Supercritical Water: A Simulation Study to Unravel the Heterogeneity of Its Molecular Structures.

(1) Background: In the quest to accurately model the radiolysis of water in its supercritical state, a detailed understanding of water's molecular structure, particularly how water molecules are arranged in this unique state, is essential. (2) Methods: We conducted molecular dynamics simulations using the SPC/E water model to investigate the molecular structures of supercritical water (SCW) over a wide temperature range, extending up to 800 °C. (3) Results: Our results show that at a constant pressure of 25 MPa, the average intermolecular distance around a reference water molecule remains remarkably stable at ~2.9 Å. This uniformity persists across a substantial temperature range, demonstrating the unique heterogeneous nature of SCW under these extreme conditions. Notably, the simulations also reveal intricate patterns within SCW, indicating the simultaneous presence of regions with high and low density. As temperatures increase, we observe a rise in the formation of molecular clusters, which are accompanied by a reduction in their average size. (4) Conclusions: These findings highlight the necessity of incorporating the molecular complexity of SCW into traditional track-structure chemistry models to improve predictions of SCW behavior under ionizing radiation. The study establishes a foundational reference for further exploration of the properties of supercritical water, particularly for its application in advanced nuclear technologies, including the next generation of water-cooled reactors and their small modular reactor variants that utilize SCW as a coolant.

Understanding the bulk and interfacial structures of ternary and binary deep eutectic solvents with a constant potential method: a molecular dynamics study.

In the last decade, deep eutectic solvents (DESs) have emerged as promising electrolytes in supercapacitors and rechargeable batteries due to their unique properties, wide electrochemical windows, low viscosity, and high ionic conductivity. The molecular structural behavior of these solvents, which plays an important role in their efficiency is not deeply understood. Therefore, in this work, by considering two types of DES electrolytes, we investigate their bulk and interfacial structures at the molecular level using molecular dynamics studies. In this regard, two different DESs-a binary DES including choline chloride and urea in a 1 : 2 molar ratio, and a ternary DES containing choline chloride, urea, and ethylene glycol with a molar ratio of 1 : 1 : 2-are considered. For the bulk phase, the partial site-site and center of mass radial distribution functions (RDFs), the mean square displacement (MSD), and the self-diffusion coefficient of the ternary system are explored. We demonstrate that in deep eutectic solvents, in addition to hydrogen-bonding and long-range and short-range correlations, a variety of neutral species play crucial roles in the properties of the bulk phase. Furthermore, considering two graphene sheets as electrodes on both sides of the DES samples, the profiles of the number density, charge density, orientational order parameter, and electrostatic potentials at different potential conditions near the electrodes are investigated. The results reveal the presence of multilayers of the neutral species in the vicinity of electrodes in addition to the ionic components of both DES systems. Finally, the computed differential capacitances (Cd) for DESs disclose that the positive electrode capacitance is higher than that of the negative electrode, and in the ternary system, the total capacitance is greater than in the binary system. Our findings give a better perspective of a new generation of electrolytes at the molecular level for electric double-layer capacitors.

The effect of Stone-Wales defects and roughness degree on the lubricity of graphene on gold surfaces.

In this study, the lubricity of perfect and defective graphene on the gold substrate (Au (111)) has been investigated by using molecular dynamics simulations. The influence of surface morphology as well as the Stone-Wales (SW) defects concentration on the friction of graphene on the gold surface is explored. The SW defects in the range of 0-2.55% are randomly distributed into the graphene. Furthermore, the self-affine fractal method is employed to generate realistic rough surfaces. The effect of the external force, F E , in the range of 0.25-1.0 nN, on the drag coefficients is also investigated. It is shown that the friction force slightly depends on the sliding velocity for all systems. We show that by increasing the defect concentration, the lubricity of graphene nano-sheet slightly decreases. Moreover, it is shown that the friction is almost insensitive to the roughness degree, within the range studied. Both of these effects can be rationalized through variations in the real atomic contact area. Graphical abstract By increasing the SW defect concentration of the graphene, the shape of the deformation is different from a sine wave profile, the real contact area, and the friction increases.