittency of the jump dynamics intensify, eventually culminating in a glass transition.

Electron confinement promoted the electric double layer effect of BiOI/β-Bi2O3 in photocatalytic water splitting

In the realm of photocatalysis, understanding the interface issues (solid/solid and solid/liquid) inherent in heterojunction at the atomic level is the ultimate for engineering an efficient photocatalyst. Herein, an electrophoretic deposition technique is adopted to synthesize BiOI/β-Bi2O3 heterojunction, exhibiting superior photocatalytic activity and stability in H2 evolution (91.5 μmol g-1 h-1) and H2O2 production (11.3 mg L-1 h-1). Combined with the experimental and computational results, a lower free energy of hydrogen evolution reaction (252.4 meV) has been observed contrast to BiOI and β-Bi2O3 samples. A carrier transfer process of like S-scheme heterojunction is proposed based on density of states (DOS) and carrier distribution calculations. The theoretical calculations illustrate the transition dipole moment, migration and accumulation of carrier in BiOI/β-Bi2O3 heterojunction. Subsequent ab initio molecular dynamics (AIMD) results of solid/liquid interface systems (BiOI/β-Bi2O3/H2O and β-Bi2O3/H2O) unravel the interface H2O (solvent) behaviors. The local aggregation of photo-generated electrons in BiOI/β-Bi2O3/H2O leads to a large potential drop, high proton migration rate and the steady electric double layer (EDL) structure compared to the β-Bi2O3/H2O, which facilitates the occurrence of photocatalytic reactions in solution. In addition to offering new insights into the hydrogen evolution and proton transfer in the EDL model and the association between the heterojunction effect and EDL structure, this work also introduces a novel design strategy for Bi-based heterojunctions.

Revealing the hidden dynamics of confined water in acrylate polymers: Insights from hydrogen-bond lifetime analysis

Polymers contain functional groups that participate in hydrogen bond (H-bond) with water molecules, establishing a robust H-bond network that influences bulk properties. This study utilized molecular dynamics (MD) simulations to examine the H-bonding dynamics of water molecules confined within three poly(meth)acrylates: poly(2-methoxyethyl acrylate) (PMEA), poly(2-hydroxyethyl methacrylate) (PHEMA), and poly(1-methoxymethyl acrylate) (PMC1A). Results showed that H-bonding dynamics significantly slowed as the water content decreased. Additionally, the diffusion of water molecules and its correlation with H-bond breakage were analyzed. Our findings suggest that when the H-bonds between water molecules and the methoxy oxygen of PMEA are disrupted, those water molecules persist in close proximity and do not diffuse on a picosecond time scale. In contrast, the water molecules H-bonded with the hydroxy oxygen of PHEMA and the methoxy oxygen of PMC1A diffuse concomitantly with the breakage of H-bonds. These results provide an in-depth understanding of the impact of polymer functional groups on H-bonding dynamics.

Water Diffusion Proceeds via a Hydrogen-Bond Jump Exchange Mechanism

The self-diffusion of water molecules plays a key part in a broad range of essential processes in biochemistry, medical imaging, material science, and engineering. However, its molecular mechanism and the role played by the water hydrogen-bond network rearrangements are not known. Here we combine molecular dynamics simulations and analytic modeling to determine the molecular mechanism of water diffusion. We establish a quantitative connection between the water diffusion coefficient and hydrogen-bond jump exchanges, and identify the features that determine the underlying energetic barrier. We thus provide a unified framework to understand the coupling between translational, rotational, and hydrogen-bond dynamics in liquid water. It explains why these different dynamics do not necessarily exhibit identical temperature dependences although they all result from the same hydrogen-bond exchange events. The consequences for the understanding of water diffusion in supercooled conditions and for water transport in complex aqueous systems, including ionic, biological, and confined solutions, are discussed.

Comparing Microscopic and Macroscopic Dynamics in a Paradigmatic Model of Glass-Forming Molecular Liquid

Glass transition is a most intriguing and long-standing open issue in the field of molecular liquids. From a macroscopic perspective, glass-forming systems display a dramatic slowing-down of the dynamics, with the inverse diffusion coefficient and the structural relaxation times increasing by orders of magnitude upon even modest supercooling. At the microscopic level, single-molecule motion becomes strongly intermittent, and can be conveniently described in terms of “cage-jump” events. In this work, we investigate a paradigmatic glass-forming liquid, the Kob–Andersen Lennard–Jones model, by means of Molecular Dynamics simulations, and compare the macroscopic and microscopic descriptions of its dynamics on approaching the glass-transition. We find that clear changes in the relations between macroscopic timescales and cage-jump quantities occur at the crossover temperature where Mode Coupling-like description starts failing. In fact, Continuous Time Random Walk and lattice model predictions based on cage-jump statistics are also violated below the crossover temperature, suggesting the onset of a qualitative change in cage-jump motion. Interestingly, we show that a fully microscopic relation linking cage-jump time- and length-scales instead holds throughout the investigated temperature range.

Dielectric relaxation of water: assessing the impact of localized modes, translational diffusion, and collective dynamics

A multitude of distinct physical processes and molecular mechanisms have been introduced in the past in an effort to understand the unusual dielectric loss spectrum of water with its pronounced peak at roughly 20 GHz. Our computer simulations including ab initio molecular dynamics provide no evidence for a major impact of cage dynamics or local-diffusive motion on the lineshape below 200 GHz. We also show that the collective motion of hundreds of water molecules and/or their significant diffusive displacements are not required. Instead, the dielectric relaxation of water up to about 200 GHz can be quantitatively described in terms of two unimodal and smoothly decaying spectral contributions.

Breakdown of the Stokes-Einstein relation in supercooled liquids: A cage-jump perspective

The breakdown of the Stokes-Einstein relation in supercooled liquids, which is the increase in the ratio τ_ατ_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_ατ_D∝⟨t_p⟩⟨t_c⟩, where ⟨t_p⟩ and ⟨t_c⟩ 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⟩⟨Δr_J ²⟩ and τ_D∝⟨t_c⟩⟨Δr_J ²⟩ effectively connect the macroscopic and microscopic timescales, with the mean square jump length ⟨Δr_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.

Transition pathway of hydrogen bond switching in supercooled water analyzed by the Markov state model

In this work, we examine hydrogen-bond (H-bond) switching by employing the Markov State Model (MSM). During the H-bond switching, a water hydrogen initially H-bonded with water oxygen becomes H-bonded to a different water oxygen. MSM analysis was applied to trajectories generated from molecular dynamics simulations of the TIP4P/2005 model from a room-temperature state to a supercooled state. We defined four basis states to characterize the configuration between two water molecules: H-bonded ("H"), unbound ("U"), weakly H-bonded ("w"), and alternative H-bonded ("a") states. A 16 × 16 MSM matrix was constructed, describing the transition probability between states composed of three water molecules. The mean first-passage time of the H-bond switching was estimated by calculating the total flux from the HU to UH states. It is demonstrated that the temperature dependence of the mean first-passage time is in accordance with that of the H-bond lifetime determined from the H-bond correlation function. Furthermore, the flux for the H-bond switching is decomposed into individual pathways that are characterized by different forms of H-bond configurations of trimers. The dominant pathway of the H-bond switching is found to be a direct one without passing through such intermediate states as "w" and "a," the existence of which becomes evident in supercooled water. The pathway through "w" indicates a large reorientation of the donor molecule. In contrast, the pathway through "a" utilizes the tetrahedral H-bond network, which is revealed by the further decomposition based on the H-bond number of the acceptor molecule.

Strongly diluted dimethyl-imidazolium chloride–alcohol solutions: solvents are structurally different but dynamic heterogeneities are similar

Based on the analysis of dynamic properties of ionic liquid solutions, the descriptions of diffusion mechanisms are built for dimethylimidazolium chloride (dmim⁺/Cl⁻)–alcohol solute systems and the influence of the monohydric alcohols' molecular structure on their diffusion mechanisms in dmim⁺/Cl⁻–alcohol at T = 400 K by molecular dynamics simulations are studied. From the analysis of radial distribution functions, MSDs, velocity autocorrelation function, and autocorrelation functions of dispersion we found that the motion of all components in IL dmim⁺/Cl⁻–alcohol (ethanol, propanol) systems at T = 400 K occurs in the sub-diffuse regime and that the dynamics of the dmim⁺/Cl⁻–alcohol (ethanol, propanol) systems is heterogeneous. The increase of the alkyl chain length of the alcohol molecule does not affect the motion of the ionic liquid components; instead, it increases the characteristic times describing the model representation of alcohol molecule diffusion at short and medium times, without affecting diffusion mechanisms.

The increase of the alkyl chain length of the alcohol molecule does not affect the motion of the ionic liquid components.